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| "...we come from a long line of failures. We are apes, a group that almost went extinct fifteen million years ago in competition with the better-designed monkeys. We are primates, a group that almost went extinct forty-five million years ago in competition with the better-designed rodents. We are chordates, a phylum that survived in the Cambrian era 500 million years ago by the skin of its teeth in competition with the brilliantly successful arthropods. Our ecological success came against humbling odds." |
We humans have always held a special fascination with our place in the evolutionary pageantry. Where did we come from? That we share close common ancestry with the apes is not in doubt. But what kind of an ape are we? How long ago did our lineage separate from the other apes? Are we still evolving? What can molecular genetics tell us about our history and our future?
Concerning our place among the apes, Thomas Huxley (known as "Darwin's bulldog" because of his popularization of Darwin's theory of evolution) provided an early correct answer in 1863. Huxley placed humans with chimpanzees and gorillas (the great apes of Africa), and separate from orangutans and gibbons. Numerous lines of evidence have since strongly supported this view. Morphologically, we share many derived traits with the African apes, including enlarged brow ridges, elongated skulls, shortened canine teeth, and enlarged mammary glands. We are also much more similar at the DNA level to the African apes than we are to any other species.
Scientists have identified three species of African apes: the gorilla (Gorilla gorilla) and two species of chimpanzees. Pan troglodytes, the common chimpanzee, is the larger of the two. Pan paniscus, which had previously been called the "pygmy chimpanzee," is now named the "bonobo." In addition to size, the two chimpanzees differ in social structure and temperament; the bonobos appear to be more peaceful and egalitarian than the common chimpanzees. Both species of chimpanzees use tools; however, the types of tools vary in different populations of both species.
By about 1980 a combination of crude molecular techniques - based on the divergence of certain proteins and the fossil record - allowed us to determine that humans, chimps, and gorillas last shared a common ancestor approximately five million to eight million years ago. Those methods, however, were not able to ascertain the order in which the three species split. Several questions remained. For instance, are chimpanzees and gorillas each other's closest relatives? Or is the closest relationship between humans and chimpanzees? Or is it between humans and gorillas?
We could encapsulate the outdated view of evolution as progress up a ladder of changes. First the ability to walk upright (bipedalism) appeared. Soon after, the lineage leading to humans (the hominids) split off from the other African apes. Many fossils of the genus Australopithecus demonstrate that the earliest bipedal hominids did not substantially differ from chimpanzees in brain size. In the outdated view of human evolution, there was a slow steady increase in brain size as Australopithecus afarensis (better known as "Lucy") evolved into Homo habilis, and then into Homo erectus. Brain size continued to increase until the appearance of Homo neanderthalis (the Neanderthals), who looked much like us but had larger brow ridges.
This older view of human evolution is not so much incorrect as it is incomplete and misleading. New fossil evidence demonstrates that the hominid lineage, our family tree, is more bush-like than ladder-like. Studies of these fossils show that several species of hominids co-existed for long periods of time. New molecular genetic evidence allows us to address which two of the three species - chimpanzee, gorilla, and human - represents the two closest relatives. Genetic data also can address the patterns of variation within and among human population. In addition, the molecular genetic data demonstrate how infectious disease has shaped genetic variation in humans.