Monday, June 21, 2010
Ardipithecus fades into the past
The long awaited publication of the partial hominin skeleton from Woranso-Mille, Ethiopia (shown on the left) should send shock waves through paleoanthropology. Yohannes Haile-Selassie et al. describe the specimen, which is dated by radiometric and paleomag techniques to 3.58 mya, in the on-line edition of PNAS now available. Older than “Lucy” by nearly 400 ky and slightly younger than the Laetoli footprints, the specimen includes key skeletal elements that clarify the anatomical adaptations for bipedality in early australopith hominins. According to the authors they are surprisingly modern, supporting contributing author Owen Lovejoy’s long-standing argument that A. afarensis had a modern striding gait and had lost functional adaptations for arboreality. At nearly 3.6 mya the Woranso-Mille hominin is less than one million years removed from Ardipithecus ramidus (~4.4 mya), but shows a near modern upper and lower limb morphology. Ar. ramidus on the contrary shows an extremely primitive post-cranial anatomy that is in many ways more Proconsul-like than modern ape-like, with well-developed arboreal adaptations. It would require a very quick evolutionary makeover to account for the necessary transformation of Ar. ramidus into Au. afarensis as described in the paper. Before this publication the degree of bipedalism that the partial skeleton illustrates was thought to be restricted to the genus Homo. If we reject the ancestor/descendant relationship between Ar. ramidus and Au. afarensis, where does that leave Ardipithecus as a hominin? It is most likely that Ar. ramidus represents a relic species, phenetically little removed from the last common ancestor of the African apes and humans. This conclusion thus leaves unanswered the question, from whence came Australopithecus?