|BRT-VP-2/73 foot bones. Look at that fat, abducted hallux! And too-long 4th metatarsal! (fig. 1 from the paper)|
|World's greatest left foot.|
This is an immensely exciting find. The fossils are from a site in Ethiopia called Burtele dating to around 3.4 million years old. This is 1 million years after Ardipithecus ramidus from Aramis (also in Ethiopia), and contemporaneous with Australopithecus afarensis (also Ethiopian, viz. sites like Maka, Dikika and the earlier parts of the Hadar formation). With its divergent, grasping big toe, we can be pretty sure this foot did not belong to Au. afarensis, the maker of the famous Laetoli Footprints which are a few hundred thousand years older than the Burtele foot. Other aspects of the foot, however, like the round, "domed" heads of the metatarsals and the upward-angling of the proximal toe-bones do suggest this thing may have been bipedal in light of its grasping big toe (or shall we say, "foot-thumb"). Now, this upward canting of proximal toe bones' proximal ends is associated with bipedalism, but what it most basically reflects is hyper-dorsiflexing (or hyperextension) of the toes - this movement doesn't necessarily have to come solely during bipedalism, and we have some baboon proximal toe bones in our lab that have slight angling (admittedly, though, not as strongly as in humans).
Another interesting thing revealed by Haile-Selassie et al.'s analyses is that Burtele's fourth metatarsal is extremely long, unlike African apes (including humans), but more similar to Old World monkeys and the 20 million-year-old early ape Proconsul. The authors take this to suggest that a long 4th metatarsal is the primitive condition for apes, which is quite reasonable. But another question you could raise is, why can't this mean that Burtele is a giant monkey and not an ape or hominid at all? After all, some hand bones that turned out to belong to a giant colobus monkey were initially thought to belong to the type specimen of Homo habilis (OH 7). I'm certainly not saying this is what I think about the fossil, and it's very possible that this question is quashed somewhere in the paper's 35-page online supplement. Nevertheless, you'll notice that throughout this post, I've refrained from referring to BRT-VP-2/73 as an ape, a hominid, or a monkey. In the absence of other parts of the skeleton I don't think we can be too sure what we have here.
And so what I think is so exciting and important about the Burtele fossils is that they further demonstrate that we have a ton to learn about human (and other apes') evolution via the fossil record (not that the recent Ardipithecus ramidus, Australopithecus sediba and the Woranso-Mille A. afarensis skeletons haven't told us this, too). The authors say the Burtele fossils demonstrate a second kind of bipedalism in a hominid lineage separate from the contemporaneous A. afarensis. But since we have no idea what the rest of this animal looked like, it raises the intriguing possibility that we may finally (F*ING FINALLY!) have a fossil ancestor to a living African ape. I've long been suspicious that nearly every single ape-like (including humans) fossil found in Africa younger than 7 million years is attributed to the hominid line. I'd be very pleased if this turned out to be a non-hominid ape. (though again I don't necessarily think that's what the Burtele fossils are)
Put this in your pipe and read it. Then smoke it.
Haile-Selassie, Y., Saylor, B., Deino, A., Levin, N., Alene, M., & Latimer, B. (2012). A new hominin foot from Ethiopia shows multiple Pliocene bipedal adaptations Nature, 483 (7391), 565-569 DOI: 10.1038/nature10922