Cephalization
Cephalization
Cephalization is the process in animals by which nervous and sensory tissues become concentrated in the "head." The evolution of a head allows scientists to distinguish between the head end, or anterior end of an animal's body, and the opposite end, the posterior . Although cephalization is associated primarily with bilaterally symmetrical species, even some of the more primitive, radially symmetrical animals show some degree of cephalization.
Cephalization evolved several times within the animal kingdom, suggesting that it offers certain inherent advantages. In particular, with the evolution of cephalization, the anterior end of the animal became most likely to first encounter food, predators, and other important features of the external environment. Because brain and sense organs are also concentrated in the anterior end, the organism is well prepared to deal with these features.
Cephalization in the Animal Kingdom
Even hydras, which are primitive, radially symmetrical cnidarians , show some degree of cephalization. They have a "head" where their mouth, photoreceptive cells, and a concentration of neural cells are located.
Flatworms (phylum Platyhelminthes) are the most primitive animals with bilateral symmetry. They also have a fairly advanced degree of cephalization, with sense organs (photosensory and chemosensory cells) and a brain concentrated at the anterior end. Consequently, scientists believe that cephalization characterized all bilaterally symmetrical animals from their origins. However, flatworms differ from more advanced animals in that their mouths are in the center of their bodies, not at the anterior end.
In arthropods, cephalization progressed with the incorporation of more and more trunk segments into the head region. Scientists believe this was advantageous because it allowed for the evolution of more effective mouth-parts for capturing and processing food.
Cephalization in vertebrates, the group that includes mammals, birds, and fishes, has been studied extensively. The heads of vertebrates are complex structures with many features not found in close relatives such as the cephalochordates. The cephalochordate Branchiostoma (formerly called Amphioxus), which is the closest relative of vertebrates, is a burrowing marine creature which lacks most of the head structures that are so distinct in vertebrates, such as distinct sense organs; a large, multilobed brain; teeth; and a tongue.
There was a persistent debate during the twentieth century as to whether the vertebrate head is "old" or "new." Scientists who champion the idea of an "old" head suggest that the vertebrate head resulted from the evolution of important modifications to a previously existing head. The idea of a "new" vertebrate head was proposed originally by American vertebrate morphologists Carl Gans and Glenn Northcutt in 1983. They suggested that the vertebrate head is a new structure, which has no corresponding structure in close relatives such as Branchiostoma.
Evidence to support a "new" vertebrate head comes from the observation that most important features of the head are derived from neural crest cells, embryonic cells found only in vertebrates. The neural crest cells are of ectodermal origin—rather than mesodermal or endodermal—and arise during the process of neurulation, the time at which the dorsal hollow nerve cord forms.
Neural crest cells are exceptional in that they are highly mobile, migrating in streams throughout the head region and the rest of the body, and because they give rise to an unusual diversity of features. The neural crest cells are responsible for forming the bones of the face and jaws, the structures of the tongue and larynx, the teeth, and portions of the eye. Experiments in which the neural crest was removed from developing animals confirmed that these critical head structures failed to develop without it.
Scientists hypothesize that increased cephalization in vertebrates, including the evolution of many of their novel head features, is related to adaptations for predation. Sensory structures—the jaw and large brain—are all requirements for a successful existence as a predator.
Losses of Cephalization
Cephalization has been lost in some groups. One example comes from the echinoderms, the phylum that includes the starfishes and sea urchins. These species have lost bilateral symmetry and returned to a radially symmetrical body plan. However, only adult starfishes and sea urchins are radially symmetrical. The larval stage remains bilaterally symmetrical and is characterized by cephalization. Other echinoderms, the sea cucumbers, have regained bilateral symmetry in the adult. Thus the phylum has been characterized by multiple instances of the acquisition and loss of bilateral symmetry and cephalization.
Mollusks represent another group in which cephalization has been lost and regained. For example, bivalves are not particularly cephalized (although some scientists have argued that they are "all head"). However, as with the echinoderms, certain mollusks regained cephalization. In particular, the appropriately named cephalopods (the group that includes the squid and octopus) are characterized by an advanced degree of cephalization. Their sense organs, including well-developed eyes and a brain, are concentrated in a distinct head region. Interestingly, as with vertebrates, the evolution of an advanced degree of cephalization in cephalopods was associated with the evolution of a predatory lifestyle.
The Origin of the Head
Although cephalization appears to have evolved multiple times, in the last ten years molecular biological work implies that the distinction between head and the rest of the body may actually be quite ancient. In particular, certain genes expressed only in the head region appear to determine the boundary between head and trunk. These genes are present in diverse animal phyla, including arthropods, chordates, and annelids (other groups have yet to be studied). This broad distribution of the multiple genes indicate that they may have been present in the common ancestor of most animals. Studies of the hydra have shown that it too possesses some of these same genes, suggesting that the distinction between head and trunk is rather ancient in the animal kingdom because the hydra is a member of a primitive lineage (the cnidarians).
see also Body Plan.
Jennifer Yeh
Bibliography
Gould, James L., and William T. Keeton, with Carol Grant Gould. Biological Science, 6th ed. New York: W. W. Norton & Co., 1996.
Hickman, Cleveland P., Larry S. Roberts, and Allan Larson. Animal Diversity. Dubuque, IA: William C. Brown, 1994.
Hildebrand, Milton, and Viola Hildebrand (ill.). Analysis of Vertebrate Structure. New York: John Wiley & Sons, 1994.