Though my better sense tells me not to say this, researchers announced in Nature today the discovery of a 3.4 million-year-old foot that doesn’t “toe the hominid line.” Dammit I regret that already. Anyway, Ethiopian paleoanthropologist Yohannes Haile-Selassie and colleagues have found the foot of a creature whose big toe was oriented away from the rest of the foot and capable of grasping, like all primates (including Ardipithecus ramidus) except hominids. See for yourself:
|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.|
To help you orient yourself, the left third of the above figure (labeled with a tiny “a”) is a top-view of the ‘articulated’ right foot of this mystery animal. To the right is an X-ray (or “roentgenogram,” if you’re so inclined) of my left foot. This is from two years ago – I’ve been running in Vibrams for about a year now, so I’d really like to see what this X-ray would look like today. And just look at my big toe, having an identity crisis and trying to get away from the rest of the 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).
From the metric analyses of the foot, a few major things stick out. First, where the Burtele foot is similar to humans, both species are also extremely similar to gorillas. The plots at right, from the paper, show the height of the first metatarsal’s (foot-thumb’s) base relative to its length (a), and relative to the base height of the second metatarsal (b). The first plot shows that, compared with chimpanzees and Old World monkeys, the foot-thumb’s base is fairly tall relative to its length. Here, the fossil is smack within the highly-overlapping human and gorilla ranges. The second plot shows that, compared with monkeys, all apes (including humans) and the fossil have tall first metatarsal bases relative to the height of the second metatarsal. Notice that the human and gorilla ranges overlap, though humans are a little higher; here the fossil is at the far end of the human range with a very tall foot-thumb base. Finally, in a principle components analysis of foot bone ratios, humans and gorillas overlap a bit, to the exclusion of chimpanzees and monkeys, and the fossil plots within the gorilla (but not human) range. What really gets me here is the remarkable similarity between humans and gorillas. Since metric analyses indicate that the gorilla-human similarities are largely confined to the aspects foot-thumb, I’d imagine the similarity is due to (1) humans’ putting greater force on our big toes because we walk on two legs, and (2) gorillas’ putting lots of force on their foot-thumbs because they are massive, massive animals. It’s not clear why, though, the Burtele foot-thumb is so similar to both of us.
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