Recently I reviewed the many doubts among scientists about claims that the human-ancestor du jour, Australopithecus sediba, was in fact a human ancestor at all. Now the New Scientist elaborates on those doubts, stating that even sediba‘s discoverer and main advocate, Lee Berger, is softening his claim that sediba was our direct ancestor. Berger still thinks sediba was close to being ancestral, but as New Scientist explains:
A. sediba, critics are quick to point out, is everything that H. habilis is not: It’s a small-brained australopith living in southern Africa 2 million years ago — a good 300,000 years after the larger-brained H. habilis first appeared in East Africa. They say A. sediba is the wrong hominin in the wrong place at the wrong time to be our direct ancestor. “It’s just too young to lead to Homo,” says [paleoanthropologist Fred] Spoor.
(Colin Barras, “The unexpected ape,” New Scientist, Issue 2925: 34-37 (July 13-19, 2013))
Berger, naturally, disagrees, and he thinks that other traits make it clear that sediba was closer to being our ancestor. But forcing sediba into our lineage has the unconventional and unexpected implication that the crucial trait of increasing brain-size isn’t diagnostic of the line leading to Homo:
What this means, he [Berger] says, is that large brains evolved twice. A small-brained East African australopith evolved into the larger-brained H. habilis around 2.3 million years ago, but this lineage died out. A little later, a southern African australopith closely related to A. sediba evolved into the large-brained H. erectus, and this lineage went on to give rise to the rest of humanity. So Berger is not claiming that A. sediba itself is our direct relative, but that our actual ancestor was a very similar australopith that lived in around the same region at around the same time. (emphasis added)
This is a crucial admission, because it shows that even Lee Berger is backing away from claims that sediba was a direct ancestor of humans. New Scientist explains why A sediba is failing to convince the skeptics that it is closely related to humans:
Berger’s ideas have met with strong criticism. [UC Berkeley paleoanthropologist Tim] White, for instance, dismisses Berger’s claims for A. sediba. Where Berger sees a suite of Homo-like characteristics, White sees a peculiar mixed anatomy that couldn’t possibly serve as the blueprint for our genus.
In the end, the article explains why the evolutionary origin of humans is still an enigma:
What [chemical analyses of hominin teeth] doesn’t do is resolve the issue of whether H. habilis or an A. sediba-like australopith was our direct ancestor. Only the discovery of clear intermediates will help settle this argument. But if the past few years are anything to go by, new finds are likely to raise more questions than they answer.
But as I discussed last year, there are many problems with claims that habilis was a human ancestor. Clear intermediates are elusive as ever, and so the mystery remains.