Hominidae II (Humans)

views updated

Hominidae II

(Humans)

Class Mammalia

Order Primates

Family Hominidae

Subfamily Homininae

Thumbnail description
Large mammals; obligate bipeds; largest brain to body size ratio among terrestrial mammals; moderate degree of sexual dimorphism; species-specific vocal communication (language); obligate reliance on tool behavior and technology; complex sociality

Size
Variable, depending upon population. Normal adult stature: 53.5–72.8 in (136–185 cm); normal adult weight: 83.8–198.4 lb (38–90 kg)

Number of genera, species
1 genus, 1 species, 1 subspecies

Habitat
All terrestrial habitats, aided by domestication of animals and plants, technology, and extensive environmental modification

Conservation status
Not threatened

Distribution
Cosmopolitan; exploration of outer space and the solar system is now proceeding apace; colonization of other worlds within the solar system will probably take place within the foreseeable future

Evolution and systematics

Humans are members of the primate infraorder Catarrhini. This infraorder encompasses the Old World monkeys (family Cercopithecidae), lesser apes (family Hylobatidae), and great apes and humans (family Hominidae). It has been clear since the 1930s that all of the living catarrhines comprise a closely related group of organisms that is both morphologically and physiologically very similar.

The taxonomy for humans is Homo sapiens Linnaeus, 1758, Uppsala, Sweden. All living humans belong to the subspecies Homo sapiens sapiens.

Humans have 46 chromosomes, in contrast to the 48 chromosomes of pongids. DNA-DNA hybridization studies initially highlighted the close genetic relationship between humans and common chimpanzees. However, in general, there is a high degree of genetic similarity between humans and other mammals. The genetic similarity between human and mouse is approximately 90%. Sequencing of the human genome was completed in 2001. A 2002 comparison of human and mouse genomes showed the existence of about 30,000 genes in both organisms. The same genetic elements can be rearranged, and appear on different chromosomes. The mouse genome has evolved 2–5 times more rapidly than the human genome, probably because the shorter generation length of mice allows for greater rates of change. Mouse genes appear to be more subject to physical reordering, and mouse genes in different locations on the same chromosome can evolve at different rates. About one-third of the genes shared between human and mouse do not encode proteins. Some of these may encode RNA, while others may serve regulatory functions. Studies of evolutionary development in humans and other vertebrates demonstrate the existence of conservative Hox genes that are responsible for establishing the embryonic blueprint.

Hominins (members of the subfamily Homininae) are descendants of an unknown pongid from the late Miocene of Africa. The first hominin may be the late Miocene Sahelanthropus chadensis, dating to 6–7 million years ago (mya), from Chad, in Central Africa. However, this species is known only from cranial and dental remains. Orrorin tugenensis is a slightly more recent (6 mya) fossil species from western Kenya with postcranial remains. Femurs of Orrorin indicate that it had bipedal locomotion, which is the hallmark of the hominid family. A climatic trigger for hominin origins is often invoked. A period of late Miocene aridity in Africa is thought to have eliminated forests and caused the spread of extensive open-country grasslands, and thus created selection pressures for the origins of terrestrial bipedal hominins. However, Sahelanthropus, Orrorin, and later hominins that are well known postcranially are found in environmental mosaics that include

forested areas. The origins of terrestrial bipedal locomotion, therefore, cannot be simply linked to the disappearance of forest and the spread of grasslands.

The poorly known species Ardipithecus ramidus occurs between 5.8 mya and 4.4 mya, but the density of hominin fossils increases later, after 4.4 mya. A suite of hominin species appears in East Africa during this time range. Hominin species also occur at South African sites, although these sites lack volcanic materials, and are therefore more difficult to date. However, the South African species Australopithecus africanus and Australopithecus robustus appear to be later in time than East African material. These South African species were the first fossil hominins recognized from Africa, and are now among the most well known fossil hominins from the Plio-Pleistocene.

The genus Australopithecus alone contains eight species of hominin. Members of the genus occur principally in East and South Africa, and date from 4.4–1.2 mya. The longest-lived species (Australopithecus boisei) has a million year span, dating from 2.2–1.2 mya. It is clear that an evolutionary radiation of hominins occurred during the late Miocene through the early Pleistocene. Furthermore, there is definite evidence of sympatric species, indicating that niche differences allowed species to divide the shared resource space.

Besides the possible hominin Sahelanthropus, there is an additional hominin species recognized from Chad. This is Australopithecus bahrelghazali, dating to about 3 or 3.4 mya. Its principal importance lies in the fact that the site lies 1,550 mi (2,500 km) west of the East African rift. These fossils demonstrate that hominins had a wide geographic distribution, and excellent dispersal abilities even at this early date. This fact might not be obvious from the plethora of human fossils that come from the rift. The richness of the fossil finds from the East African rift is a taphonomic accident, and is caused by the fact that the rift is a sediment trap with the potential for excellent fossil preservation, as well as chronometrically datable volcanic materials. A wide geographical range of hominins at this date indicates that intrinsic biological properties are contributing to dispersion, and not necessarily complex sociality or cultural behavior.

The site of Laetoli, in Tanzania, has hominin footprints laid down in trackways dating to 3.6 mya. These footprints were preserved in a gentle fall of volcanic ash that was deposited by rain. The importance of this site lies not only in its unequivocal record of bipedal locomotion, but also in its documentation that three hominins made the trackways—this is the earliest record of hominin sociality. Because fossils of Australopithecus afarensis occur at Laetoli, hominins belonging to this species were apparently responsible for the trackways. This agrees with traits that are unequivocal adaptations for bipedality in the vertebral column, pelvis, and lower limb of this species. Slightly later in time, Australopithecus afarensis is also found at localities in Hadar, Ethiopia. As of 2002, the remains of 17 contemporary individuals of this species have been found at the Hadar locality AL 333. A sudden, unknown event—not a flood—was responsible for the mass mortality. This material is also important in documenting sociality, because these individuals were apparently members of the same social group.

Although a large brain relative to body size was long considered the hallmark of the Homininae, by 2003 it became clear that the earliest hominins had a brain to body size ratio comparable to those of living pongids. Brain size increases only with the appearance of genus Homo. However, because there is a concomitant increase in body size, the relative increase in brain size does not become obvious until the late Pleistocene.

What of archaeology, which is the evidence of hominin behavior? The earliest stone tools, belonging to the Oldowan Industry, appear in Africa at 2.5–2.6 mya. Stone tools thus occur long after hominin origins. Besides the stone tools themselves, animal bones that show hominin modifications, such as cut-marks or percussion marks, yield a record of hominin behavior. Some early archeological sites contain no stone tools at all, but only modified bone. The earliest archaeological evidence occurs without substantial brain size increase. For example, the Ethiopian site of Bouri, dating to 2.5 mya, contains the hominin species Australopithecus gahri, along with modified bone. This species has a brain size of 450 cc, which is equivalent to that of a pongid, and smaller than that of most australopithecines. Hominin tool behavior is thus not dependent on brain size.

In 1999, a major taxonomic revision of Plio-Pleistocene hominins collapsed two early species of genus Homo (H. habilis and H. rudolfensis) into the genus Australopithecus, reserving genus Homo for material that unequivocally showed an increase in body size, had modern human proportions, and had no traits indicating a retention of climbing or arboreal adaptations. Some researchers argue that six or more species of genus Homo coexisted in the early Pleistocene, only to be winnowed out with the advent of Homo sapiens. However, it is unlikely that early genus Homo was speciose. One can assess the species richness of early Homo in contrast to other mammalian genera by examining the species richness of extant mammalian genera with a similar body size. Using this method, one or two hominin species is the number expected for a mammal genus of 66–143 lb (30–65 kg), which is the size range usually estimated for early Homo fossils.

African Homo erectus, appearing at 1.8 mya, is the first unequivocal member of genus Homo. Postcranial fossils indicate that body size has increased in this species. A higher quality or more predictable diet must underlie this increase in body size. Details of tooth enamel formation demonstrate that Homo erectus matured quickly, in an ape-like fashion. Sexual maturity may have been reached by females at 8–9 years, and by males at 10–12 years. This faster maturation may be a major factor in the dispersal abilities of this species, which was the first hominin to emerge from Africa to penetrate other regions of the Old World.

Slow maturation, equivalent to that of modern humans, appears only with the Neanderthals. Neanderthal fossils date from 300,000–28,000 years ago, and occur in Europe, Central Asia, and the Middle East. They are the most well known of fossil humans, because of the completeness of their skeletal remains. Nearly all researchers agree that this completeness results from deliberate burial of remains, rather than accidental preservation. Neanderthals possess a distinctive suite of skeletal traits. These traits (especially traits in the nasal region) appear to be adaptations to extremely cold, dry conditions. Neanderthals had highly carnivorous diets, as established by the bone chemistry of these fossil humans and contemporary animals. The taxonomic status of Neanderthals has been problematic since the discovery of the first Neanderthal fossils in the middle of the nineteenth century. As of 2003, most researchers assign them to a different species (Homo neanderthalensis), but many argue that they are distinct only at a subspecies level (Homo sapiens neanderthalensis). The argument is not trivial, because it affects discussions of whether modern human populations incorporate genetic material from earlier, non-modern humans, or represent descendants

of a completely novel small founding population that completely replaces earlier humans.

Mitochondrial DNA (mtDNA) evidence initially seemed to support the origin of anatomically modern humans from a very small late Pleistocene founding population in sub-Saharan Africa. This idea became a prominent feature in many textbooks, where it was categorized as the "Out of Africa" or "Complete Replacement" model, because it seemed to imply that modern humans completely replaced their predecessors

in the Old World, who went extinct without issue. However, Templeton in 2002, using mtDNA and nuclear DNA from both autosomes and sex chromosomes, demonstrated that the evolutionary picture is substantially more complex, with a series of migrations out of Africa and another migratory vector out of Asia. There was no single small founding population for modern humans, during either the middle or late Pleistocene. Mitochondrial DNA has also failed to elucidate lower level questions about human evolution and dispersal. For example, it is clear in 2003 that mtDNA from Native Americans cannot illuminate crucial questions about the peopling of the Americas, such as the number or timing of migration events, or the source of the founding populations.

Global dispersal

With the advent of the fossil species Homo erectus, humans emerged from sub-Saharan Africa and rapidly colonized broad areas of the Old World. By 1.7 mya, several specimens of this taxon are found at the site of Dmanisi, in the Republic of Georgia. Abundant fossil remains of Homo erectus have been recovered from the island of Java. Two of the sites from Java have very early dates (1.8 mya and 1.6 mya), and much of the Javanese fossil material dates from about 1 mya. Pleistocene human fossils occur at a later period in Iberia, England, northern, southern, and eastern Europe, Central Asia, and China.

When anatomically modern humans appear, additional continental expanses were penetrated. The continent of Australia was reached between 46,000 and 50,000 years ago, and quickly settled. A global fall in sea level during the Pleistocene allowed humans to travel on dry land between areas that are now separated by water. They crossed from Asia to North America via a now submerged land bridge in the Bering Straits. Humans were in the Americas by 14,000 years ago, as shown by the important archaeological site of Monte Verde in Chile. These migrants may have used a narrow coastal passage along the western continental margins to penetrate quickly to the south. Boating technology, navigational techniques, and logistical preparedness for deliberate colonization allowed humans to settle the South Pacific islands. Settlers from Indonesia crossed the entire expanse of the Indian Ocean to reach Madagascar around 1,200 years ago. The North and South Islands of New Zealand were settled 1,000 years ago. This represents the last major human migration event using traditional modes of transportation.

Physical characteristics

Bipedal locomotion is the hallmark of the hominin family. Both the morphology and the orientation of bones and joints must be extensively altered from the ancestral pongid condition in order to accommodate bipedality. These alterations affect the foot, leg, pelvis, and vertebral column. Extensive biomechanical analysis of bipedalism has been conducted in living humans. This analysis demonstrates that there is very little electrical activity in muscles when subjects walk at a normal pace and are unencumbered by burdens. Hence, although it is slow, bipedalism is a very energy efficient mode of locomotion. A human walking at a normal speed uses only about

87% of the energy used by a similarly sized, generalized quadrupedal mammal moving at the same speed.

Normal humans carry large amounts of subcutaneous fat. This is peculiar for terrestrial mammals, which typically accumulate fat only before breeding, migrating, or hibernating. Unlike marine mammals, humans do not need this fat for maintaining the core temperature of the body. Furthermore, given constant supplies of abundant food and little physical activity, humans can quickly increase their store of subcutaneous fat. The most likely explanation for this human peculiarity is that it evolved to allow humans to survive periods of starvation or near-starvation. Indeed, seasonal calorie restriction is documented today for hunter-gatherers, as well as for agriculturalists. Many contemporary humans experience famine, if rainfall is low, or inadequate emergency stores of food have been set aside. There may be no extra food to cache for emergencies. Seasonal want appears to be the norm for humans, and thus natural selection has provided a built-in reserve of fat to tide humans over the inevitable lean period.

The surface of the human body is virtually hairless. With high ambient temperatures, sweat evaporates from this hairless skin. The temperature of the human body surface is thus lowered through evaporative cooling. This physiological adaptation is seen in all humans. It is a species-specific trait, because it is based on the presence of eccrine sweat glands on the surface of the skin. Hairlessness promotes evaporation. The abundance and density of eccrine sweat glands are unique to humans among other mammals. These glands are mainly restricted to the bottoms of the paws and adjacent regions in other mammals. Eccrine sweat glands do not produce the fatty secretions that are associated with scent and scent-marking in mammals. Instead, eccrine glands produce abundant watery secretions that contain salt, potassium, and calcium. The human sweating response is entirely dependent upon access to abundant fresh water, because any water lost through sweating must be quickly replaced. If this water is not replaced, death, caused by shock through loss of blood volume and heat stroke, can occur within a single day. A normal human sweating rate is 0.5–1 liter/hour, but this can be increased to 2 or sometimes 3 liters/hour in working humans accustomed to high temperatures. This rate, however, cannot be sustained.

Human body build shows climatic adaptation to extremes of temperature. This has been noted since the nineteenth century, and confirmed in many studies during the twentieth century. In 1847, Bergmann observed that endothermic animals had heavier bodies in cold climates, and lighter body builds in hot climates. In 1877, Allen observed that endothermic animals had shorter extremities in cold climates, and longer extremities in hot climates. Humans conform to Bergmann's and Allen's rules. In 1994, Ruff established that human pelvic breadth, which is a good proxy for body width, is correlated with temperature. Pelvic breadth is wide in cold climates, and narrow in hot climates.

High altitude also affects humans, principally through low oxygen pressure. However, cold temperature, high winds, rough terrain, poor soils, and impoverished ecosystems also exercise a profound affect on humans living at high altitudes.

Humans entering high altitude areas from the lowlands gradually increase the number of red blood cells in their body. These cells carry hemoglobin, which binds to oxygen, and transports it through the system. This response is caused by reduced oxygen at higher elevations. Humans born at high altitudes have a larger heart and lungs, and grow more slowly. Human populations that adapt to high altitude through evolutionary time have larger placentas, and consequently develop better contact between the blood supply of fetus and mother. Newborns of these populations have a higher birth weight, and greater survivorship than infants from other groups that are new migrants to the region. Native people in Tibet, who may have evolved high altitude adaptations through the longest time, have a genetically based variant hemoglobin that has enhanced oxygen binding properties.

Human populations differ in skin pigmentation. Since the 1930s, a relationship has been documented between skin pigmentation and latitude. Darker skin occurs at low latitudes, and lighter skin at high latitudes. The pigment melanin, produced by melanocytes in deep layers of the skin, is responsible for variation in human skin color. The skin also synthesizes vitamin D when it is exposed to sunlight. Vitamin D has an important role in calcium metabolism, which affects not only skeletal density, but also proper functioning of the nervous system. The adaptive significance of melanin in the skin appears to involve maintaining critical amounts of vitamins—vitamin D synthesis and the preservation of adequate amounts of folate, necessary for normal development of the fetal nervous system. Light skin allows more vitamin D to be synthesized in high latitudes where sunlight is weak. Dark skin decreases vitamin D synthesis and preserves folate in low latitudes where sunlight is intense.

Human adaptation to extreme climates is dependent upon culture and technology. Culture and technology allow humans to create pleasant or balmy microhabitats in which to live. Fire, clothing, shelter, transportation, food acquisition, processing, and caching, water storage, and complex behavioral adaptations underlie and complement human morphology and physiological response.

Since the early 1950s, physical anthropologists have studied human populations, not races. This reflects an understanding of the importance of variation within populations, and an overriding interest in natural selection, adaptation, and other evolutionary processes. The earlier approach, defining races and human types, was typological in nature. It categorized humans, devised schemes for human classification, and was relatively indifferent to evolution. In 2003, race is principally

used by forensic anthropologists in the analysis of human DNA and skeletal and soft-tissue traits, where the ancestry of forensic material needs to be ascertained.

Distribution

Humans are global in distribution. They are not restricted by major geographic barriers, because of the use of technology to travel over water and land, and through the air. This dispersal ability is not new. By 1.7 mya, humans occupied an Old World geographic range that extended from the East African rift to the island of Java. This early broad distribution was accomplished without the benefit of transportation technology. It is based on intrinsic biological properties for ranging and foraging that allowed humans to expand their geographic distribution. Anatomically modern humans occupied and rapidly penetrated the continent of Australia between 46,000 and 50,000 years ago. The Australian evidence demonstrates that humans were able to cross a substantial water gap by this time, and could rapidly disperse through the entire continent.

Habitat

Humans occupy all terrestrial habitats. Only human ectoparasites and endoparasites or vermin that attend humans occupy a comparably broad range of habitats. Yet, humans have experienced no speciation in spite of a vast array of occupied habitats. Therefore, the wide distribution of humans is associated with biological factors underlying good dispersability and a very broad niche. These factors include a wide tolerance for habitat diversity and pronounced seasonal variation. Humans therefore fall into the category of r-selected organisms, in spite of their large body size, longevity, and low intrinsic rate of increase.

Nutritional ecology

Humans are omnivorous. Humans were eating wild plant foods from the origins of the subfamily Homininae about 6 mya until the inception of agriculture 11,000 years ago. Comparisons with omnivorous, widespread non-human primates such as baboons make it likely that the earliest hominins consumed a variety of plants and plant parts, and also consumed insects, eggs, and small animals like birds and hares. Bone chemistry analyzing stable carbon isotopes shows that South African australopithecines were omnivores. This is true even for the species Australopithecus robustus, which had been considered highly vegetarian since the mid-1950s. Neanderthal bone chemistry shows that these fossil humans were highly carnivorous, as one might expect, given that they lived in

highly seasonal environments where carbohydrates were impoverished during certain periods. Humans lack the high complex molar teeth or ruminant stomachs that allow ungulates to process grass, and they lack the ability to detoxify secondary compounds in mature leaves or other plant parts. Only the advent of food processing or cooking technology allows humans to compensate for these biological restrictions, and to incorporate certain plants into their diets.

Cut-marks and percussion marks made by stone tools on animal bones show that vertebrate meat, fat, and marrow were incorporated into the hominin diet beginning at 2.5–2.6 mya. Tools are necessary to cut through the tough skin of a carcass, sever tendons and dismember a carcass, remove meat from bones, and break open bones to extract marrow. It is likely that hominins first acquired meat, fat, and marrow by scavenging carcasses brought down by large mammalian carnivores. By 1.8–1.6 mya, however, some archaeologists argue for the definite presence of either confrontational scavenging (where hominins displace large carnivores at a fresh and relatively intact carcass) or the hunting of vertebrate prey.

In 1968, the social anthropologist Marshall Sahlins famously described living hunter-gatherers as having the "Original Affluent Society." The depiction of hunter-gatherers as experiencing a leisurely and affluent lifestyle is no longer considered accurate. Detailed information on living hunter-gatherer groups shows that nutritional intake can be extremely variable between groups. Seasonal variation in total caloric intake or nutrient quality can be quite marked.

The domestication of animals and plants is a milestone in human history, and represents a fundamental difference in the human ability to alter ecosystems on a global scale. Animal and plant domestication occurs when humans intervene in the reproduction of other species. This intervention gradually becomes deliberate, and humans consciously select for certain phenotypic traits in the domesticated species. Dogs are the first domesticated species. Unequivocally domesticated dogs appear in the Natufian of the Middle East at 14,000 years ago. Goats, sheep, pigs, cattle, and donkeys follow. Evidence of farming first appears in the Middle East, about 11,000 years ago. Food crops have multiple centers of origin in both the New and Old Worlds.

The body mass index is widely used to study human body build and the relationship between nutrient intake and activity levels. This index is weight divided by height (BMI = kg/m²). A BMI of less than 18.5 indicates a chronic energy deficiency. Harsh environments increase the probability of insufficient calories, at least seasonally. The body mass index is rising in nearly all populations that are experiencing industrialization. This is caused by an ever more sedentary lifestyle, in which decreased physical activity is accompanied by an abundance of readily available, high calorie foods. As of 2003, this trend is becoming so pronounced, and has such deleterious health consequences, that many medical and governmental agencies are investigating ways to halt the increase in human obesity.

Reproductive biology

Humans have diverse mating systems. There is no consensus about which, if any, mating system is the oldest, and the triggers initiating human pair-bonding remain obscure. Many ideas are not testable. Species-specific mating systems occur in many mammals, and are often affected by the degree of sexual dimorphism. However, unlike other mammals, there appears to be no relationship between the degree of sexual dimorphism and a particular mating system in humans. Formal social rules often govern the choice of mate, and elaborate marriage customs can exist. Incest taboos forbidding the mating of relatives are widespread. These taboos are most effective in maintaining genetic diversity when group size is small. Human partners may travel significant distances after marriage to live with the spouse's family. This activity promotes gene flow, while increasing inter-group contacts and the dispersal of ideas. Both sexes can leave their natal group. Some genetic analysis tracking male (Y chromosome) versus female (mitochondrial DNA) dispersion indicates that females may disperse more.

Humans have no breeding season, and human females experience no estrous cycling. Singleton births are the norm, but some families and populations have an elevated frequency of dizygotic twinning, because more than one egg can be released and fertilized. The human sex ratio is usually skewed at conception and birth to favor males. The neonatal sex ratio is highly responsive to a variety of local influences. Male mortality exceeds that of females, and so the sex ratio gradually declines with age. The sex ratio is approximately equal at reproductive maturity; after this, females tend to outnumber males. Male mortality caused by violence and accident exceeds that of females. Male mortality caused by infectious diseases is also higher than that of females, and parasite load is higher in males. Higher male mortality caused by violence and the increased male parasite susceptibility appear to be the evolutionary consequences of sexual selection.

In comparison to other catarrhine primates, where males may be more than twice the size of females, humans have only a small degree of sexual dimorphism. Depending upon the population, humans have 4–7% statural dimorphism. Statural dimorphism differences are higher in populations with tall stature, and lower in populations with small stature. Human body weight dimorphism averages about 11%. Much human sexual dimorphism involves soft-tissue characters. Subcutaneous fat patterning, seen especially in breast, thigh, and buttock fat depots, is markedly different in human males and females. Females also carry a larger percentage of subcutaneous fat than males do. Even in hunter-gather groups, where humans are very active and lean, subcutaneous body fat as measured by skinfold thickness is 5–15% in males and 20–25% in females.

Humans mature slowly, so that the onset of puberty is delayed relative to pongids and other catarrhines. In females, the onset of puberty is signaled by menarche, or first menstruation. This is triggered by a critical amount of body fat. The hormone leptin, released by fat, appears to trigger menarche. Reduction of body fat in a cycling female suppresses menstruation.

Relative brain size and intelligence

Humans have the largest brain to body size ratio among terrestrial mammals, rivaled only by the smaller odontocete whales. The modern human brain has nearly tripled in size since the origins of the subfamily Homininae. The brain reaches its modern size relative to body size at approximately 300,000 years ago, which is late in human evolutionary history. Brain size reaches its apogee among the Neanderthals, where the average cranial capacity was about 300 cc more than that of the average for living humans (1,200 cc).

Using other primates for comparison, many researchers argue that human brain size increase is associated with social intelligence, driven by complex social interactions and the ability to predict and manipulate the behavior of other members of the social group (Machiavellian intelligence). However, tool behavior also must be a factor that contributes to human technical intelligence and innovation. Furthermore, humans have an ability to understand and manipulate the behavioral ecology of other species, and understand the physical properties of inanimate objects. This ability distinguishes humans from other primates, whose intelligence is oriented towards conspecifics.

Humans have the ability to use symbols and engage in symbolic behavior. In living humans, this powerfully affects all social and economic interactions. Artifacts can have symbolic properties. Archaeologists have tried to study the beginnings of symbolic behavior by investigating symmetry and other properties of stone tools. Art and bodily ornamentation are widely considered to signal the unequivocal beginning of human symbolic behavior. Pigments like red ochre and signs of pigment processing are found in archaeological sites dating to 250,000 years ago. Representational art and ornaments occur much later, and do not become abundant until about 40,000 years ago.

Species-specific behaviors: Language, tool behavior, and technology

Humans are characterized by language, which is a species-specific type of vocal communication. Special neuroanatomical centers, usually located in the left cerebral hemisphere, underlie human language abilities. Human sign languages, which are non-vocal, also utilize these centers. Although other mammals and birds possess complex vocal communication with referential signaling, human language has the unique property of recursion. This is the ability to create an infinite number of expressions by permutations of discrete components such as words or numbers. Consequently, there is no limit to the possible communications based on language or numbers. A critical period for the acquisition of human language occurs during infancy, and infants who are not exposed to language during this time fail to develop normal language abilities later in life, despite intensive training. The human infant's ability to reproduce the sounds of its native language depends on imitation. Imitation is also responsible for the faithful reproduction and cultural transmission of other human behaviors. Imitation is found in some other animals, but appears to be lacking in non-human primates, where the

transmission of behavior occurs through emulation or goal-directed behavior, and faithful reproduction is absent. Humans can acquire multiple languages, although the ease of acquisition is affected by age.

Using all available genetic information, Cavalli-Sforza et al. discovered that genetic differences between human groups are frequently, but not always, associated with language differences. This implies that language often functions as a reproductive barrier between humans, and can lead to a reduction in gene flow and subsequent population demarcation. However, a 2000 study of Y chromosome haplotypes in Europe showed that geography influences genetic diversity more than language does, at least in males.

Reliance on tools and technology is another species-specific human behavior. With the exception of the New World monkey genus Cebus, tool behavior is rare in wild non-human primates, in comparison to other animals, such as birds, where tool behavior may be much more frequent. Human tool behavior is not based solely on the ability to manipulate objects. All catarrhine primates have truly opposable thumbs, but the mere existence of truly opposable thumbs does not generate tool behavior. Nevertheless, fossil human hand anatomy has been scrutinized. Fossil hand bones presumably belonging to the taxon Australopithecus robustus have been recovered from the site of Swartkrans in South Africa. These bones date to 1.5 mya and indicate frequent manipulation. Bone digging tools also have been recovered from the site. The recovery of tools from a time before relative brain size increase indicates that hominin tool behavior is also not predicated on brain size.

The ability to control fire—i.e., to maintain and transport it away from a naturally occurring source, such as a brush fire caused by a lightning strike or volcanic event—was a milestone in human evolutionary history. Some researchers argue that control of fire may have begun as early as 1.6 mya in East Africa, although this date is controversial. The control of fire meant that, unlike other higher primates, humans did not need to seek shelter at night in trees or cliffs, where they would be safe from nocturnal predators. Fire further permitted humans to remain active after nightfall, and provided warmth at higher altitudes or in colder habitats. Fire permitted humans to cook foods and drive hunted animals, and thus expanded human dietary range. Fire also allowed humans to modify ecosystems in a profound fashion, as they burned grasslands, cut down trees, and burned forests. The control of fire marks the beginning of human modification of the earth's surface, the signs of which are now universal. In fact, charcoal lenses appearing in pollen spectra or sediment horizons are sometimes used by archaeologists as a signature of

human presence, even if human skeletal material or cultural remains are absent.

Human behavioral ecology

Humans mature slowly, although this trait appears late in human evolutionary time, appearing first among the Neanderthals. This slow maturation necessitates that adult caretakers must rear the young, even after the young are independently mobile and completely weaned. Human social complexity also mandates long periods to acquire recondite social knowledge. Consequently, much human social behavior is geared towards care, protection, and teaching of the young. Biological kin, as well as non-related individuals, engage in these care-taking behaviors.

Food acquisition and processing can be a major influence on human social organization. Relatively subtle dietary shifts may underlie significant transitions in human history. The abundance and predictability of critical food resources influences the complexity of traditional societies. Humans have the ability formally to exchange resources. Barter, trade, and economic transactions are universal. Formal marriage systems and other alliance systems promote harmony between groups. However, humans also exhibit aggression and violence. Inter-personal aggression, raiding, and warfare occur in all human groups, and they are found deep in human prehistory. They are not the fruits of an all-corrupting civilization.

Human population increase and population aggregation lead to social complexity. Five general levels of human social complexity are usually recognized. These levels are the band, the tribe, the chiefdom, the state, and the nation-state.

At the band level, humans are hunter-fisher-gatherers, living in groups of 30–100 people. There are no permanent settlements, but temporary aggregations can occur for specific reasons (e.g., seasonal hunting). The society is egalitarian, and only shamans (individuals with an ability to contact and control supernatural forces) exhibit any degree of specialization. Inter-group aggression over scarce, defensible resources can occur. Tribes exist either when hunting-fishing-gathering occurs in a rich environment, or when agriculture or pastoralism exists. Multiple kin-groups are found in a tribal society, and some division of labor takes place. Raiding is possible.

Chiefdoms exist when populations are large, and when hunting-fishing-gathering occurs in a rich environment, or when agriculture or pastoralism exists. A more sedentary lifestyle and permanent aggregations can lead to village life. Villages appear before agriculture in both the Old and New Worlds. For example, an intensive use of wild grass seeds, a sedentary lifestyle, and villages appear among the Natufian people of the Middle East, who lived between 14,000 and 11,000 years ago. Chiefdoms have food storage, hoarding, and formalized distribution of food and other resources. There is

centralized leadership vested in a chief, ranking or hierarchical division, and an artisan class. Elaborate trade networks can exist, and formalized warfare is possible.

States occur with the advent of large populations, agriculture, and the rise of urban life in cities. Cities are large, permanent aggregations of people that have multiple activity areas. Cities serve as organizational centers for a broad region. States possess a complex bureaucracy with centralized power vested in a ruling class, often a royal or noble class. The state has many nonagricultural specialists, including religious specialists for formal religion. There are central services, with a complex organization of labor, goods, and services. Complex record-keeping, culminating in the invention of writing systems, assists in facilitating these intricate activities and exchanges. Complex trade networks can occur over long distances, and monumental architecture appears.

Finally, nation-states occur with large populations, agriculture, and cities. The nation-state first emerged more than 5,000 years ago in pre-dynastic Egypt, when the unification of Upper and Lower Egypt took place. The nation-state has a broad geographic spread, and incorporates many different habitats. Multiple ethnic groups, languages, and religions exist within a nation-state. Elaborate organized warfare and conquest are possible.

Resources

Books

Cavalli-Sforza, L. L., P. Menozzi, and A. Piazza. The History and Geography of Human Genes. Princeton, NJ: Princeton University Press, 1994.

Frisancho, A. R. Human Adaptation and Accommodation. Ann Arbor: University of Michigan Press, 1993.

Klein, R. G. The Human Career: Human Biological and Cultural Origins. 2nd ed. Chicago: University of Chicago Press, 1999.

Panter-Brick, C., R. H. Layton, and P. Rowley-Conwy, eds. Hunter-Gatherers: An Interdisciplinary Perspective. Cambridge, UK: Cambridge University Press, 2001.