StW 89
StW 89 is the designation of a recently described 2-2.6 million year old second metatarsal from the Sterkfontein formation in South Africa. As a metatarsal, it’s an important fossil since very few pre-Homo metatarsus have been recovered because they’re very small. As such, it’s a fascinating find despite the fact it’s only a single bone. It’s even more fascinating given just how strange it is.
Another hominin metatarsal has been recovered from the Sterkfontein formation and dated to the same age as StW 89. They also share many anatomical similarities, suggesting that StW 89 belongs to the same species as this other metatarsal: Australopithecus africanus. This hominin is younger than the famous Au. afarensis (of which AL 288-1 is a member) and more human like, with a larger brain and more human-esque face. As such many argue that this species is closely related to modern humans, perhaps even a direct ancestor.
How metatarsals are numbered. Purely for reference, I’m not calling you dumb. Fyi, the toe is on the left.
As an Australopithecine, Au. africanus also had extensive bipedal capabilities with a foramen magnum which permitted the skull to sit atop the head, legs shaped to be stable whilst upright and a pelvis which could support a vertical torso, amongst other things. Sure enough, StW 89 also has many attributes that indicate its owner was bipedal. The joint with the rest of the toe is domed permitting a greater range of movement. This means the owner could’ve stood on its tippy toes and “toe-off” during bipedal locomotion. There is also indication that the big toe (hallux) would’ve faced the same direction as the other toes, enabling them all to work together during walking.
All of these are traits present in humans and most of their hominin ancestors. So, case closed right? Well no, this is where things start to bizarre. Whilst there are all these traits indicating StW 89 belonged to a bipedal creature, there’s a key feature pointing in the opposite direction. The metatarsus of bipedal hominins are straight so that as much of the toe is in contact with the ground as possible. Unlike them, StW 89 is twisted which would place this metatarsal in opposition to the hallux. This is a crucial adaptation for the grasping (prehensile) big toe seen in chimps and other apes. Such an adaptation would contradict all the bipedal traits of the toe, making this a ruddy mysterious find.
StW 89 compared to a human and chimp metatarsal. Of particular note are the large articular surface (2) so the toe has a large range of movement, roughness which suggests the big toe lies in the same direction as this metatarsal (3) and the twisting (5)
This isn’t a missing link or “transitional” form, half ape/half man. This is a bone which contradicts itself. Some parts are traits adapted for bipedal locomotion, others appear more consistent with a non-bipedal ape. To try and explain how this freak toe could exist, researchers tried to find reasons why a bipedal toe would be twisted.They realised that if the arches of the foot were shaped in a certain way, the twisting would ensure that as much of the toe was in contact with the ground as possible – a bipedal adaptation.
The foot has various arches which perform a variety of roles, such as transmitting force from locomotion to where it should go. The arches in the human foot, for example, funnel the pressure to the strongest bones which can withstand them. Since other apes walk differently, they have smaller and differently shaped arches. However, if a creature had an arch shaped like an apes, but bigger (although not as big as a humans) then the metatarsal twisting would be useful for bipedalism.
A comparison of human and chimp arches, along with the hypothetical arch of StW 89. The gray boxes represent the metatarsal bases, and the darker circles the metatarsal heads.
However, this raises another problem. Such a foot is quite archaic, being more similar to Ardipithecus or the recently found Burtele foot than Au. africanus. Yet this foot is younger than all of them (StW 89 is 2 million years old whilst Ardi is >4) and contains more adaptations to bipedal locomotion than they do. As such StW 89 seems to be a “living fossil” of sorts, a member of an older species which survived (and kept evolving) for an extra few million years.
Alternative explanations include an ape which looks funny or an Au. africanus which looks even funnier. However these explanations can offer no real justification as to why they contain the contradictory features and so the living fossil hypothesis – in my view at least – is the superior explanation. However, we need more metatarsus to work out just how unique StW 89 is and thus whether an archaic species is the best candidate for it.
This bone has caused all sorts of mystery, fascination and intrigue. And it’s only a single bone!
| Desilva JM, Proctor DJ, & Zipfel B (2012). A complete second metatarsal (StW 89) from Sterkfontein Member 4, South Africa. Journal of human evolution PMID: 22762740 |
