Overview of Human Evolution
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Two versions of the Hominid Family Tree



One example of the classification controversy is illustrated in the chart above left: Donald Johanson regards A. afarensis as the common ancestor of humans and ape hominids. Richard Leakey, looking at the same evidence, disputes this claim and regards the ape hominids, including A. afarensis, as a collateral line, not ancestral to humans, as indicated in the chart on the right. In Leakey's view, the most important "missing link" is still to be found. Other interesting controversies are flourishing as well.

It is difficult, for example, to pinpoint where late Homo erectus becomes early Homo sapiens or to determine whether the Neanderthals were closely related to modern humans. Are they a subspecies of Homo sapiens?--or should they be classified as the most recent subspecies of Homo erectus? Our present classification of the hominids is therefore likely to change as we acquire new knowledge. Among the fossil specimens already known, for instance, there may be five or more australopithicine species.

Currently, new discoveries and re-examination of older scientific dating techniques are challenging the view that Homo erectus is only 1.6 or 1.7 million years old. Some of the new but controversial dates may require extending the species life of H. erectus well beyond 2 million years. A new dating technique makes some specimens found in Asia appear much older than previously thought.

One limiting factor in the surviving evidence is worth noting. Often physical remains can be dated with some degree of precision, so it is possible to establish a fairly reliable chronology--though chronologies are always subject to revision, as new information becomes available. Physical evidence (such as fossil bones) establishes anatomical similarities between specimens and groups of specimens. But similarities in structure do not necessarily establish relatedness, the actual lines of descent. Carefully observed structural similarities only allow us to make informed guesses about kinship or descent. Furthermore, species are not absolute entities in nature, especially viewed over time.

Only time will tell.

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