MR.EVIL

Human Evolution

The third major trend in hominine development is the gradual decrease in the size of the face and teeth. All the great apes are equipped with large, tusklike canine teeth that project well beyond the level of the other teeth. The earliest hominine remains possess canines that project slightly, but those of all later hominines show a marked reduction in size. Also, the chewing teeth—premolars and molars—have decreased in size over time. Associated with these changes is a gradual reduction in the size of the face and jaws. In early hominines, the face was large and positioned in front of the braincase. As the teeth became smaller and the brain expanded, the face became smaller and its position changed; thus, the relatively small face of modern humans is located below, rather than in front of, the large, expanded braincase.      The fossil evidence for immediate ancestors of modern humans is divided into the genera Australopithecus and Homo, and begins about 5 million years ago. The nature of the hominine evolutionary tree before that is uncertain. Between 7 and 20 million years ago, primitive apelike animals were widely distributed on the African and, later, on the Eurasian continents. Although many fossil bones and teeth have been found, the way of life of these creatures, and their evolutionary relationships to the living apes and humans, remain matters of active debate among scientists. One of these fossil apes, known as Sivapithecus, appears to share many distinguishing features with the living Asian great ape, the orangutan, whose direct ancestor it may well be. None of these fossils, however, offers convincing evidence of being on the evolutionary line leading to the hominid family generally or to the human subfamily in particular. Comparisons of blood proteins and the DNA of the African great apes with that of humans indicates that the line leading to modern people did not split off from that of chimpanzees and gorillas until comparatively late in evolution. Based on these comparisons, many scientists believe a reasonable time for this evolutionary split is 6 to 8 million years ago. It is, therefore, quite possible that the known hominine fossil record, which begins about 5 million years ago, extends back virtually to the beginnings of the human line. Future fossil discoveries may permit a more precise placement of the time when the direct ancestors of the modern African ape split off from those leading to modern people and human evolution can be said to begin.