Summary

The clade (a.k.a. twig of the Tree of Life) that includes modern humans includes all of the extinct species that are judged, on the basis of their morphology or their genotype, to be more closely related to modern humans than to chimpanzees and bonobos. Taxic diversity with respect to the hominin clade refers to evidence that it included more than one species at any one time period in its evolutionary history. The minimum requirement is that a single ancestor-descendant sequence connects modern humans with the hypothetical common ancestor they share with chimpanzees and bonobos. Does the hominin clade include just modern human ancestors or does it also include non-ancestral species that are closely related to modern humans? It has been suggested there is evidence of taxic diversity within the hominin clade back to 4.5 million years ago, but how sound is that evidence?

The main factor that would work to overestimate taxic diversity is the tendency for paleoanthropologists to recognize too many taxa among the site collections of hominin fossils. Factors that would work to systematically underestimate taxic diversity include the relative rarity of hominins within fossil faunas, the realities that many parts of the world where hominins could have been living are un- or under-sampled, and that during many periods of human evolutionary history, erosion rather than deposition predominated, thus reducing or eliminating the chance that animals alive during those times would be recorded in the fossil record. Finally, some of the most distinctive parts of an animal (e.g., pelage, vocal tract, scent glands) are not likely to be preserved in the hominin fossil record, which is dominated by fragments of teeth and jaws.