Did Lucy walk, climb, or both? Australopithecine ancestors—arboreal versus terrestrial habitat and locomotion

Dec 31, 2012
Men in Twa society from Uganda regularly climb trees to gather honey. Credit: Nathaniel Dominy

Dartmouth researchers investigate tree-climbing behavior of modern hunter-gatherers to elucidate our fossil ancestors' terrestrial versus arboreal preferences.

Much has been made of our ancestors "coming down out of the trees," and many researchers view terrestrial as the hallmark of "." After all, most of our living —the great apes, specifically—still spend their time in the trees. Humans are the only member of the family devoted to the ground, living terrestrial rather than arboreal lives, but that wasn't always the case.

The shows that our predecessors were arboreal habitués, that is, until Lucy arrived on the scene. About 3.5 million years ago in Africa, this new creature, Australopithecus afarensis, appeared; Lucy was the first specimen discovered. Anthropologists agree that A. afarensis was bipedal, but had Lucy and her legions totally forsaken the trees? The question is at the root of a controversy that still rages.

"Australopithecus afarensis possessed a rigid ankle and an arched, nongrasping foot," write Nathaniel Dominy and his co-authors in (PNAS). "These traits are widely interpreted as being functionally incompatible with climbing and thus definitive markers of terrestriality," says Dominy, an associate professor of anthropology at Dartmouth.

But not so fast; this interpretation may be a rush to judgment in light of new evidence brought to light by Dominy and his colleagues. They did what anthropologists do. They went out and looked at modern humans who, like Lucy, have feet adapted to terrestrial bipedalism, and found these people can still function as effective treeclimbers.

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Anthropologist Nathaniel Dominy, Dartmouth College, questions the abruptness of our ancestral transition from the trees to the ground, more than 3.5 million years ago. Credit: Dartmouth College

Co-authors Vivek Venkataraman and Thomas Kraft collaborated with Dominy on field studies in the Philippines and Africa that inform their PNAS paper. Venkataraman and Kraft are Dartmouth graduate students in the Ecology and PhD program in the Department of Biological Sciences, and are supported by National Science Foundation graduate research fellowships.

The studies in Uganda compared Twa hunter-gatherers to their agriculturalist neighbors, the Bakiga. In the Philippines, the researchers studied Agta hunter-gatherers and Manobo agriculturalists. Both the Twa and the Agta habitually climb trees in pursuit of honey, a highly nutritious component of their diets. They climb in a fashion that has been described as "walking" up small-diameter trees. The climbers apply the soles of their feet directly to the trunk and "walk" upward, with their arms and legs advancing alternately.

Among the climbers, Dominy and his team documented extreme dorsiflexion—bending the foot upward toward the shin to an extraordinary degree— beyond the range of modern "industrialized" humans. Assuming their leg bones and ankle joints were normal, "we hypothesized that a soft-tissue mechanism might enable such extreme dorsiflexion," the authors write.

They tested their hypothesis using ultrasound imaging to measure and compare the lengths of gastrocnemius muscle fibers—the large calf muscles—in all four groups—the Agta, Manobo, Twa and Bakiga. The climbing Agta and Twa were found to have significantly longer muscle fibers.

"These results suggest that habitual climbing by Twa and Agta men changes the muscle architecture associated with ankle dorsiflexion," write the scientists, demonstrating that a terrestrially adapted foot and ankle do not exclude climbing from the behavioral repertoire of human hunter- gatherers, or Lucy.

In their conclusions, the Dartmouth team highlights the value of modern humans as models for studying the anatomical correlates of behavior, both in the present and in the dim past of our fossil ancestors.

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A_Paradox
not rated yet Jan 01, 2013
That all sounds perfectly feasible. What surprised me was the chimpanzee foot he waved around; I guess I just never thought that their ankle [lower wrist] is habitually at about 90 deg.

The big question though, is whether the carrying of stick tools in the hand was the major reason for walking bipedally. I tend to think it was and that other aspects, like it takes less energy to balance on two legs was a helpful synergy rather than the main reason.
Torbjorn_Larsson_OM
not rated yet Jan 01, 2013
The habitual comparison with chimps and sometimes gorillas is not beneficial here, since they have both independently evolved knucklewalk locomotion (partly arboreal respectively all ground). Probably because of mass, as gibbons with or without arm use do a pretty similar bipedal locomotion between trees as we do. (There are interesting youtubes outside and inside walk labs.)

And both gibbons, if you take out their long arms and specialized ball wrist, and humans, if you take out their specialized feet, hips and large mass, are pretty generic monkeylike hominoids. I think we resisted specialization longer, but we got more able and interested in hunting-gathering (meat and roots) than the chimps (say) and that started it.

At first gathering roots and then get away from predators back into trees made bipedalism fitter. Tool carrying could have been later, as mobility increased and it was awkward to find or remake your digging (roots) and carving (root, carrion, fresh hunt) tools.
Torbjorn_Larsson_OM
not rated yet Jan 01, 2013
Maybe I shouldn't say we resisted specialization, as there are pretty clear signs that we socialized amiably early on, to the degree we rapidly lost large ape canines. AFAIK some have proposed that as definition for our Homo lineage.

Apparently we didn't go the bonobo route, but started to go for monogamous relationships and hidden estrous and less sexual dimorphism instead. Each to their own.

So that was perhaps our earliest specialization. Of course, loss of canines promoted fitness of later tool use! (Defensive or aggressive stones and sticks.)
grondilu
5 / 5 (1) Jan 01, 2013
less sexual dimorphism instead.


Are you sure about that? It really seems to me that sexual dimorphism is quite accentuated in our species.

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