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Saturday, April 10, 2010

A new hominid ancestor


In 2008, paleontologist Lee Berger's nine year old son found an interesting fossil in a South African cave. Consequently, study showed that the fossil was from one of two partial hominid skeletons belonging to a hitherto unknown species, now named Australopithecus sediba. ‘Sediba’ means ‘fountain’ or ‘wellspring’ in the local language of Sotho.

Figure 1

Australopithecus sediba

CREDIT: L. BERGER ET AL., SCIENCE

Researchers used uranium-lead radiometric dating (by two independent labs), as well as paleomagnetic dating of the sediments around the fossils to date the skeletons to just under 2 million years old, making them over a million years younger than Lucy, an Australopithecus afarensis.

The specimens have traits in common with the older Australopithecines as well as with the earliest Homo species. In fact, the scientists believe that Au. sediba descended from Au. africanus (itself a descendent of Au. afarensis), and may be an ancestor of Homo erectus or Homo habilis.

I should point out that the hominid family tree is extremely ‘bushy’. There are a large number of species on the tree, many of which were once thought to lead directly to Homo sapiens sapiens (us), but which now appear to be side branches that were eventually trimmed off by evolution. Add to that the fact that many species existed for long stretches of time, overlapping later species, and it becomes extremely difficult to establish an exact evolutionary timetable. For example, because Au. sediba has certain traits in common with Homo erectus but not with Homo habilis, Au. sediba may replace H. habilis as our ancestor. On the other hand, both species may be ancestors of ours, but may need to be temporally realigned.

Besides the Au. sediba skeletons, at least 25 other species were found in the same cave. The scientists researchers were able to make a detailed analysis of the surrounding conditions, and speculate that the hominids and other creatures may have entered the cave looking for water and then either fell to their deaths or gotten hopelessly lost.

The two skeletons are of boy and an adult woman. The children of South Africa have been invited to give the juvenile skeleton a common name.