THE RECORD OF HUMAN EVOLUTION: AN OVERVIEW

The human twig, or clade, separated from the rest of the Tree of Life between 5 Myr to 10-14 Myr ago. The human twig (or hominin clade) came off a common branch we share with chimpanzees. Modern human and chimpanzee skeletons can readily be distinguished, but it is more difficult to distinguish the earliest members of the hominin and chimp (or panin) clades. Morphology consistent with habitual bipedalism, small canine tooth crowns and relatively large chewing teeth are features that would encourage researchers to interpret fossil evidence as hominin as opposed to panin. This, of course, assumes that the only African higher primate clades that existed between 5-10 Myr ago were the hominin and panin clades, and the clade that included their common ancestor. Statements about how many species are sampled in the hominin fossil record are hypotheses. All species have a “beginning” (the result of a speciation event), a “middle” (that lasts as long as the species persists), and an “end” (either extinction or participation in another speciation event). We observe living species during an instant in geological time during its evolutionary history, much as a single still photograph is only a partial record of a long distance running race. In the hominin fossil record the same species may be sampled several times so, to return to our metaphor, the hominin fossil record may well be providing us with more than one still photograph of the same race. Taxonomists must work out how to tell several photographs of the same race apart from photographs of different races. If a punctuated equilibrium model of evolution is adopted along with a branching, or cladogenetic, interpretation of the fossil record, then researchers will tend to divide the hominin fossil record into a larger rather than a smaller number of species. Conversely, researchers who favor a more gradualistic or anagenetic interpretation of evolution, emphasizing morphological continuity instead of morphological discontinuity, and who see species as longer-lived and more prone to substantial changes in morphology through time, will resolve the hominin fossil record into fewer, more inclusive, species. Examples of both types of taxonomy will be presented, and attention will be drawn to significant events in hominin evolution that are common to both taxonomic interpretations.