Hominid Species Time Line

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The course of hominid and human development is, in many aspects, quite well preserved in the fossil record. The level of detail unearthed about early hominids in the past 80 years is remarkable, given the presumed rarity of our ancestral family of species.  The discoveries of the last decade have pushed our knowledge of our family history back to 7 million years ago and filled in many gaps in the record. 

By a fortunate circumstance, most early hominid species lived in east Africa, along the Great Rift Valley, which, in recent geologic time, has experienced repeated episodes of volcanism that left distinctive layers of ash amid the usual accumulations of strata.  Volcanic ash is datable by measuring the potassium-argon decay rate and other radiometric processes.  Fossils lying between ash layers are therefore datable in relative terms:  a given specimen is younger than the ash layer just beneath, and older than the one lying above.  So we not only have a relative abundance of fossil evidence of species leading to or related to modern humans, but a rare degree of precision regarding much of the chronology of that story as well. 

The last two decades especially have witnessed a rich trove of new evidence that is, at the same time, productive of controversies.  The issue is where the new finds (and some older ones) fit into the larger picture of hominid family history.  Many species names and even some genus classifications are still in contention, and the discoveries have also stimulated new hypotheses about human development. In the meantime, new discoveries continue to be made every year. 

Nevertheless, significant gaps in the record remain, and until they are filled we cannot be certain about critical details of human evolution. As in all inquiries of an historical nature, complete evidence is almost always out of reach.

To use a metaphor:  at present, the evidence provides an array of “dots” (representing points of information on a few specimens) and some “dashes” (where multiple finds of a single species are available), which together suggest the shape of our hominid family tree.  At this moment, however, we cannot “connect the dots” with confidence. On the other hand, there is no reasonable doubt that the dots do connect.  The business of connecting these dots falls to the modern science of phylogenetics, which provides several recent hypotheses of relationships within hominid evolution. As new fossils and new data become available, they will be incorporated into our current knowledge and a more detailed family tree will emerge.

Much of what we would like to know about the past — our past — may simply not be preserved in the fossil record — or has not been discovered yet.  However, to reiterate my original point, the broad outlines of human descent are remarkably well established and, in a sense, the arguments are about details.