Human Evolution

Determining when a fossil find is an early human

The hominid fossil record

Appearance of modern-looking humans

Resources

The history of how the modern human species, Homo sapiens sapiens, evolved is reconstructed by evidence gathered by paleontologists, anthropologists, archeologists, anatomists, biochemists, behavioral scientists, and other professions. Evidence comes from the record left by fossils and by extrapolation from modern primates, human hunter-gatherer tribes, and (in recent years) rapidly increasing knowledge of DNA (deoxyribonucleic acid), both human and other.

Fossils are evidence of past life. In practice, human fossils are mostly bones and teeth, which are the parts of the human body least likely to decompose. Most individuals never fossilize; it is extremely unusual for bodies to be subjected to all of the conditions necessary for fossilization. Nevertheless, there is an unusually rich record of transitional fossils showing the gradual formation of human features from those of our immediate prehuman apelike ancestors. The growth of the skull, the shrinking of the jaw, and the shift to upright posture are all well documented.

Scientists date fossils by one of several techniques, including carbon-14 dating, which measures the ratio of radioactive carbon to stable carbon, and potassium-argon dating, which measures the ratio of a radioactive form of the element potassium to its breakdown product, argon. Before these methods were available, index fossils of a particular geologic period were used to give an approximate date to other fossils. More recent dating methods include thermoluminescence, electron spin resonance, and fission track dating.

Paleontologists try to recreate the entire animal from sparse bone fragments by comparing the fossil fragments with similar animals, both now living or fossil, of which more information is known. Since complete fossils are rarely found, anatomists recreate the entire skeleton by comparing it with other individuals from the same species or with closely related species. Muscles are reconstructed over the skeleton based on a knowledge of anatomy, and the animal is positioned based upon how a similar living animal would move.

Studies of the DNA of humans and the great apes indicate that the closest living relatives of humans are chimpanzees and gorillas. Humans are not thought to be direct descendants of apes, rather we have descended from a common ancestor. Initial studies comparing chimpanzee and human DNA estimated that the similarity is 98.5%. However, recent studies showed that this similarity is more likely to be lower and is estimated at 95%. The final verdict will be delivered in a few years when the chimpanzee genome project undertaken by the Riken Institute, is finished. Despite being closely related and having some things in common (number of bones) there are distinct differences between humans and chimps. These include the human’s larger brain, ability to speak due to a differently built larynx, ability to walk upright on two legs instead of swinging or knuckle-walking, and greater manual dexterity, due to the opposable thumb that enables humans to manipulate small tools with precision. The faces of humans are flattened, or reduced compared to the apes. The human skeleton is similar to that of a chimp or a gorilla, but is modified for walking upright on two legs. At some point in our development, humans began to rely more on learned behavior (which creates culture) than on genetically fixed or instinctive behavior. This cultural development might be indicated by remains other than bones or teeth, including objects such as stone tools. The first appearance of those traits in the fossil record indicate that those animals were nearly as human as us, which makes them a possible ancestor.

Determining when a fossil find is an early human

What is it that makes us essentially human? Our name, Homo sapiens, means “wise man.” Intelligence is the quality most widely seen as making humans unique. Fossil evidence of intelligence is based upon brain size measured in volume (cubic centimeters, cc). Human brains are three times larger than any comparable primate of a similar weight. Although they grow after birth at a rate that is average for a mammal, they continue to grow for much longer than other animals. Our brains also have different proportions than other primates. Particular areas of the human brain have developed in unique ways, especially the parts of the brain responsible for speech. The other physical traits that we have uniquely acquired include an upright posture, walking on two feet, and an opposable thumb. Finally, human young are cared for over a longer period of time than any other primate.

Other animals besides humans use tools, such as chimpanzees, who fashion twigs into devices to poke termites from termite mounds. However, humans make and preserve tools with anticipation of using them in the future. Some species of animals communicate using complex sounds or show evidence of aiding another, such as dolphins and whales. Although other primates do not use symbolic language, where the meaning of words is learned, they are capable of leaning our system of symbols; chimpanzees and gorillas have been taught to use and understand American Sign Language. Humans are unique in having developed written languages—but not all human societies have done so. In general, it is difficult to point to any single quality or ability in human beings that is not shared with some other species, especially with primates, but the degree to which human beings modify themselves and their environment through cultural behaviors has no parallel.

The hominid fossil record

The first pre-human fossil to be named was Australopithecus africanus, meaning the southern ape of Africa. The fossils were found at a site called Taung in South Africa by Raymond Dart, who recognized it as being intermediate between apes and humans. The fossils are dated at three million years old. Additional fossils of A. africanus were discovered at Sterkfontein and at Makapanskat in South Africa. The bones from other animals found along with A. africanus were interpreted as meaning that our ancestors were hunters. Other scientists determined that those bones were actually the leftover meals from leopards and hyenas. It is now believed that A. africanus was primarily a vegetarian, and probably did little, if any, hunting. Teeth wear patterns indicate that A. africanus ate fruit and foliage. No stone tools were found with any of these fossils, so there is no evidence that Australopithecus made or used tools, or used fire.

In 1912, William Dawson discovered pieces of a skull and jaw along with stone tools and index fossils at Piltdown in England. The jaw was apelike, but the skull was humanlike. British anthropologists at the time judged the find to be authentic, perhaps because it appeared to support a cherished belief that humans had first developed a big brain, and then later developed other human characteristics. It was subsequently discovered that Piltdown man was a hoax, composed of a human braincase and the jaw from an orangutan, modified to look old.

Australopithecus afarensis, the southern ape of the Afar region in Ethiopia, was discovered more recently and found to be the oldest known humanlike animal to have walked upright. The most famous of these fossils, nicknamed “Lucy,” was found near Hadar, Ethiopia, by a team of anthropologists led by Dr. Donald Johanson. Lucy lived about 3.5 million years ago, and had a skull, knees and a pelvis more similar to ours than to the apes. Her brain size was about 350 cc, which was less than one third of the brain size of modern humans (1,400 cc), yet larger than any apelike ancestor to have come before. She would have stood at a height of about 3.5 ft (1 m) tall, with long arms, a v-shaped jaw, and a large projecting face.

Fossils of several male and female Australopithecus have been found together. There is some uncertainty as to whether these are A. afarensis or another closely related species. This group find gives evidence that they were social animals. Two of these early humanlike ancestors also left a trail of footprints at Laetoli in East Africa in what was then volcanic ash that later became fossilized. These were discovered by Mary Leakey (1913–1996), the wife of the pioneer paleontologist, Louis Leakey (1903–1972). The fossil footprints look very similar to modern human prints and add further proof that our ancestors walked upright.

The reasons our ancestors started to walk upright are not known. Possibly, it was a response to environmental changes; as tropical forests were beginning to shrink, walking might have been a better way to cross the grasslands to get to nearby patches of forest for food. We can get some ideas of possible advantages of upright posture to our ancestors by studying modern apes. When chimpanzees or gorillas become excited, they stand in an upright posture and shake a stick or throw an object. By standing upright, they appear bigger and more impressive in size than they normally are. This would be useful to help protect the group against predators. Also, the ability to stand up and get a wider view of the surroundings gives an animal an advantage in the tall grasses. Walking upright frees up the hands to carry objects, such as tools.

Two other species of Australopithecus are A. robustus and A. boisei. Australopithecus robustus, from South Africa, was named for its massive jaws and large flat chewing teeth. This species also had a bony ridge along the top of its skull (the sagittal crest) similar to that of an adult male gorilla, which served as a site of attachment for massive jaw muscles. Its skull had the brain capacity of 500 cc. Living about 1.9–1.5 million years ago, the diet of A. robustus probably consisted of tough gritty foods, such as plant tubers. Australopithecus robustus was probably not a direct ancestor of modern humans. The other Australopithecus species, A. boisei, was discovered by Louis Leakey at Olduvai Gorge in Tanzania, a site that has been famous for hominid fossils for more than 60 years. Sediments and fossils are exposed in the walls of the gorge that represent almost two million years of evolutionary history. Australopithecus boisei had huge flat grinding teeth, a very long face, and a large elongated cranium, with a brain capacity of 530 cc and a sagittal crest atop the skull.

The record of animals that were ancestral to Australopithecus is poor. An apelike animal (Ramapithecus ), lived in Africa some 12 million years ago and is thought to have been the first representative of the line leading to humans. Ramapithecus lived on the forest fringe, near rivers and lakes, and began to make the transition to life on the more open savanna. Very few remains of Ramapithecus have been found, only fragments of upper and lower jaws and teeth. Its dental pattern was unique among other fossil finds from that time. The canine teeth were fairly small, indicating that its diet may have included seeds and other tough plant material that required being torn apart before eaten. A five million-year gap in the fossil record between the time of Ramapithecus and Australopithecus has been recently partially filled by new finds in Africa, although it is not yet clear where exactly on the human evolution tree these fossils will be placed. Remains of a hominid from six million years ago were found at Kapsomin by a French and Kenyan anthropological group led by Martin Pickford and Brigitte Senut in 2001. It was named Orrorin tugenensis. However, in July 2002, Professor Michael Brunet with an international group of scientists found an even older (seven million-year-old) skull in Chad, called Sahelanthropus tchadensis, nicknamed Toumai. The opinions on whether it is the skull of a hominid or an ancient gorilla are divided. Independent of the final verdict, the fact that the skull was nearly intact is very important for further comparative analysis.

Australopithecus is similar enough to humans to be considered an ancestor, but different enough to be assigned to a separate genus. On the other hand, Homo habilis, which lived about 1.5–2 million years ago, is similar enough to modern humans be included in the genus Homo. The braincase of H. habilis was appreciably larger than that of Australopithecus, with a brain capacity of 750 cc. Homo habilis individuals were short and made stone tools from pebbles about 5 in (12.7 cm) long, formed from flakes of rock. The flakes had been broken off the pebble by blows from another stone and were probably used for cutting.

Homo erectus is generally thought to have been our direct ancestor. Homo erectus lived about 1.7 million years ago, and had a brain capacity of 950 cc. The first fossil of Homo erectus was found in Java; it was nicknamed Java man. Similar fossils found in China were dubbed Peking man. Recently, an entire skeleton of a closely related species, Homo ergaster, was found in Kenya. Walking with a fully upright posture, tall and slender, the fossils were found with sophisticated stone tools. They were probably hunters and also scavengers. Bones found along with the fossils have been studied closely; they carry the remains of tooth marks from predators, like leopards, as well as hominid tooth marks. Homo erectus probably scavenged from kills made by large predators, breaking bones to eat the rich marrow. The presence of charcoal provides evidence that H. erectus used fire, probably to scare off predators.

H. erectus was thought to be the first hominid to leave Africa. This notion was recently (2001) challenged by David Lordkipanidze’s group finds in the Georgian village of Dmanisi. The skulls found were much smaller (estimated brain size 600–780 cc) than those of H. erectus but had enough similarities to be classified in the same species. The fact that they were dated to 1.7–1.8 million years ago challenges the notion that long-legged, large-brained H. erectus left Africa around one million years ago. A hypothesis was made by Vekua and colleagues that Dminisi hominids might have evolved from H. habilis outside Africa. Confirmation of such a hypothesis, however, will require further fossil evidence.

Neanderthal man (Homo sapiens neanderthalensis ) was the first human fossil to be found. It was discovered in 1856 in Germany’s Neander Valley and is the source of the caveman stereotype. Neanderthals first appeared 300,000 years ago in what is now Europe, lived throughout the ice ages, and were thought to disappear about 35,000 years ago, but recently remains from 28,000 years ago were found in Croatia. Neanderthals had a large brain capacity about (1,500 cc), a strong upper body, a bulbous nose, and a prominent brow ridge. There is general agreement today that the Neanderthals were not our direct ancestors, but a cousin species. In 1997, Krings and colleagues based on the analysis of mitochondrial DNA from Neanderthal remains concluded that modern humans and Neanderthals were not related. However, just a year later in 1998 professor Erik Trinkaus discovered a skeleton of a four-year-old boy from 25,000 years ago that showed a mixture of Neanderthal and modern features suggesting that the two species interbred. But interbreedability would be consistent with a recent common ancestor species, and does not necessarily show lineal descent of humans from Neanderthals.

Recent excavations in Israel, Portugal, and Croatia show clearly that Neanderthals were contemporary with modern Homo sapiens sapiens. The two hominids apparently survived independently of each other for tens of thousands of years. Some anthropologists see this as evidence that Neanderthals were not our direct ancestors; other anthropologists speculate the two types of humans may have interbred and Neanderthal became genetically absorbed by more modern humans. We do not know why Neanderthals died out, nor what the nature of their interaction with modern Homo sapiens sapiens might have been.

Neanderthal man made a number of crafted flint tools with many different uses. Judging from the hearths found at many sites, Neanderthals had mastered the art of making fire. Fossil bones show signs of old injuries that had healed, indicating the victim had been cared for. Some Neanderthal caves contain burial sites. The Shanidar cave in Iraq held the remains of a Neanderthal buried 60,000 years, with bunches of flowers. Several of the flowers discovered were species used today as herbal medicines. It is therefore possible that Neanderthals had an elaborate culture, were aware of the medicinal properties of plants, and ritually buried their dead. One anthropologist in Israel found what he believed to be evidence that Neanderthals had the capacity for speech, a fossil bone from the throat (the hyoid), which anchors the muscles connected to the larynx and tongue, and which permit speech in modern humans.

Appearance of modern-looking humans

Although all of the ancestors described thus far first evolved in Africa, there is uncertainty as to where modern humans, Homo sapiens sapiens, first appeared. There are two theories to explain this process. The first is the “multi-regional” model, which proposes that Homo sapiens sapiens evolved in Europe, Asia, and Australia from Homo erectus after the latter left Africa about one million years ago. The second model, called Out of Africa, suggests that modern humans evolved only once, in Africa, leaving there within the last 200,000 years in a rapid global expansion. They replaced other populations of older human forms in Europe and Asia, including the Neanderthals. Variations and combinations of these two theories have also been proposed.

The oldest fossils of modern human beings, Homo sapiens sapiens, are 100,000–125,000 years old, appearing at the time of the first of the great ice ages. Homo sapiens sapiens are identified by a large brain (1,400 cc), a small face in proportion to the size of the skull, a small chin, and small teeth. In addition, they were tall and relatively slender in build.

The first fossil of modern Homo sapiens sapiens was found at Cro-Magnon in France, which gave that name to all early Homo sapiens sapiens. Cro-Magnon remains have been found along with the skeletons of woolly mammoth, bison, reindeer, and with tools made from bone, antler, ivory, stone and wood, indicating that Cro-Magnon hunted game of all sizes. Cro-Magnons also cooked their food in skin-lined pots heated with stones. Pieces of amber from the Baltic found in southern Europe together with Cro-Magnon

KEY TERMS

Hominid —A primate in the family Homidae, which includes modern humans.

Primate —An animal of the order Primata, which includes lemurs, monkeys, apes, and humans.

fossils indicate these humans traded material over vast distances. Cro-Magnon humans buried their dead with body ornaments such as necklaces, beaded clothing, and bracelets.

Cro-Magnon humans lived at the mouth of caves under shelters made of skins or in huts made of sticks, saplings, stones, animal skins, or even bones. A mammoth-bone hut 15,000 years old has been excavated at a site in the Ukraine. Anthropologists interpret some of the fossil findings of early Homo sapiens sapiens, by making comparisons with present-day hunter-gatherer tribes such as the Kalahari or Kung bushmen. These nomadic people live in relatively small bands of about 25 people. A larger group of about 20 bands makes up a community of people who all speak the same dialect and occasionally gather in large groups. The groups disperse into smaller bands during the wet season, and establish clusters around permanent water holes in the dry season. The men hunt in cooperative bands when the game is plentiful. The women gather plant material; about two thirds of their diet is made up of plant food. Since only a small portion of time is spent hunting or gathering, there is plenty of time for visiting, entertaining, and sewing. The same might be true for the hunting people living in Europe before 12,000 years ago.

Ice age humans were artists, producing hauntingly beautiful cave art. Carefully rendered pictures of animals, human and mythical representations, and geometric shapes and symbols were created using charcoal and other pigments. The remains of stone lamps found deep within these caves suggested that the caves were visited often. Carvings of stone, ivory and bone have also been discovered in these caves, including female figures. We cannot know what the significance of this art was to them, other than that it was a reflection of how early humans perceived the world around them.

The end of the ice ages brought changes in climate and ecosystems. In Europe, the vast grasslands were replaced by forests, and animal populations shifted from reindeer and bison to red deer and boar. The focus of cultural innovation shifted from Europe to the Middle East, where settled cultivation began.

See also Dating techniques; Fossil and fossiliza-tion; Genetics.

Resources

BOOKS

Arsuaga, Luis, et al. Chosen Species: The Long March of Human Evolution. Malden, MA: Blackwell Publishing, 2005.

Dobzhansky, T., and E. Boesiger. Human Culture: A Moment in Evolution. New York: Columbia University Press, 1983.

Stringer, Chris and Peter Andrews. The Complete World of Human Evolution. Thames and Hudson, 2005.

PERIODICALS

Balter, Michael. “Are Human Brains Still Evolving? Brain Genes Show Signs of Selection.” Science. 309 (2005): 1662-1663.

Brunet M et al, “A New Hominid from the Upper Miocene of Chad, Central Africa.” Nature (July 2002):145–151

Ponting, Chris P. and Gerton Lunton. “Evolutionary Biology: Human Brain Gene Wins Genome Race.” Nature. 443 (2006): 149-150.

Senut, Brigitte, Pickford, Martin, Gommery, Dominique, Mein, Pierre, Cheboi, Kiptalam, Coppens, Yves, “First Hominid from the Miocene (Lukeino Formation, Kenya).” C.R.Acad.Sci.Paris, Earth Planet Sci. (January 2001) 332:137–144

Vekua, A, Lordkipanidze, D, Rightmire, G.P, Agusti, J, Ferring, R, Maisuradze, G, et al., “A New Skull of Early Homo from Dmanisi, Georgia.” Science (July 2002):85–89

OTHER

Institute of Human Origins, Arizona State University. “Becoming Human: Paleoanthropology, Evolution and Human Origins.” 2006. <www.becominghuman.org> (accessed November 2, 2006).

Marion Dresner

Human evolution

The history of how the human species , Homo sapiens sapiens, evolved is reconstructed by evidence gathered by paleontologists, anthropologists, archeologists, anatomists, biochemists, behavioral scientists, and many other professions. The evidence comes from the record left by fossils and by extrapolation from modern primates and human hunter-gatherer tribes. Fossils are evidence of past life. In practice, human fossils are mostly bones and teeth, which are the parts of the human body least likely to decompose. Most types of fossils are rare; it is extremely unusual for bodies to be subjected to all of the favorable conditions necessary for fossilization. Scientists date fossils by one of several techniques; carbon-14 dating, which measures the ratio of radioactive carbon to stable carbon, and potassium-argon dating, which measures the ratio of a radioactive form of the element potassium to its breakdown product, argon. Before these methods were available, index fossils of a particular geologic period were used to give an approximate date to other fossils. More recent dating methods include thermoluminescence, electron spin resonance , and fission track dating.

Paleontologists try to recreate the entire animal from sparse bone fragments by comparing the fossil fragments with similar animals, both now living or fossil, of which more information is known. Since complete fossils are rarely found, anatomists recreate the entire skeleton by comparing it with other individuals from the same species or with closely related species. Muscles are reconstructed over the skeleton based on a knowledge of anatomy , and the animal is positioned based upon how a similar living animal would move.

Studies of the DNA of humans and the great apes indicate that the closest living relatives of humans are chimpanzees and gorillas . Humans are not thought to be direct descendants of apes, rather we have descended from a common ancestor. Initial studies comparing chimpanzee and human DNA estimated that the similarity is 98.5%. However, recent studies showed that this similarity is more likely to be lower and is estimated at 95%. The final verdict will be delivered in a few years when the chimpanzee genome project undertaken by the Riken Institute, is finished. Despite being closely related and having some things in common (number of bones) there are distinct differences between humans and chimps. These include the human's larger brain , ability to speak due to a differently-built larynx, ability to walk upright on two legs instead of swinging or knuckle-walking, and greater manual dexterity, due to the opposable thumb that enables humans to manipulate small tools with precision. The faces of humans are flattened, or reduced compared to the apes. The human skeleton is similar to that of a chimp or a gorilla, but is modified for walking upright on two legs. At some point in our development, humans began to rely more on learned behavior (which creates culture) than on genetically fixed or instinctive behavior. This cultural development might be indicated by remains other than bones or teeth, including objects such as stone tools. The first appearance of those traits in the fossil record indicate that those animals were nearly as human as us, which makes them a possible ancestor.

Determining when a fossil find is an early human

What is it that makes us essentially human? Our name, Homo sapiens, means "wise man." Intelligence is the quality most widely seen as making humans unique. Fossil evidence of intelligence is based upon brain size measured in volume (cubic centimeters). Human brains are three times larger than any comparable primate of a similar weight. Although they grow after birth at a rate that is average for a mammal, they continue to grow for much longer than other animals. Our brains also have different proportions than other primates. Particular areas of the human brain have developed in unique ways, especially the parts of the brain responsible for speech . The other physical traits that we have uniquely acquired include an upright posture, walking on two feet, and an opposable thumb. Finally, human young are cared for over a longer period of time than any other primate.

Many paleontologists argue that evidence of culture, such as making fire and tools, using spoken language, and having self awareness are some of the less tangible but important qualities that differentiated human ancestors from other animals. Other animals besides humans use tools, such as chimpanzees that fashion twigs into devices to poke termites from a termite mound. However, humans make tools with anticipation of using them in the future. In addition, we have become advanced in our tool making capabilities that we can extend our powers of observation beyond our senses. Some species of animals communicate using complex sounds or show evidence of aiding another, such as dolphins and whales. Although other primates do not use symbolic language, where the meaning of words is learned, they are capable of leaning our system of symbols; chimpanzees and gorillas have been taught to use and understand American Sign Language. Humans are unique in having developed written languages.

There is no direct evidence of when or how self-awareness first arose in humans, but some indirect evidence is available. It appears that only humans exhibit awareness that someday they will die. This death awareness leads humans to bury their dead, and intentional burial leaves a trace in the fossil record.

The hominid fossil record

The first pre-human fossil to be named was Australopithecus africanus, meaning the southern ape of Africa . The fossils were found at a site called Taung in South Africa by Raymond Dart, who recognized it as being intermediate between apes and humans. The fossils are dated at three million years old. Additional fossils of A. africanus were discovered at Sterkfontein and at Makapanskat in South Africa. The bones from other animals found along with A. africanus were interpreted as meaning that our ancestors were hunters. Other scientists determined that those bones were actually the leftover meals from leopards and hyenas. It is now believed that
A. africanus was primarily a vegetarian, and probably did little, if any, hunting. Teeth wear patterns indicate that A. africanus ate fruit and foliage. No stone tools were found with any of these fossils, so there is no evidence that Australopithecus made or used tools, or used fire.

In 1912, William Dawson discovered pieces of a skull and jaw along with stone tools and index fossils at Piltdown in England. The jaw was ape-like, but the skull was humanlike. British anthropologists at the time judged the find to be authentic, perhaps because it appeared to support a cherished belief that humans had first developed a big brain, and then later developed other human characteristics. It was subsequently discovered that Piltdown man was a hoax, composed of a human braincase and the jaw from an orangutan, modified to look old.

Australopithecus afarensis, the southern ape of the Afar region in Ethiopia, was discovered more recently and found to be the oldest known humanlike animal to have walked upright. The most famous of these fossils, nicknamed "Lucy," was found near Hadar, Ethiopia, by a team of anthropologists led by Dr. Donald Johanson. Lucy lived about 3.5 million years ago, and had a skull, knees and a pelvis more similar to ours than to the apes. Her brain size was about 350 cc, which was less than one third of the brain size of modern humans (1,400 cc), yet larger than any ape-like ancestor to have come before. She would have stood at a height of about 3.5 ft (1 m) tall, with long arms, a v-shaped jaw, and a large projecting face.

Fossils of several male and female Australopithecus have been found together. There is some uncertainty as to whether these are A. afarensis or another closely related species. This group find gives evidence that they were social animals. Two of these early humanlike ancestors also left a trail of footprints at Laetoli in East Africa in what was then volcanic ash that later became fossilized. These were discovered by Mary Leakey, the wife of the pioneer paleontologist, Louis Leakey. The fossil footprints look very similar to modern human prints and add further proof that our ancestors walked upright.

The reasons our ancestors started to walk upright are not known. Possibly, it was a response to environmental changes; as tropical forests were beginning to shrink, walking might have been a better way to cross the grasslands to get to nearby patches of forest for food. We can get some ideas of possible advantages of upright posture to our ancestors by studying modern apes. When chimpanzees or gorillas become excited, they stand in an upright posture and shake a stick or throw an object. By standing upright, they appear bigger and more impressive in size than they normally are. This would be useful to help protect the group against predators. Also, the ability to stand up and get a wider view of the surroundings gives an animal an advantage in the tall grasses . Walking upright frees up the hands to carry objects, such as tools.

Two other species of Australopithecus are A. robustus and A. boisei. Australopithecus robustus, from South Africa, was named for its massive jaws and large flat chewing teeth. This species also had a bony ridge along the top of its skull (the sagittal crest) similar to that of an adult male gorilla, which served as a site of attachment for massive jaw muscles. Its skull had the brain capacity of 500 cc. Living about 1.9–1.5 million years ago, the diet of A. robustus probably consisted of tough gritty foods, such as plant tubers. Australopithecus robustus was probably not a direct ancestor of modern humans. The other Australopithecus species, A. boisei, was discovered by Louis Leakey at Olduvai Gorge in Tanzania, a site that has been famous for hominid fossils for more than 60 years. Sediments and fossils are exposed in the walls of the gorge that represent almost two million years of evolutionary history. Australopithecus boisei had huge flat grinding teeth, a very long face, and a large elongated cranium, with a brain capacity of 530 cc and a sagittal crest atop the skull.

The record of animals that were ancestral to Australopithecus is poor. An ape-like animal (Ramapithecus), lived in Africa some 12 million years ago and is thought to have been the first representative of the line leading to humans. Ramapithecus lived on the forest fringe, near rivers and lakes, and began to make the transition to life on the more open savanna . Very few remains of Ramapithecus have been found, only fragments of upper and lower jaws and teeth. Its dental pattern was unique among other fossil finds from that time. The canine teeth were fairly small, indicating that its diet may have included seeds and other tough plant material that required being torn apart before eaten. A five-million-year gap in the fossil record between the time of Ramapithecus and Australopithecus has been recently partially filled by new finds in Africa, although it is not yet clear where exactly on the human evolution tree these fossils will be placed. Remains of a hominid from six million years ago were found at Kapsomin by a French and Kenyan anthropological group led by Martin Pickford and Brigitte Senut in 2001. It was named Orrorin tugenensis. However, in July 2002, Professor Michael Brunet with an international group of scientists found an even older (seven million-year-old) skull in Chad, called Sahelanthropus tchadensis, nicknamed Toumai. The opinions on whether it is the skull of a hominid or an ancient gorilla are divided. Independent of the final verdict, the fact that the skull was nearly intact is very important for further comparative analysis.

Australopithecus is similar enough to humans to be considered an ancestor, but different enough to be assigned to a separate genus. On the other hand, Homo habilis, which lived about 1.5–2 million ago, is similar enough to modern humans be included in the genus Homo. The braincase of H. habilis was appreciably larger than that of Australopithecus, with a brain capacity of 750 cc. Homo habilis individuals were short and made stone tools from pebbles about 5 in (12.7 cm) long, formed from flakes of rock. The flakes had been broken off the pebble by blows from another stone and were probably used for cutting.

Homo erectus is generally thought to have been our direct ancestor. Homo erectus lived about 1.7 million years ago, and had a brain capacity of 950 cc. The first fossil of Homo erectus was found in Java; it was nicknamed Java man. Similar fossils found in China were dubbed Peking man. Recently, an entire skeleton of a closely related species, Homo ergaster, was found in Kenya. Walking with a fully upright posture, tall and slender, the fossils were found with sophisticated stone tools. They were probably hunters and also scavengers. Bones found along with the fossils have been studied closely; they carry the remains of tooth marks from predators, like leopards, as well as hominid tooth marks. Homo erectus probably scavenged from kills made by large predators, breaking bones to eat the rich marrow. The presence of charcoal provides evidence that H. erectus used fire, probably to scare off predators.

H. erectus was thought to be the first hominid to leave Africa. This notion was recently (2001) shaken by David Lordkipanidze's group finds in the Georgian village of Dmanisi. The skulls found were much smaller (estimated brain size 600–780 cc) than those of H. erectus but had enough similarities to be classified in the same species. The fact that they were dated to 1.7–1.8 million years ago challenges the notion that long-legged, large-brained H. erectus left Africa around one million years ago. A hypothesis was made by Vekua and colleagues that Dminisi hominids might have evolved from H. habilis outside Africa. Confirmation of such a hypothesis, however, will require further fossil evidence.

Neanderthal man (Homo sapiens neanderthalensis) was the first human fossil to be found. It was discovered in 1856 in Germany's Neander Valley and is the source of the caveman stereotype. Neanderthals first appeared 300,000 years ago in what is now Europe , lived throughout the ice ages , and were thought to disappear about 35,000 years ago, but recently remains from 28,000 years ago were found in Croatia. Neanderthals had a large brain capacity about (1,500 cc), a strong upper body, a bulbous nose, and a prominent brow ridge. There is disagreement as to whether or not Neanderthals were the direct ancestors of modern humans. The controversy has been fueled even further by disagreement between genetic and morphological evidence. In 1997, Krings and colleagues based on the analysis of mitochondrial DNA from Neanderthal remains concluded that modern humans and Neanderthals were not related. However, just a year later in 1998 professor Erik Trinkaus discovered a skeleton of a four-year-old boy from 25,000 years ago that showed a mixture of Neanderthal and modern features suggesting that the two species interbred.

Recent excavations in Israel, Portugal, and Croatia show clearly that Neanderthals were contemporary with modern Homo sapiens sapiens. The two hominids apparently survived independently of each other for tens of thousands of years. Some anthropologists see this as evidence that Neanderthals were not our direct ancestors; other anthropologists speculate the two types of humans may have interbred and Neanderthal became genetically absorbed by more modern humans. We do not know why Neanderthals died out, nor what the nature of their interaction with modern Homo sapiens sapiens might have been.

Neanderthal man made a number of crafted flint tools with many different uses. Judging from the hearths found at many sites, Neanderthals had mastered the art of making fire. Fossil bones show signs of old injuries that had healed, indicating the victim had been cared for. Some Neanderthal caves contain burial sites, indicating that Neanderthals were probably self-aware. The Shanidar cave in Iraq held the remains of a Neanderthal buried 60,000 years, with bunches of flowers. Several of the flowers discovered were species used today as herbal medicines. It is therefore possible that Neanderthals had an elaborate culture, were aware of the medicinal properties of plants, and ritually buried their dead. One anthropologist in Israel found what he believed to be evidence that Neanderthals had the capacity for speech, a fossil bone from the throat (the hyoid), which anchors the muscles connected to the larynx and tongue, and which permit speech in modern humans.

Appearance of modern-looking humans

Although all of the ancestors described thus far first evolved in Africa, there is uncertainty as to where modern humans, Homo sapiens sapiens, first appeared. There are two theories to explain this process. The first is the "multi-regional" model, which proposes that Homo sapiens sapiens evolved in Europe, Asia , and Australia from Homo erectus after the latter left Africa about one million years ago. The second model, called Out of Africa, suggests that modern humans evolved only once, in Africa, leaving there within the last 200,000 years in a rapid global expansion. They replaced other populations of older human forms in Europe and Asia, including the Neanderthals. Variations and combinations of these two theories have also been proposed.

The oldest fossils of modern human beings, Homo sapiens sapiens, are 125,000-100,000 years old, appearing at the time of the first of the great ice ages. Homo sapiens sapiens are identified by a large brain (1,400 cc), a small face in proportion to the size of the skull, a small chin, and small teeth. In addition, they were tall and relatively slender in build.

The first fossil of modern Homo sapiens sapiens was found at Cro Magnon in France, which gave that name to all early Homo sapiens sapiens. Cro Magnon remains have been found along with the skeletons of woolly mammoth , bison , reindeer, and with tools made from bone, antler, ivory, stone and wood , indicating that Cro Magnon hunted game of all sizes. Cro Magnons also cooked their food in skin-lined pots heated with stones. Pieces of amber from the Baltic found in southern Europe together with Cro Magnon fossils indicate these humans traded material over vast distances. Cro Magnon humans buried their dead with body ornaments such as necklaces, beaded clothing, and bracelets.

Cro Magnon humans lived at the mouth of caves under shelters made of skins, or in huts made of sticks, saplings, stones, animal skins or even bones. A mammoth-bone hut 15,000 years old has been excavated at a site in the Ukraine. Anthropologists interpret some of the fossil findings of early Homo sapiens sapiens, by making comparisons with present-day hunter-gatherer tribes such as the Kalahari or Kung bushmen. These nomadic people live in relatively small bands of about 25 people. A larger group of about 20 bands makes up a community of people who all speak the same dialect and occasionally gather in large groups. The groups disperse into smaller bands during the wet season, and establish clusters around permanent water holes in the dry season. The men hunt in cooperative bands when the game is plentiful. The women gather plant material; about two thirds of their diet is made up of plant food. Since only a small portion of time is spent hunting or gathering, there is plenty of time for visiting, entertaining, and sewing. The same might be true for the hunting people living in Europe before 12,000 years ago.

Ice age humans were artists, producing hauntingly beautiful cave art. Carefully rendered pictures of animals, human and mythical representations, and geometric shapes and symbols were created using charcoal and other pigments. The remains of stone lamps found deep within these caves suggested that the caves were visited often. Carvings of stone, ivory and bone have also been discovered in these caves, including female figures. We cannot know what the significance of this art was to them, other than that it was a reflection of how early humans perceived the world around them.

The end of the ice ages brought changes in climate and ecosystems. In Europe, the vast grasslands were replaced by forests, and animal populations shifted from reindeer and bison to red deer and boar. The focus of cultural innovation shifted from Europe to the Middle East, where settled cultivation began.

There remain many unanswered questions. For example, Homo sapiens sapiens is the only species of hominid now existing. What happened to the preceding older species after the newer human form appeared and replaced it? How will Homo sapiens sapiens evolve in the future? Can humans consciously help to shape future evolution? Humans are the only species with the potential to consciously direct future evolution.

See also Dating techniques; Fossil and fossilization; Genetics.

Resources

books

dobzhansky, t., and e. boesiger. human culture: a moment inevolution. new york: columbia university press, 1983.

johanson, donald l., and maitland b. eddy. ancestors: thesearch for our human origins. new york, random house, 1994.

leakey, richard, and roger lewin. origins: what new discoveries reveal about the emergence of our species and its possible future. new york: viking penguin, 1991.

tattersall, ian. the human odyssey; four million years ofhuman evolution. englewood cliffs, nj: prentice hall, 1993.

willis, delta. the hominid gang: behind the search forhuman origins. new york: viking penguin, 1991.

periodicals

brunet, m, et al. "a new hominid from the upper miocene of chad, central africa." nature (july 2002): 145–151.

duarte, mauricio, c., j. p.b. pettitt, p. souto, e. trinkaus, h.plicht van der, j. zilhao, "the early upper paleolithic human skeleton from abrigo do lugar velho (portugal) and modern human emergence in iberia." proc.natl.acad.sci. usa (june 1999): 7604–7609.

krings, m., a. stone, w. schmitz, h. krainitzki, m. stoneking,s. paabo. "neandertal dna sequences and the origins of modern humans" cell (july 1997):19–30.

Senut, Brigitte, Martin Pickford, Dominique Gommery, Pierre Mein, Kiptalam Cheboi, Yves Coppens. "First Hominid from the Miocene (Lukeino Formation,Kenya)." C.R.Acad.Sci.Paris, Earth Planet Sci. (January 2001) 332: 137–144.

Vekua, A., D. Lordkipanidze, G.P. Rightmire, J. Agusti, R. Ferring, G. Maisuradze, et al. "A New Skull of Early Homo from Dmanisi, Georgia." Science (July 2002): 85–89.

Wood, Bernard, and Mark Collard. "The Human Genus." Science (April 1999): 65–71.

Marion Dresner

KEY TERMS

Hominid

—A primate in the family Homidae, which includes modern humans.

Primate

—An animal of the order Primata, which includes lemurs, monkeys, apes, and humans.

Human Evolution

Human evolution is the lengthy process of change by which people originated from apelike ancestors starting nearly five million years ago. The modern scientific study of human evolution is called paleoanthropology. A subfield of anthropology, this discipline searches for the roots of human physical traits, culture, and behavior. It attempts to answer questions: What makes us human? When and why did we begin to walk upright? How did our brains, language, art, music, and religion develop? By approaching these questions from a variety of directions, using information learned from other disciplines such as molecular biology, paleontology, archaeology, sociology, and biology, we continue to increase knowledge of our evolutionary origins.

Most cultures throughout human history have myths, stories, and ideas about how life and culture came into existence. Although the current theory of evolution, based on the ideas of Charles Darwin, is accepted by a majority of scientists in our time, it is important to remember that many earlier ideas were recognized as well.

Darwin's books, On the Origins of the Species by Natural Selection (1859) and The Descent of Man (1872), expressed his theory of evolution and revolutionized the study of life and human origins.

Darwin presented evidence showing that natural species including humans have changed, or evolved, over long spans of time. He also argued that radically new forms of life develop from existing species. He noted that all organisms compete with one another for food, space, mates, and other things needed for survival and reproduction. The most successful individuals in this competition have the greatest chance of reproducing and passing these characteristics on to offspring. Over hundreds of thousands of generations, one form of life can evolve into one or more other forms. Darwin called this process natural selection.

Modern science now understands that the mechanism for evolutionary change resides in genes , the basic building block of heredity . Genes determine how the body, and often the behavior, of an organism will develop over the course of its life. Certain information in genes can change, and over time this genetic change can actually alter a species' overall way of life.

In recent decades, biological and social scientists have made impressive strides in understanding our complex physical and cultural origins. Their research has revealed gradual alterations in our genetic structure, as well as shifts in culture and behavior, that have transformed humankind into the planet-dominant species.

Scientists estimate that our human ancestors began to diverge from the African primates between eight million and five million years ago. This figure is the result of studying the genetic makeup of humans and apes, and then calculating approximately how long it took for those differences to develop. Using similar methods of comparing genetic variation among human populations around the world, it is thought that all people living today share a common genetic ancestor.

Early Life in Africa

The human story begins in one of the most geologically fascinating areas on Earth, the Great Rift Valley of Africa. It is an enormous split torn into Earth's crust that runs from the forests in Tanzania to the deserts of Ethiopia. In some places the rift is thousands of feet deep and exposes the last fifteen million years of the earth's history. Here, fossil remains of our earliest ancestors can be found. Humankind appears to have first evolved in Africa, and the fossils of early humans, or hominids, who lived between five million and two million years ago, come entirely from Africa.

Starting with the modern human skull, it is possible to trace our ancestry back millions of years. As we travel back in time, our ancestors look less and less like us and begin to resemble our closest relatives, the African apes. Because our physical and genetic characteristics are similar, evolutionary theory offers evidence that ancestral humans had a very close relationship to a group of primates, the apes. Humans, chimpanzees, gorillas, and the large apes of Africa share a common ancestor that lived between eight million and five million years ago.

Humans, or hominids, belong to the scientific order named Primates, a group of more than 230 species of animals that includes the monkeys, lemurs, and apes. Modern humans have a number of physical characteristics resembling our ape ancestry. The social systems of humans also share similarities with the African apes and other primates, such as baboons, chimpanzees, and rhesus monkeys. Chimps live, groom, feed, and hunt together and form strong family bonds. Early humankind probably had a similar lifestyle.

Scientists now know that nearly 98 percent of the genes in humans and chimpanzees are identical, making chimps the closest living biological relative of humans. However, there are fundamental differences between modern humans and their primate relatives. The human brain is larger and more complex, giving humankind the ability to communicate through language, art, and symbols, to walk upright, and to develop a throat structure that makes speech possible.

One of the earliest defining human traits is bipedalism, the ability to walk upright on two legs. This characteristic evolved over four million years ago. Other important human characteristics, such as a large and complex brain, the ability to make and use tools, and the capacity for language and culture, developed more recently. Many of what we consider advanced traits, such as art, religion, and different expressions of cultural diversity, emerged during the past 100,000 years.

Most paleoanthropologists today recognize ten to fifteen different groups of early humans. They do not agree, however, about how they are related or which ones simply died out along the way. Researchers also disagree about how to describe, identify, and classify these early human species, and what factors influenced the evolution and extinction of each species.

Early Humans: Evolution of Australopithecines

Nearly five million years ago in Africa, an apelike species evolved with two important traits that distinguished it from the apes. This species had small canine teeth (next to the four front teeth), and it was bipedal , meaning it could walk on two legs instead of four. Scientists refer to these earliest human species as australopithecines, or australopith for short.

The fossil record shows that there is not an orderly sequence leading from one form to another. Several groups lived at the same time and characteristics developed at different rates; therefore the human family tree suggests a long and complex past.

Fossils from several early australopith species that lived between four million and two million years ago clearly demonstrate a variety of adaptations that mark the transition between ape to human. Prior to four million years ago, fossil remains are scarce and incomplete; where available, however, they do show a primitive combination of ape and human features.

Most of the key characteristics that stand out as distinctly human are related to their bipedal stance. The australopiths had an S-shaped spine that allowed for balance when standing. The opening through which the spinal cord attached to the brain was positioned more forward, allowing for the head to be balanced over the upright spine. The pelvic bone was shorter and broader than in apes, giving the pelvis a bowl shape that supported the internal organs when standing or walking upright. The upper legs angled inward allowing the knees to support the body while standing or walking. Shorter and less flexible toes functioned as rigid levers for pushing off the ground with each step.

Most early species had small canine teeth, a projecting face, and a small brain. They weighed between 22 and 37 kilograms (60 to 100 pounds), and were 0.9 to 1.5 meters (3 to 5 feet) tall. Males were generally larger than females. Both had curved fingers and long thumbs with a wide range of movement. The apes, in comparison, have longer, more curved, and stronger fingers that make them well adapted for hanging and swinging from branches. Apes also have short thumbs, which limits their ability to manipulate small objects.

There were at least two major groups of australopithecine, one with very large teeth and heavy jaw muscles referred to as robust, and another referred to as gracile. The main difference was in the size of the jaws and teeth. Beyond that, there was no appreciable difference in body size. The evidence suggests that the large-toothed robust group ate primarily plant foods, where as the gracile group concentrated on a more diverse diet that included meat. Details known about each group are delineated below.

Early Australopiths or Gracile Group

• Ardipithecus ramidus. Discovered in 1994 and estimated at 4.4 million years old. This ancient line suggests a close relationship with apes and chimps because of the enamel found on the teeth. Whether or not it walked upright is unknown.
• Australopithecus anamensis. Discovered in 1995 and estimated at four million years old. Jaws were apelike but the legs were humanlike; it may have walked upright.
• Australopithecus afarensis. Discovered in 1974 by Donald Johanson and known as "Lucy." Estimated at 3.9 to 3.1 million years old. Thought to walk upright and bipedal, these may have left footprints in volcanic ash in Laetoli 3.7 million years ago. Fossils show sexual differences, and suggest that they were adept at climbing trees.
• Australopithecus africanus. First found in 1924 by Raymond Dart, this was the first known australopith. Dating from 3 to 2.4 million years ago, it had forelimbs longer than legs and walked upright. Many feel this is the best candidate as ancestor to early Homo species.

Later Australopiths or Robust Group

• Australopithecus aethiopicus. Found in 1985, this group dates from 2.7 million years ago. The skull, known as "the black skull," shows a possible relationship with A. afarensis.
• Australopithecus boisei. This group lived over a long period of time, between 2.3 and 1.2 million years ago. This skull has the most specialized features of the robust group, with a massive, wide face capable of withstanding extreme chewing forces.
• Australopithecus robustus. This group lived between 1.8 and 1.3 million years ago in the same region as A. africanus. This group had jaws, teeth, and habitat similar to A. boisei, but the groups appear to not be related.

Evolution of Modern Humans

Homo habilis.

After researchers unearthed the australopithecines, the next major "missing link" to be found was Homo habilis, an early representative of modern humankind. Found by Louis and Mary Leakey at Olduvai Gorge in Tanzania, these fossils date to between 2.5 and 1.7 million years ago. This creature was bipedal, fully upright, and had the capacity to use forearms for handling tools and weapons.

These fossil specimens show an increased brain size of 600 cubic centimeters (37 cubic inches), and a jaw and tooth size more closely resembling modern humans. Any residual physical traits for climbing had also disappeared. Cut marks on bones suggest the use of tools to prepare meat. They probably retained some of the skeletal characteristics of the australopithecines that made them great climbers. They may have spent considerable time in trees foraging, sleeping, and avoiding predators. They were the first of our relatives to have opposable thumbs, and the fossil skulls show physical traces of asymmetrical brain development, which is reflected in the way that stone tools were shaped.

Some researchers feel that Homo habilis had a large enough brain to have the rudimentary capacity for speech that may have encouraged cooperation and sharing amongst members of a group. That our distant H. habilis ancestors were able to produce such tools demonstrates that they had manual dexterity but also a capacity for planning, as well as knowledge about what kinds of stones to use and where to find them. The technology of these first toolmakers existed for more than 800,000 years.

Homo ergaster and Homo erectus.

Next in the story of human evolution, we find a group represented by Homo ergastor, a recently recognized African link between Homo habilis and Homo erectus. This group lived from about 1.8 million to 1 million years ago, when Homo erectus and other forms replaced it. Homo erectus fossils found in Java and the Republic of Georgia at 1.9 million years old and 1.6 million years old, respectively, indicate an early migration of Homo ergaster from Africa followed by Homo erectus evolving in Asia and spreading to other areas.

A fossil skeleton of Homo ergaster found in Kenya in 1984 became popularly known as Turkana Boy. This skull led researchers to believe this group may have been the first "naked ape." This specimen suggested no body fur, a dark pigmented skin, and no evidence of living in trees. This species may have reached up to 1.8 meters (6 feet) in height; they appear to have had a near modern size brain and a striding gait. They may have been the first to make and wear clothing of some kind.

Homo ergaster made stone tools, including well-made hand axes and cleavers for the butchering and processing of hunted animals. This technology appeared in Africa and was later carried into western Asia and Europe by Homo ergaster or its descendants. This technology was widespread and used until the end of the Early Stone Age, only a few hundred thousand years ago.

It now appears certain that Homo ergaster was the direct ancestor to the first inhabitants of Eurasia, including Homo erectus in the Far East, as well as the predecessor of Homo sapiens and Homo neanderthalensis in Europe. Homo ergaster led to Homo erectus, the famous missing link, which is our first ancestor to occupy territory from what is now northern China in Asia, to southern Great Britain and Spain in Europe, and all of Africa.

Emergence of Modern Human Beings

Neanderthals and Modern Homo sapiens.

The origin of modern humans is still controversial. The debate centers on whether modern humans have a direct relationship with Homo erectus or the Neanderthals, a well-known, more modern group of humans who evolved within the last 300,000 years. Some researchers feel that modern humans originated separately in Asia, Europe, and Africa. Others feel that modern humans originated in Africa and after migrating into Europe and Asia they replaced the Neanderthals or archaic Homo sapiens found there.

For many years, scientists believed that Neanderthals were the direct descendants of modern humankind. In the 1960s an interesting theory proposed that different groups of Homo ergaster gave rise to numerous groups of Homo sapiens, including a group known as Neanderthals. This theory suggested that the Neanderthals had disappeared because of being outcompeted by and having interbred with Homo sapiens sapiens, sometimes referred to as Cro-Magnon people. However, more recent evidence suggests a different story.

In a landmark study conducted in 1997, scientists examined the mitochondrial DNA of a Neanderthal fossil and a modern human. This analysis done by molecular biologists provides evidence about when two populations of people last had a common ancestor. The results concluded that it is unlikely that Neanderthals were related to modern humans. Instead it is thought that Neanderthals were a distinct species that evolved side-by-side with early Homo sapiens for hundreds of thousands of years. In addition, the earliest version of Homo sapiens, one with the characteristics that would link it with a common ancestor for Neanderthals and modern humans, Homo sapiens sapiens, had evolved in Africa from Homo ergastor at least 600,000 years ago.

The scientists further calculated that, while Neanderthals and modern humans did indeed share a common ancestor, Homo ergastor, the two lineages had diverged sometime between 550,000 and 690,000 years ago. This established that the Neanderthals evolved in Europe and evolved from archaic Homo sapiens and go back perhaps nearly 300,000 years. It appears that the Neanderthals almost made it to the present, as they appear to have died out only 30,000-40,000 years ago, for reasons not fully understood at this time.

Compared with Homo sapiens, Homo neanderthalensis was barrel-chested with massive brow ridges, a nose that protruded forward, a low sloping forehead, a lower jaw without much of a chin, thick arm and leg bones, and heavier muscles in the shoulder and neck. The brain was actually larger than that of modern humans, possibly because of the additional capacity needed to control the extra musculature. Although the brain size of Neanderthals overlapped with early and modern Homo sapiens, the shape of the cranium was different, suggesting that perhaps the frontal cortex, which controls "higher thought," was restricted.

The Neanderthals also appear to have been culturally quite advanced. While most lived in caves, it is possible that some may have begun to build house-like structures. They manufactured a variety of stone tools, including spear points, scrapers, and knives. They used and controlled fire, which probably helped in cooking frozen meat and in keeping warm. Evidence that they buried their dead with flowers and tools suggests that perhaps they had a form of religion.

It appears, then, that modern human beings are direct descendants of a group known as Cro-Magnon Homo sapiens that appeared in Europe and Asia 100,000 years ago. Although they overlapped with the Neanderthals, they were physically unrelated. They appeared thoroughly modern, with a high forehead and a well-defined chin. Artifacts and stone tools demonstrate that they had mastered the art of making tools and useful instruments from stone, bone, and ivory, and they may have used spears. A number of colored paintings left on cave walls suggest an evolving, rich, and complex cultural life. They hunted cooperatively and were perhaps the first to have a language.

Conclusion

In March 2001 a new fossil, known as Kenyanthropus platyops, was added to the family tree of early humans. Thought to be 3.5 million years old, it is considered a new genus and species of an early human ancestor that lived in the same area and time of Lucy. This recent find is an example of the ways in which our long and complex past slowly reveals itself to us as we come to recognize and understand more about our human ancestors, piece by piece and fossil by fossil.

see also Biological Evolution; Primates.

Brook Ellen Hall

Bibliography

Conroy, Glenn. Reconstructing Human Origins: A Modern Synthesis. New York: W. W. Norton, 2000.

Ehrlich, Paul, E. Human Natures, Genes, Cultures, and the Human Prospect. Washington, DC: Island Press for Shearwater Press, 2000.

Gore, Rick. "The Dawn of Human: The First Steps." National Geographic 191, no. 2(February 1997): 72-100.

Johanson, Donald, and Edgar Blake. From Lucy to Language. New York: Simon and Schuster, 1996.

Internet Resources

Becoming Human: Paleoanthropology, Evolution, and Human Origins. The Institute of Human Origins. <http://www.becominghuman.org>.

Darwin's theory of natural selection is commonly known as "survival of the fittest."

Evolution, Human

Human evolution is a field of science that falls within the larger area of physical anthropology. Human evolutionary studies are broadly synonymous with paleoanthropology, although paleoanthropology is a slightly wider concept that covers the host of fields contributing to the understanding of the human biological past in all its varied aspects. The central concern of human evolution involves sorting anatomical and behavioral differences within and among hominid species in order to delineate their ranges of variation through geological time and across geographical space. Hominid is often used as a colloquial term to indicate membership of fossil forms in the family Hominidae, the taxonomic group that includes anatomically and behaviorally modern humans and their precursors of the last six million years. The term human is a more subjective notion, whose limits can be debated. Some writers use it to include all members of the hominid family, while others restrict it to the genus Homo or to the species Homo sapiens.

In pre-evolutionary times, the Swedish naturalist Carolus Linnaeus (1707–1778), in his first edition of the Systema Naturae (1735), classified all organic organisms into a natural order using a hierarchical system with binominal nomenclature. He included humans (under the genus Homo and the species sapiens, derived from the Latin words for "man the wise"), along with lemurs, monkeys, and apes, in the order Primates. Intriguingly, in place of supplying physical characteristics to define this new species, Linnaeus avoided controversy by simply writing nosce te ipsum ("know thyself"). More than two and a half centuries later, physical anthropologists are still unable to agree on what constitutes modern humanity.

In terms of the morphological definition of modern humans, only a small number of unique anatomical characteristics stand out: (1) Homo sapiens is the only surviving member of the family Hominidae, a group anatomically committed to terrestrial bipedalism; (2) Members of this species have (not uniquely) relatively large brains—averaging 1,350 milliliters—with the most complex neocortex of all primates; (3) their chin-bearing faces are small compared to their neurocrania; and (4) they have a brow region structured into two parts. Behaviorally, modern humans are identified by the unique presence of: (1) a spoken language; (2) the cognitive faculties to generate mental symbols, as expressed in art; (3) the ability to think, reason, and plan; and (4) a bizarre inability to sustain prolonged bouts of boredom. Are anatomically modern humans and behaviorally modern humans the same thing? Not entirely. Anatomically and behaviorally modern humans appear in the archaeological and fossil records at different times.

Approximately one hundred thousand years ago, or perhaps somewhat earlier, anatomically modern humans appear in the fossil records of the Middle East and Africa; they are similar both cranially and postcranially to modern humans today, yet these earliest forms left no archaeological evidence to lead us to believe they had incorporated a modern behavioral repertoire. At seventy to fifty thousand years ago, we detect no change in the morphology of early anatomically modern humans, but there is dramatic evidence of a change in behavior. Splendid murals painted on the walls and ceilings of caves, musical instruments, and elaborate notations, together with a complex technology of stone and bone, are known from western Europe beginning about thirty thousand years ago. But these dramatic expressions were rather late, compared to the suggestions of similar symbolic behaviors known from as long ago as seventy thousand years, and maybe even more, in Africa. Similarly, modern humans had arrived in Australia by sixty thousand years ago, and an effectively modern level of cognition must have been present in these people to have allowed them to cross at least fifty miles of open ocean to get there. Obviously, a cognitive gulf was breached at some time after about seventy thousand years ago (perhaps earlier). This arose first of all in Africa, and spread thence to other parts of the world. Once Homo sapiens was in this behavioral mode, the speed of technological and other behavioral innovation (formerly episodic and rare) increased out of all proportion to what had gone before. At what point religious awareness was acquired is not known, but it was probably part of an overall biological potential for modern cognition that was achieved as a single "package." The huge range of behaviors made possible by this potential was only gradually discovered—and indeed, Homo sapiens is still enlarging its behavioral range today.

The human species and religious doctrine

By nature humans are inquisitive beings with an unquenchable thirst to understand and explain the meaning of life, especially their own. Since the days of the ancient Greek philosopher Aristotle (384–322 b.c.e.), the organic world had been looked upon as stable and unchanging, ascending steadily from the simplest forms to the most complex. Under the doctrine of the "Great Chain of Being," humans were perceived as godly creations and were positioned just below the angels on the top branch of a nicely ordered tree of life. All flora and fauna were designed for the purposes in nature that they were perceived to fulfill. The humanistic ideas of the Renaissance period centered all philosophy on human values and exalted human autonomy and superiority to the rest of nature. By the late seventeenth century, René Descartes's (1596–1650) philosophical idea that animals were complex machines with no higher sense of purpose had been expanded by French and German philosophers to create new foundations for a human social order. Morality was no longer considered to descend from an absolute truth enshrined in Christian beliefs, nor was the notion of accountability in the afterlife. The study of human nature became the key to understanding moral order in decent, complex societies. At a later period some struggled to integrate humans and nature with materialistic philosophy, but this view lost support during the turmoil of the French Revolution.

From Cuvier to Darwin

It would not be until the eighteenth century that the study of human origins became an approachable, but still controversial, topic within the budding science of natural history. Doubts raised by some natural historians questioned the interpretations of biblical literalists as to how humans came to exist on Earth, especially as increasing fossil discoveries in recognizably ancient sediments came to reveal that Earth's fauna did indeed appear to have a biological past. It was evident to naturalists that the Earth bore scars of an ancient history that contained puzzling geological phenomena, such as fossil fish on the tops of mountains, that were inexplicable within the boundaries even of the rudimentary scientific understanding that then existed.

It was impossible, then, to avoid the question as to where humans fitted into the picture. In 1830, the French naturalist Georges Cuvier's (1769–1832) treatise on fossil fauna and flora that was discovered in ancient geologic strata reported no evidence of human fossils coeval with these ancient genera. Since the geologic deposits involved varied greatly from one layer to the next, with bony evidence of once living creatures present in places where they had either gone extinct or now existed only on other continents, Cuvier reasoned that divinely instigated catastrophes and re-creations were responsible for the many extinction and replacement events he perceived. He argued that human fossils could be found if one were to look under the deepest of oceans, as suggested by the Old Testament's story of the great flood. Other naturalists, like Etienne Geoffroy Saint-Hilaire (1772–1844) and Jean Baptiste de Lamarck (1744–1829), provided strict evolutionary reasons for the drastic changes observed in the fossil record. Lamarck, for example, postulated that anatomical and behavioral changes acquired in a creature's lifetime might be passed on to its descendants. However, the Lamarckian paradigm of evolution would shift when two important events took place: (1) the 1858 announcement of Charles Darwin's (1809–1882) and Alfred Russel Wallace's (1823–1913) mechanism of natural selection to explain how species gradually change over time; and (2) the 1856 discovery (and the 1864 naming) of an extinct human species.

Charles Darwin, who rejected the basic tenets of the inheritance of acquired characteristics, enormously popularized a different evolutionary explanation for life on Earth with his the On the Origin of Species, published in 1859. Darwin proposed that biological organisms gradually evolve over time by adapting to their environments. Those individuals who are optimally suited to their environments end up producing more descendants than those who are not. If the features that make them better "adapted" are passed along by biological inheritance to their offspring, those features will become more common in the population, whose aspect will thus change over time. Keenly aware of the controversy it would generate, the retiring Darwin minimized any reference to humans in his publication, and did not broach the problem of human origins until many years later. Darwin's theory of "descent with modification" generated a great deal of controversy within religious and scientific communities. The highly public and politico-religious uproar that resulted centered on the distasteful suggestion that humans and apes share a common ancestor, especially in view of the long held belief that other animals are unable to think and are effectively nothing more than soulless automatons. Coming to Darwin's defense, Thomas Henry Huxley (1825–1895) fervently defended the tenets of Darwinian evolution, most publicly in his debate with Bishop Samuel Wilber-force (1805–1873) in 1860. In his influential 1863 publication of a series of public lectures titled Evidence as to Man's Place in Nature, Huxley argued that humans should be seen as biological organisms, and subject to the same natural laws that all other organic entities obey.

Interpreting the hominid fossils

The second epochal event for human evolutionary studies was the 1856 discovery of a fossil human at the Feldhofer Grotto in the Neander Valley, Germany. Most authorities of the day dismissed this find as the remains of a "barbarous" type of Homo sapiens. However, in 1864 the anatomist William King named the new form Homo neanderthalensis, thereby implying that there had been at least one ancient human extinction and speciation event. With further discoveries of the remains of extinct fossil humans, evolutionary concepts were more palatably applied to modern humans. The British geologist Charles Lyell (1797–1875), once a firm believer in God's role, abandoned many of his theological notions and accepted Darwin's theory of descent with modification after examining the remains of the Feldhofer Neanderthal.

At the turn of the twentieth century, the rediscovery of Mendelian genetics provided a basis for Darwin's evolutionary mechanism. Nonetheless, some paleontologists continued the attempt to integrate Christian beliefs with the idea of evolution. One such was the French Jesuit Pierre Teilhard de Chardin (1881–1955). While in Jesuit training in England, Teilhard also trained in paleontology and archaeology, and became embroiled in the Piltdown controversy that was just erupting. In 1912, he was invited to the Piltdown site in Sussex, which had yielded fossil bones including those of a human, and flint tools. Upon arrival he found a tooth. Reconstruction of the fragmentary hominid pieces seemingly offered the perfect transitional candidate from apes to humans—perhaps too perfect.

In 1912, "Piltdown Man" was introduced to the world as Eoanthropus dawsoni. At that time, the large brain was considered to be the hallmark of humanity; and for forty years British anatomists would disregard many significant fossil human discoveries because of their prized and large-brained Piltdown fossil. Teilhard later continued his paleontological research at the "Peking Man" site of Zhoukoudian in China. The Chinese fossils helped Teilhard to reconcile his now expansive knowledge of the human fossil record with his Christian beliefs. In The Phenomenon of Man (1938–1940), Teilhard proposed a theory of human evolution in which humans were evolving towards a final spiritual unity, also known as Finalism. This notion elicited the disapproval of his Jesuit superiors.

Early in the 1950s, Piltdown was exposed as a hoax—the doctored remains of a human and orangutan—and Teilhard has even been fingered as the hoaxer, though he remains only one of the more unlikely suspects of many. By the late 1950s the human fossil record had greatly expanded, as had the plethora of names used to describe it. A tidying-up was in order, and this was gradually achieved under a gradualist and progressivist model of human evolution.

In the 1970s and 1980s, new systematic methods began to transform the understanding of the constantly expanding human fossil record. Further, molecular studies were providing new perspectives. In particular, the "molecular clock" shortened the ape-human divergence to as little as five to six million years ago (from maybe twelve to fourteen). From around 1970 researchers uncovered bipedal but otherwise rather apelike hominids from sites in eastern Africa. These joined the Australopithecus fossils already known from southern Africa in the 2.5 to 1.5 million years ago range, and dated mostly from about 3.5 to 2.0 million years ago. Interpreted using an underlying gradualist model, these archaically-proportioned fossil hominids mostly reflected the search for an "earliest ancestor."

The situation at the beginning of the twenty-first century

Over the following few decades, hundreds of fossil human discoveries offered fuel for systematic debates. The "single species hypothesis," which stated that the human ecological niche was so wide that only one species of hominid could have existed at any one time, was rapidly invalidated by new finds, but still lingers in models of human origins that find deep roots in time for contemporary geographical groups of humankind. Evolutionary theory, as well as the rather sparse fossil record, imply in contrast that the species Homo sapiens must have had a single origin at one time and in one place, probably Africa. All of the human diversity familiar today has apparently appeared within the past 150 thousand years or so.

Despite minor differences of opinion, it is clear that the diversifying pattern of human evolution is similar to that of other mammalian taxa. Hominid phylogeny is a story of evolutionary experimentation, with multiple speciations and extinctions. The hominid family comprises at least five genera and eighteen known species (see Fig. 1, p. 302), some of which shared territories in both time and space. At present, all geographical varieties of modern humans occupy the single surviving twig of what appears once to have been a densely branching bush.

See also Anthropology; Evolution; Evolution, Biocultural; Evolution, Biological; Evolution, Theology of; Paleoanthropology; Paleontology; Sociobiology; Teilhard de Chardin, Pierre

Bibliography

bowler, peter j. evolution: the history of an idea. berkeley: university of california press, 1984.

darwin, charles. the origin of species (1859). new york: bantam classic, 1999.

eldredge, niles. the triumph of evolution and the failure of creationism. new york: w. h. freeman, 2000.

huxley, thomas henry. the major prose of thomas henry huxley, ed. alan p. barr. athens: university of georgia press, 1997.

johanson, donald, and blake, edgar. from lucy to language. new york: simon and schuster, 1996.

schwartz, jeffrey and tattersall, ian. extinct humans. boulder, colo.: westview press, 2000.

tattersall, ian. the fossil trail: how we know what we think we know about human evolution. new york: oxford university press, 1995.

tattersall, ian. becoming human: evolution and human uniqueness. new york: harcourt, 1998.

tattersall, ian. the monkey in the mirror: essays on the science of what makes us human. new york: harcourt, 2001.

teilhard de chardin, pierre. the phenomenon of man. new york: harper, 1976.

wolpoff, milford. paleoanthropology, 2nd edition. boston: mcgraw-hill, 1999.

kenneth mowbray ian tattersall

Human Evolution

It is remarkable that the unique qualities of humans—language, advanced thought, and culture—evolved through the same processes that shaped the adaptations of all other creatures on earth: natural selection. How this came about is perhaps the most fascinating question of all time. The direct evidence for human evolution has increased enormously since the early 1990s via the discovery of hundreds of new fossils, including three new genera and even more new species; and via the comparisons of modern and ancient deoxyribonucleic acid (DNA). Great leaps forward are being made in science because of this proliferation of information.

Humans and our ancestors are called hominins, going back to the time of the split from the lineage of human's closest relatives, the chimpanzees. (Until recently the term hominid was used, but based on genetic relatedness and the rules of zoological nomenclature, the word hominid should apply to both the chimpanzee and human clades [lineages].) Humans have evolved various traits that have diverged from the typical great ape collection of characters; notable is human's commitment to standing on two legs (bipedalism) rather than four, increased fine motor dexterity making possible extensive use of tools, prolonged period of infancy and childhood, increased brain size, language, great cultural complexity and economic interdependence.

Ape Ancestors

During the Miocene epoch (23 to 5 million years ago), when our super-family, the Hominoidea (apes) flourished and speciated, more than twenty genera and about twice that number of species in the Hominoidea family. Many of these species went extinct, and today there are only five genera of apes: gibbons, orangutans, gorillas, chimpanzees/bonobos, and humans. The origins of the hominins remain obscure. Genetic comparisons and the molecular clock suggest that the human-chimp lines split in the late Miocene about 6 million years ago. The fossil record is very incomplete at this time, and neither fossils of ancestral chimpanzees nor fossils that are clearly ancestral to both lines are known. However, field research since the mid-1990s has resulted in several fossils from this time period, and researchers hope that this trend of discovery will continue and scientists' understanding of the origin of hominins will improve over the twenty-first century.

Scientists do know this: Hominins evolved from an ape that was similar to the living African apes, and the human lineage did not spread beyond Africa into Eurasia until about 1.8 million years ago. The early hominins lived in an array of habitats, but most evidence points to wooded savannas (a type of grassland) as a principal one. There is solid evidence that significant amounts of bipedal behavior preceded other major events such as increased brain size or tool use.

Earliest Hominins

The earliest named hominin, Orrorin tugenensis, was discovered in 2000 in eastern Kenya, and is believed to have lived 6 million years ago. Thigh bones suggest that this species spent a significant amount of time bipedally. In the 5-million-year time range there are several fragmentary fossils from East Africa, but they are difficult to associate with particular species. At 4.4 million years ago, another genus and species, Ardipithecus ramidus, is present. This species has many traits, especially in the teeth, that are between chimpanzees and later hominins (australopithecines) in size and character (for example canine size, enamel thickness, and size of the cheek teeth). Discovery of a third new early hominin, Kenyanthropus platyops, was published in 2001 by Meave Leakey and colleagues.

The newly discovered Kenyanthropus platyops dates from the same era as the Australopithecus but belongs to a previously unknown species. The new skull has a more humanlike face with much smaller teeth. Its name means "the flat-faced human from Kenya."

Between 3.9 and 2.0 million years ago there was a proliferation of australopithecine species, including the "gracile" australopithecines of the genus Australopithecus, with five commonly recognized species, and the robust australopithecines of the genus Paranthropus, with three commonly recognized species. These australopithecine species are variable, but general trends include: significant amounts of terrestrial bipedalism, with some species also retaining significant arboreal adaptations; brains still very close to chimpanzees' in size; tool use generally minimal; very large grinding cheek teeth and chewing muscles, consistent with a largely vegetarian diet including many tough foods; canine teeth reduced in size; and male body size much larger than females (that is, high degrees of sexual dimorphism).

Origins of Homo

The origins of the human genus Homo is also being reexamined in the early twenty-first century. There are many fossils in the 2.4- to 1.6-million-year time range in Africa that have variously been assigned to Homo habilis, Homo rudolfensis, or Homo species, but many still have large cheek teeth, smallish brains, and faces shaped like australopithecines. Some of these fossils even retain many arboreal adaptations. Thus the majority of these specimens will probably be moved into the genus Australopithecus; reserving the genus Homo to include only those specimens which are clearly more closely related to humans.

Considered this way, the human genus evolved nearly 2 million years ago with the appearance of Homo ergaster. Homo ergaster, personified by the amazingly well-preserved West Turkana skeleton of a twelve-year-old boy, is a markedly different creature than australopithecines, though still not modern. They were tall, lanky, long-legged with body proportions nearly the same as modern humans, committed to efficient terrestrial bipedalism at the expense of arboreality, and larger—about the size of modern humans. This increase in body size was accompanied by an increase in brain size—the largest examples having cranial capacities (brain cavities) of about 900 cubic centimeters (cc), the smaller examples closer to 650 cc. (Modern human brains are about 1300 cc, chimpanzees are about 400 cc.) Because of the increase in body size, this is not a large relative increase in brain size over earlier hominids. The trend toward increasingly large cheek teeth is reversed in our lineage: There is a reduction in cheek tooth size, generally thought to reflect consumption of higher quality foods and/or more food preparation.

Homo ergaster also used tools. The earliest archaeological sites, which contain the debris of stone tool manufacture and animal bones eaten by hominins, date to 2.6 million years ago in Ethiopia. The earliest stone tools are simply sharp flakes broken off rocks that were used as cutting tools. It is difficult to say which species made and used these earliest tools, in fact it probably included some of the australopithecine species. By 1.5 million years ago the stone tool industries include more elaborate, symmetrically flaked rocks, called Acheulian hand axes.

Spread Beyond Africa

Immediately after the evolution of Homo ergaster there was an initial, but probably short-lived, spread beyond Africa into the Near East, Far East (Java), and southern Spain. Either something about H. ergaster or the environment must have facilitated this spread. Was it the increase in body size? Did their ecological position shift up as they became more successful at acquiring meat, and therefore their home ranges needed to increase to encompass more and more space? Was it learning to control fire? Or changes in social organization? More research will focus on these questions, but whatever caused this initial spread, there is very little evidence for Homo outside of Africa again for about 1 million years, when more sites are found around Europe and Asia.

The populations in the Far East (China and Indonesia) evolved into a local species called Homo erectus, which may have survived until into the Late Pleistocene (approximately 50,000 years ago). These fossils have long low skulls with thick brows and cranial capacities near 1000 cc. Interestingly, H. erectus seems to have been contemporary with other species of archaic Homo, specifically species that evolved into Homo neanderthalensis in Europe and the Near East, and another that evolved into anatomically modern humans (AMH) in Africa. All of these species have long, low skulls, big faces, and large brow ridges, but differ in details of the cranial anatomy, and some have larger cranial capacities. They are robust with massive skeletons and muscles, reflecting a large dependency on brawn to cope with the world. Archaeological evidence for accomplished hunting in the form of wooden spears is clear by 400,000 years ago.

In Europe Neanderthals evolve into their classic form: extremely strong, stocky, and robust, with large eyes and noses and jutting faces; their bodies seem well adapted to the cold Pleistocene. Their tool kit was more elaborate, including stone-tipped spears, and archeological and isotopic evidence indicates a great reliance on meat in the diet. Nonetheless, except in a very few instances when Neanderthals were contemporary with modern humans, there is little to no evidence for art. For this reason, many anthropologists believe Neanderthals did not have modern language.

Anatomically Modern Humans

Meanwhile, archaic Homo evolved into anatomically modern humans in Africa. Throughout this time range (between 200,000 and 100,000 years ago) there are increasing bits of evidence for art and symbolism in the form of red ochre, beads, and composite tools in Africa while they are still absent in Europe, and it is during this period that genetic evidence from modern humans suggests that anatomically modern humans evolved. Indeed, the earliest anatomically modern humans fossils come from Africa and the Near East, just over 100,000 years ago. Anatomically modern humans are characterized by a reduction in skeletal robustness and strength, probably related to greater reliance on technology and culture rather than brute strength. Cranial capacity remains about the same as in the more robust predecessors, but the face and teeth are smaller, the forehead becomes high and the chin juts out. Cultural elaboration is evident in increased number of tools types, regional variation in style, more composite tools, and notably, art.

There is a minority of anthropologists who consider all the Homo specimens to be from one diverse species. In the early twenty-first century, the majority of anthropologists believe that the fossil, archaeological, and genetic evidence concur that anatomically modern humans evolved in Africa about 200,000 to 150,000 years ago, and spread out from there to Europe and Asia as recently 100,000 to 25,000 years ago (depending on where), replacing the archaic Homo species, for example, Neanderthals and Homo erectus, and these later species may have contributed relatively little genetic diversity to the human gene pool. The information scientists have thus suggests that all humans across the globe today are very closely related to one another.

see also Biology of Race; Evolution; Grassland; Leakey Family; Natural Selection; Population Genetics; Primate

Martha Tappen

Bibliography

Jones, S., R. Martin, and D. Pilbeam. The Cambridge Encyclopedia of Human Evolution. New York: Cambridge University Press, 1992.

Klein, Richard. The Human Career, 2nd ed. Chicago: The University of Chicago Press, 1999.

McBrearty, S., and A. S. Brooks. "The Revolution That Wasn't: A New Interpretation of the Origin of Modern Human Behavior." Journal of Human Evolution 39 (2000): 453–563.