Monday, April 17, 2006

A New Find in the Middle Awash

A new set of finds of Australopithecus anamensis in the Middle Awash River region have filled in some gaps in the evolutionary sequence of early hominids. As Foxnews reports, notable is the fact that eight successive species are now represented in the region. Berhane Asfaw, one of the principle excavators, states:

"We just found the chain of evolution, the continuity through time...One form evolved to another. This is evidence of evolution in one place through time."

Palaeoanthropologist Tim White also notes:

"It's like 12 frames of a home movie, but a home movie covering 6 million years."

The corresponding Nature article (White, et al., 2006. Asa Issie, Aramis and the origin of Australopithecus. Nature, 440: 883-889) is a bit more illuminating. The photo that is provided shows maxillary and mandibular remains, associated teeth and a right femoral shaft. The teeth are larger than that of Ardipithecus ramidus but are similar to those of Au. afarensis and Au. anamensis. The authors also write:

In an assessment of fossils from Kanapoi (3.9-4.2 Myr ago), the anagenetic series Ar. ramidus, Au. anamensis and Au. afarensis has been hypothesized. The evidence reported here from the Afar Rift consitutes a strong test from a single stratigraphic succession that fails to falsify this hypothesis. Middle Awash Au. anamensis is anatomically intermediate in many characters between the earlier Ar. ramidus and the later Au. afarensis from the same study area. (White, et al. 2006)

The readers are also treated to an exposition on anagenetic speciation versus punctuated equilibrium and how each can be recognized in the fossil record. This is described by Stephen Jay Gould:

We can distinguish the punctuations of rapid anagenesis from those of branching speciation by invoking the eminently testable criterion of ancestral survival following the origin of a descendant species. If the ancestor survives, then the new species has arisen by branching. If the ancestor does not survive, then we must count the case either as indecisive, or as good evidence for rapid anagenesis--but, in any instance, not as evidence for punctuated equilibrium. (Gould, S.J. 2002. The Structure of Evolutionary theory. Cambridge: Harvard University Press. p. 795)

White, et al. do not venture a guess as to which the Middle Awash deposits represent. Interestingly, according to the authors, a trigger has not been identified for the origin of the australopithecines. In other words, we have a pretty good idea when it happened. We do not, at present, know why. ID perhaps?

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