Researchers announced in Nature today the discovery of a 3.4 million-year-old foot that doesn’t “toe the hominin 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 later hominins. 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 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 bipedality, specifically 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 principal 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. Either way, 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 cercopithecine 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.

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 our past that only the fossil record can illuminate (the recent Ardipithecus ramidus, Australopithecus sediba and the Woranso-Mille A. afarensis skeletons highlight this as well). The authors say the Burtele fossils demonstrate a second kind of bipedalism in a hominin 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 have a fossil ancestor  or relative of the other apes. I’ve long been suspicious that nearly every single ape/human-like fossil found in Africa younger than 7 million years is attributed to the hominin line, so I’d be very pleased if this turned out to be a non-hominin ape (though again I don’t necessarily think that’s what the Burtele fossils are).

The paper itself:
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