Human Evolution: Overview

There is abundant evidence that the living animals most closely related to modern humans (Homo sapiens) are the chimpanzee (Pan) and the gorilla (Gorilla). Both are nonhuman in their appearance and behaviour and it was assumed that they were more closely related to each other than either was to modern humans. However, when their genetic identities are compared there is evidence that the DNA in both the nucleus and the mitochondria of the cells of modern humans and the chimpanzee are very similar. An increasingnumber of researchers are convinced that the similarities between them suggest a shared common ancestry to the exclusion of the gorilla, but othersmaintain that the relationships between Homo sapiens, Pan and Gorilla are so close that it is not possible to link twoof them to the exclusion of the third. Differences in the DNA can be used to provide an estimate of how long lineages have been independent. The molecular differences between living people and the living African apes suggest that the lineage which includes modern humans has been separate from the rest of the apes (or hominoids) for between 5 and 8Ma. Animals in the fossil record that are judged to be more closely related tomodern humans than to the apes have traditionally been called ‘hominids’, and those that are closer to the apes than to modern humans have been called ‘pongids’. Now that there is good evidence that Homo and Pan are so closely related, scientists are using ‘hominid’ to refer to the family Hominidae, which includes Homo and Pan, and they use ‘hominin’ to refer to the Tribe Hominini, which includes only modern humans and the human clade. Modern humans and their ancestors are thus known as ‘hominins’. For many years human evolution was likened to a ladder, with earlier, more primitive, species being ‘replaced’ by later, more advanced ones, with modern humans at the top of the ladder of ascent. Recent evidence suggests that the metaphor of a ladder is no longer an appropriate one. Instead, the hominin evolutionary tree is much better likened to a bush that has multiple stems leading off from close to the base, as well as closer to the crown (Figure 1). All but one of the stems stop well short of the highest point of the bush; these are hominin lineages that have no living descendants. The Earliest Hominins