Carol Ward1, William Kimbel, and Donald Johanson have published a paper in Science on the arch seen in a newly discovered fourth metatarsal of Australopithecus afarensis (AL 333-160). A lot of the popular press are publishing misleading headlines that this proves bipedalism in australopithecines. No, we’ve known they were bipedal — we just didn’t have a true idea as to what extent they were bipedal. So a find like this helps investigate the degree of bipedalism.
How does this tell us how bipedal A. afarensis were?
If you’ve ever had the pleasure of visiting a podiatrist, you’d know flat feet are not conducive to bipedalism. The two-way arch helps support upright walking and distribute recoil force. Other great apes like, chimps and gorillas have flatter feet than us. The authors of this paper confirmed this by comparing the fourth metatarsal of chimps, gorillas, and humans to AL 333-160.
On all their comparisons, AL 333-160 fell within range of humans. There were some occasions where there’s a lot of gray area which I’ll address later. Nonetheless, to the right you can see the best comparison, in my opinion, which the comparison of the arch of the diaphysis of the bone between the two species. You can have a look at the rest of the figures here.
The problem I am seeing here is that this metatarsal is not Lucy’s (AL 288-1). AL 333 is designated to fossils from the site where the “First Family” came from and not the same locality as AL 288. Nonetheless, they are not the same individual. Kimbel is quoted in the BBC News, saying,
“Lucy’s spine has the double curve that our own spine does,” Professor Kimbel said.
“Her hips functioned much as human hips do in providing balance to the body with each step, which in a biped of course means that you’re actually standing on only one leg at a time during striding.
“The knees likewise in Lucy’s species are drawn underneath the body such that the thighbone, or femur, angles inwards to the knees from the hip-joints – as in humans.
“And now we can say that the foot, too, joins these other anatomical regions in pointing towards a fundamentally human-like form of locomotion in this ancient human ancestor.”
This is a flawed association to make; a form of what I would call confounding bias. We don’t have Lucy’s 4th metatarsal to see what it looks like and unfortunately we don’t have the rest of the this specimens skeleton to say it looked like Lucy’s. In fact, we have very little australopithecine appendicular and skeletons other than AL 288-1 (most notable are AL 129-1 and STS 14). So how can Kimbel say that the foot joins other anatomical regions when we don’t know what the other regions really looked like?
See, the n of this sample is 1. Looking at the intervals in the figures, especially Fig 3 & 4, there a a significant amount of variation in humans and chimpanzees that overlap. Chimps aren’t bipedal but we are. So imagine you are a paleoanthropologist way in the future looking at one metatarsal of a now-current then-ancient chimpanzee way and comparing it to a humans — surely you could make the same conclusion as these three have. And herein lies the big issue with sensationalism… as is the problem often in paleoanthropology, we just don’t have many comparative samples but people want definitive conclusions.
- Ward, C., Kimbel, W., & Johanson, D. (2011). Complete Fourth Metatarsal and Arches in the Foot of Australopithecus afarensis Science, 331 (6018), 750-753 DOI: 10.1126/science.1201463