Hominid Species Time Line

Page 7

Ardipithicus ramidus kadabba
Dated to 5.8-5.5 million years old

 A subsequent (1997) expedition from U.C. Berkeley discovered additional remains along the Afar rift that proved to be older than the Ardipithicus ramidus bones discovered in 1994.  Eleven bone fragments, including a toe bone, and representing at least five individuals have provided additional information about this species.  These older remains are seen by some as a subspecies of Ardipithicus ramidus, and by others as a separate species, Ardipithicus kadabba.  The teeth of the two ramidus species have a good deal in common with each other and also with the teeth of O. tugenensis, which strongly suggests a family relationship among these specimens.

 A. ramidus kadabba was chimpanzee-sized, about four feet tall.  The canine teeth of ramidus kadabba resemble the teeth of later hominid species more than those of a chimpanzee.

 One of the discoverers, Yohannes Haile-Selassie, a citizen of Ethiopia and a doctoral student at the University of California, Berkeley, argued in the July 12, 2004 issue of Nature that both the shape and the wear patterns on the single toe bone discovered indicate a bi-pedal mode of locomotion.  If these inferences are correct, it would push the evolution of bipedalism back toward the 6 million year threshold.  Of course, if the Toumai species was bi-pedal, this mode of walking is much older still.  The toe bone was found in strata several hundred thousand years younger than the oldest and more complete remains.  Its identification with kadabba is thus assumed rather than proved, and therefore the claim that the species was bipedal rests on rather tenuous ground.

 Bernard Wood (“Palaeoanthropology: Hominid revelations from Chad,” Nature 418 (11 July 2002), 133-135) makes the interesting argument that the evolution of bipedalism in ape-like creatures may have occurred more than once, rather than being the single “event” that is usually assumed.  If that is the case, the “hominid family tree” (or trees) may have two or more “trunks.”