New Dates For Little Foot Push It Back to Au. afarensis Time-Period

On the heels of the Ledi jaw dates for early Homo at 2.8 million years ago, and the corresponding tentative conclusion that only Au. afarensis could emerge as a possible ancestor for early Homo, to the exclusion of any of the other australopithecines, comes a redating of the Australopithecus specimen from the cave of Sterkfontein to 3.67 million years ago.  From the story in Science Daily:

Ronald Clarke, a professor in the Evolutionary Studies Institute at the University of the Witwatersrand who discovered the Little Foot skeleton, said the fossil represents Australopithecus prometheus, a species very different from its contemporary, Australopithecus afarensis, and with more similarities to the Paranthropus lineage.

“It demonstrates that the later hominids, for example, Australopithecus africanus and Paranthropus did not all have to have derived from Australopithecus afarensis,” he said. “We have only a small number of sites and we tend to base our evolutionary scenarios on the few fossils we have from those sites. This new date is a reminder that there could well have been many species of Australopithecus extending over a much wider area of Africa.”

So, what is Au. prometheus, exactly and how does it differ from the other australopithecines in the area?  From the paper by Granger, et al.¹:

This species was named on the basis of a parietooccipital fossil from Makapansgat23. It has been suggested22 that several other Sterkfontein and some Makapansgat specimens also belong in this species making Australopithecus africanus and A. prometheus contemporaries in the assemblages of Makapansgat Member 3 and Sterkfontein Member 4. A. prometheus differs from A. africanus in features including Paranthropus-like larger, bulbous-cusped cheek teeth, a longer, flatter face, incipient supraglabellar hollowing and a more vertical rounded occiput22. (Note that we use the term hominid in the traditional sense to include humans and their ancestral relatives but exclude the great apes.)

One of the raging debates in human palaeontological studies concerns whether or not the robust australopithecines, Au. robustus and Au. boisei, represent their own clade, Paranthropus. This perspective is based on traits that are shared to the exclusion of other australopithecine species.  Opponents of this view argue that quite a few of the traits that make up this clade are functional in nature, involving mostly the chewing complex, and that as such, Au. robustus and Au. boisei are  outgrowths of Au. africanus.

The story continues, quoting Ron Clarke:

“It demonstrates that the later hominids, for example, Australopithecus africanus and Paranthropus did not all have to have derived from Australopithecus afarensis," he said. “We have only a small number of sites and we tend to base our evolutionary scenarios on the few fossils we have from those sites. This new date is a reminder that there could well have been many species of Australopithecus extending over a much wider area of Africa.”

Hope so, because right now, here is how it looks:

• P. robustus (or Au. robustus) only in South Africa
• P. boisei (or Au. boisei) only in East Africa
• Early Homo only in East and Northeast Africa
• Au. africanus only in South Africa
• Ar. ramidus only in North East Africa
• Au. afarensis only in North East Africa

As Slim Pickens would say: “What in the Wide, Wide World of Sports is a-goin' on here?”

All of this sort of leads to the question of who the last common ancestor of humans and modern apes was.  All we have to go on is a badly crushed skull from the Sahel River area in Chad that is purported to be somewhere around 7 mya, but is, in reality, a surface find, and some hominin-looking post-cranial remains from the Tugen Hills, in Kenya that are dated to between 5.6 and 6.1 mya.  By the time we get to Ar. ramidus, at least facultative bipedalism is in place, although there are still quite a few ape-like traits.  What is not clear from the report by Granger et al, is how the morphology compares to earlier hominins.  For example, can the traits observed in Au. prometheus be derived from Ar. ramidus?  If so, then it still represents a possible precursor and something like Ar. ramidus gave rise to both the australopithecines and the paranthropines.  The fact that the discoverers are calling it Australopithecus suggests that it shares enough derived traits with the australopithecines as a whole to be called that.  If the traits cannot be derived from Ar. ramidus, then it raises the possibility that the paranthropines and Ar. ramidus share a common ancestor.  At this point, until some systematic analyses can be done, we simply don't know. 

Of course, all of this is contingent on the dates holding up.  The article gives a pretty good run-down on how isochron dating works and indicates that the dates are consistent with what would be expected given the deposition.   I am sure that more will come out about this very shortly.  Until then...

¹Darryl E. Granger, Ryan J. Gibbon, Kathleen Kuman, Ronald J. Clarke, Laurent Bruxelles, Marc W. Caffee. New cosmogenic burial ages for Sterkfontein Member 2 Australopithecus and Member 5 Oldowan. Nature, 2015