Ankles of the australopithecines

Recent University of Michigan Ph.D. Jeremy DeSilva gets some nice press about his work demonstrating that fossil hominins didn't climb like chimpanzees:

"Frankly, I thought I was going to find that early humans would be quite capable, but their ankle morphology was decidedly maladaptive for the kind of climbing I was seeing in chimps," DeSilva told LiveScience. "It kind of reinvented in my mind what they were doing and how they could have survived in an African savannah without the ability to go up in the trees."

This is a good example of the comparative method in paleoanthropology. We can't observe the behavior of extinct species; we can only observe the behavior of their living relatives. We can observe the anatomy of fossil specimens, but testing hypotheses about their behavior requires us to understand the relationship between anatomy and behavior in living species. We've known about the anatomy of fossil hominid ankles for a long time, but it's not so obvious how the anatomical differences between them and chimpanzee ankles relates to behavior. The paper's abstract:

Whether early hominins were adept tree climbers is unclear. Although some researchers have argued that bipedality maladapts the hominin skeleton for climbing, others have argued that early hominin fossils display an amalgamation of features consistent with both locomotor strategies. Although chimpanzees have featured prominently in these arguments, there are no published data on the kinematics of climbing in wild chimpanzees. Without these biomechanical data describing how chimpanzees actually climb trees, identifying correlates of climbing in modern ape skeletons is difficult, thereby limiting accurate interpretations of the hominin fossil record. Here, the first kinematic data on vertical climbing in wild chimpanzees are presented. These data are used to identify skeletal correlates of climbing in the ankle joint of the African apes to more accurately interpret hominin distal tibiae and tali. This study finds that chimpanzees engage in an extraordinary range of foot dorsiflexion and inversion during vertical climbing bouts. Two skeletal correlates of modern ape-like vertical climbing are identified in the ankle joint and related to positions of dorsiflexion and foot inversion. A study of the 14 distal tibiae and 15 tali identified and published as hominins from 4.12 to 1.53 million years ago finds that the ankles of early hominins were poorly adapted for modern ape-like vertical climbing bouts. This study concludes that if hominins included tree climbing as part of their locomotor repertoire, then they were performing this activity in a manner decidedly unlike modern chimpanzees.

DeSilva's conclusion is straightforward and easy to illustrate. Chimpanzees climb vertical tree trunks pretty much like a logger does. A logger slings a strap around the trunk and leans back on it. Friction from the strap holds him up as he moves his feet upward; spikes on his boots hold him while he moves the strap.

Of course, chimpanzees don't have spikes on their feet, and they don't use a strap. Instead, their arms are long enough to wrap around the trunk, and they can wedge a foot against the trunk by flexing their ankle upward -- dorsiflexing it -- or grip the trunk by bending the ankle sideways -- inverting the foot -- around it. The paper includes a photo that shows the chimpanzee style of climbing clearly:

Chimpanzee climbing a tree

Photo of chimpanzee climbing a tree, from DeSilva (2008)

You might wonder, yeah so what? Isn't it obvious that chimpanzees climb this way?

Well, it wasn't so obvious which features of the ankle might adapt chimpanzees to this style of climbing. By watching the chimpanzees (and other apes) DeSilva was able to determine the average amount (and range) of dorsiflexion and inversion of the feet while climbing, and could also assess the extent to which dorsiflexion is accomplished at the ankle joint (as opposed to the midfoot). In this case, the observations were pretty obvious -- chimpanzees were habitually flexing their ankles in ways that would damage a human ankle. Then, by examining the bony limits on human ankle flexibility, DeSilva showed that fossil hominins shared the same constraints on ankle movement as recent people. They couldn't have climbed like chimpanzees.

Human climbing

I would say that the ankle-joint observations match the rest of the skeleton. It seems pretty obvious that Australopithecus afarensis and later hominids couldn't possibly have climbed in the chimpanzee-like manner described in DeSilva's paper, because the hominins' arms were too short. If a logger tried to climb with his arms instead of a strap, even spikes on his feet would be relatively ineffective holding him up. Dorsiflexion would be hopeless -- the normal component of force against the tree trunk would be insufficient to prevent slipping.

Humans who aren't loggers use a different strategy to climb vertical tree trunks -- they put a large fraction of the surface area of their legs directly in contact with the trunk. Wrapping legs around and pressing them together gives the necessary friction to hold the body up.

If you're like me, you'll remember this climbing strategy ruefully from gym class, where "rope climbing" is the lowest common denominator of fitness tests. The sad fact is that many otherwise-normal humans fall on the wrong side of the line between mass and muscle power. Straining my groin muscles to the max, I still could never pull my way up a rope.

There's nothing magical about getting a human to climb. Ladders, after all, are relatively easy for the large fraction of the population who can't climb a rope or tree trunk. The trick with a ladder is that friction is organized in a more effective way for our ankle mechanics and arm length. But you don't need to schlep a ladder, if you can manage a little extra arm strength and a low enough body mass.

Early hominin climbing

Australopithecines were light in mass, and from what we can tell, they had strong arms. So they had what it takes for humans today to climb trees effectively -- not like chimpanzees, but like humans. Up to A. afarensis, every early hominin we know about lived in an environment that was at least partially wooded.

In his comments about the paper, DeSilva hypothesizes a trade-off between climbing ability and effective bipedality, so that early hominins could not have effectively adapted to both. I don't think a chimpanzee-like ankle would have been any use with arms as short as australopithecines'. So I don't see the necessity of a trade-off in ankle morphology. A. afarensis -- long before any evidence of stone tool manufacture -- had very non-apelike arms, hands and thumbs.

But there's one significant question that DeSilva omits discussing: StW 573. Clarke and Tobias (1995) describe the foot of StW 573 as having a big toe that is abducted (sticks out) from the foot, intermediate between the chimpanzee and human condition. They conclude:

[W]e now have the best available evidence that the earliest South African australopithecine, while bipedal, was equipped to include arboreal, climbing activities in its locomotor repertoire. Its foot has departed to only a small degree from that of the chimpanzee. It is becoming clear that Australopithecus was not an obligate terrestrial biped, but rather a facultative biped and climber (Clarke and Tobias 1995:524).

DeSilva studied the talus, not the toe. StW 573 has a talus, and although it is not in DeSilva's sample, it probably would place very close to the other hominins in his comparison. Even Clarke and Tobias described its talus as humanlike -- their argument for an intermediate form was based mostly on the toe.

But still, it's hard to believe that australopithecines would retain a chimpanzee-like big toe, if they couldn't use that big toe by inverting or dorsiflexing their foot in any significant way. By all other accounts, an abducted hallux would only impede effective bipedality. It is of no use at all for a human-like pattern of climbing. The only remaining utility would be for small-branch grasping, but small branches would seem unlikely as a support for hominin arboreality.

One possibility is that Clarke and Tobias were simply mistaken. That appears to be the explanation favored by Harcourt-Smith and Aiello (2004:412), who cited Harcourt-Smith's 2002 thesis:

Recent multivariate analyses of the Stw 573 tarsal bones (medial cuneiform, navicular and talus) using geometric morphometric techniques demonstrate that this fossil had a very ape-like talus, a navicular that was intermediate between apes and modern humans, and a human-like medial cuneiform inferring a lack of any hallux opposability (Harcourt-Smith, 2002). This finding contrasts with the findings of Clarke & Tobias (1995), but is does not change the fact that Stw 573 would still have a different combination of morphologies in the foot than does A. afarensis.

This view was also supported by McHenry and Jones (2006), who concluded that all known hominin feet appear to lack any "ape-like ability to oppose the big toe." They also point to the Laetoli footprint trails, most observers of which agree that the big toe was adducted, not abducted.

I tend to favor that explanation -- australopithecines simply didn't have a grasping foot. But they may not have shared the medial longitudinal arch, at least not in the human configuration, and without it one might doubt that their gait featured as strong a toe-off as that of later humans. Who knows?

Meanwhile I can recommend Harcourt-Smith and Aiello's review for those who want to read more about bipedality and climbing in early hominins. It's not the last word but it is a good introduction to the literature.

UPDATE (2009/04/15): A reader writes to suggest also the 1987 paper by Bruce Latimer, James Ohman and Owen Lovejoy. I recommend it for anyone who wants to dig deeper into australopithecine ankle morphology. I've added it to the bibliography below.

References:

DeSilva JM. 2009. Functional morphology of the ankle and the likelihood of climbing in early hominins. Proc Nat Acad Sci USA 106:6567-6572. doi:10.1073/pnas.0900270106

Clarke RJ, Tobias PV. 1995. Sterkfontein Member 2 foot bones of the oldest South African hominid. Science 269:521-524.

Harcourt-Smith WEH, Aiello LC. 2004. Fossils, feet and the evolution of human bipedal locomotion. J Anat 204:403-416. doi:10.1111/j.0021-8782.2004.00296.x

Latimer B, Ohman JC, Lovejoy CO. 1987. Talocrural joint in African hominoids: Implications for Australopithecus afarensis. Am J Phys Anthropol 74:155-175.

McHenry HM. Jones AL. 2006. Hallucial convergence in early hominids. J Hum Evol 50:534-539. doi:10.1016/j.jhevol.2005.12.008