Hominid Species Time Line

Page 11

Conclusions and Questions
Uncertain Lines of Descent, Uncertainty at the Boundaries 

Three million years ago was a critical period in the development of the hominid species, and ultimately, of human beings.  It is not yet clear, however, how the members of this category of very early hominids relate to each other. That is, the lines of descent from one species to another are not yet established with certainty.  The proliferation of genus names suggests there may be two or even three hominid family lines.  Still less is it clear how these very early hominid species relate to what came after them — how species in my earliest category connect to later species across my arbitrary boundary lines.

To address one crucial example:  the species life of A. afarensis straddles the 3 million year boundary adopted for this discussion. Some investigators suggest that A. afarensis is the direct ancestor of the Homo (human) line.  Others see A. afarensis as ancestral to the australopithecines only.  Others still see it as ancestral to both the australopithecines and the genus Homo. Others are not so sure.

A. africanus is another candidate that was once suggested as the ancestor of the genus Homo, though this view has received dwindling support in recent years.

 A third possibility is that humans emerged from a line descending from O. tugenensis and/or K. platyops, as the Leakeys believe.

 A fourth possibility is that there was another, or were other, as-yet-undiscovered species of early hominids who were direct ancestors of the genus Homo and therefore of modern humans.   At this point, we don’t know.  Tracing a direct line of descent among these three categories or even within our own genus is not yet possible.

 Based on existing evidence, then, scientists offer several possible lines of descent or relationship among the known specimens of extinct hominids. All of them, it must be emphasized, are tentative constructions.

 This version of a hominid family tree has the virtue of simplicity:  it attempts to show only the genera of the hominid family (distinguished by color) and their relative chronological positions.  It does not try to draw connections (lines of descent) among the genera.

Below is a more detailed (and bolder) version of the hominid family tree by Ian Tattersall from the article “Once We Were Not Alone,” Scientific American, (January, 2000), p. 60.  While it was devised prior to the discovery of Sahelanthrops tchedensis, it shows, with due cautions, a plausible set of relationships of the species known at that time.

See also Donald Johanson’s alternative lines of descent at http://www.becominghuman.org/documentary.