Upper right third molar, Ardipithecus (Australopithecus?) ramidus
Ardipithecus ramidus is the earliest hominid found so far and was discovered in Aramis, in the Middle Awash region of Ethiopia in 1994 by Tim White and his two colleagues, Gen Suwa and Berhane Asfaw. Ardipithecus ramidus translates literally as "ground man-root" and is thought to be 4.4 to 4.5 million years old. Originally it was named as a member of the australopithecine family, but it was later decided that this species differed too much from other australopithecines and was not an ancestor.
17 different specimens were found with many bones present. These specimens included part of a child's mandible, some isolated teeth, a fragment of basi-cranium, and three bones of a left arm of a single individual. The image above shows the upper right third molar which they found on the site. However, it cannot be told whether this member of the hominid family was bipedal as all hip-bones and femurs were missing. If this was the case, then this pushes bipedal walking back even further. The origin of bipedal walking is shown in the following figure and as can be clearly seen, it is not sure whether bipedal walking can be attributed to A. ramidus.
The dentition of Ardipithecus ramidus is more primitive (more apelike) than that seen in Australopithecus afarensis, with narrower molar teeth capped with thin enamel, unlike the condition in all other known hominines; the canines are larger, but not as large as in living apes. The arm exhibits both apelike and non-apelike features, from which, White and his colleagues concluded that the mode of locomotion cannot confidently be determined.
In an unusual move, seven months after their initial publication in Nature, White and his colleagues published a note in the same journal changing the genus name to Ardipithecus (ardi means "ground floor" in the Afar language). The authors noted that Ardipithecus was probably the sister taxon of Australopithecus; in other words, both species derived from an as yet unknown common ancestor. No substantive reason was given for the change, but many observers believe that it may be based on an early analysis of a partial skeleton of ramidus that was discovered at the end of 1994. Highly fragmented and encased in a matrix, the skeleton will take a long time to prepare and analyse fully, but preliminary indications are that it might reveal a more primitive, chimp-like morphology - hence the change of genus name. However, even though the possibility has been raised that ramidus might even be an ape, it is fairly sure that it is a hominid, as the very earliest hominines were expected to be apelike (or even possibly chimp-like) in many ways such as dentition anyway. It has thus been decided that Ardipithecus ramidus was not a direct ancestor to later hominids.
Credit: photo and (edited) text courtesy of Lorraine Dallmeier. Her website, PaleoAnthropology: A Short Journey Through Time, offers a superb introduction to the subject.