It's all in the wrist: Humans lack a knuckle-walking ancestor

by Zahra Hirji
Thursday, January 5, 2012

Though counterintuitive, scientists have turned their attention away from the feet and to the wrist and forearm to better understand how humans evolved upright walking, or bipedalism. African apes are humans' closest living relatives, and because these apes knuckle-walk, some paleoanthropologists have suggested that African apes and humans share a knuckle-walking ancestor. A new study, however, reveals that lumping the locomotion of all African apes together is a mistake: Knuckle-walking may have evolved more than once in the ape lineage. Therefore, the researchers say, humans probably did not evolve from a knuckle-walker but instead from a more general tree-dweller.

All African apes — gorillas, chimpanzees and bonobos — knuckle-walk. The scientists who think that humans have a knuckle-walking heritage bolster the claim by pointing to the fact that modern and ancient humans, or hominins, such as Lucy (Australopithecus afarensis), retain several wrist and forearm features that are supposedly knuckle-walking adaptations, says Tracy Kivell, a paleoanthropologist at Duke University in Durham, N.C., and co-author of the study published in Proceedings of the National Academy of Sciences.

But not all paleoanthropologists are convinced that humans' wrists and forearms indicate knuckle-walking. Kivell and others suggest that the so-called knuckle-walking features that humans have are not related to knuckle-walking at all.

Kivell and Daniel Schmitt, also a paleoanthropologist at Duke University, looked at seven anatomical features of the wrist that have been associated with knuckle-walking in more than 200 bonobo, chimpanzee and gorilla bones. “If those bumps and ridges are so important for knuckle-walking,” Kivell says, then we should expect to find them in all apes that knuckle-walk.

Instead, they found that although chimpanzees and bonobos have all of the pre-defined knuckle-walking anatomical features, gorillas do not. In addition, the pair also studied wrist bones of non-knuckle-walking monkeys, and found that monkeys such as baboons and macaques have some of the features previously associated with knuckle-walking as well. This led Kivell and Schmitt to conclude that these features might not be related to knuckle-walking adaptations.
The researchers examined how the different African apes move. African apes, they realized, employ two different types of knuckle-walking: One is used by chimpanzees and bonobos, the other by gorillas.

The differences are probably due to differences in the animals' lifestyles, Kivell says. Chimpanzees and bonobos spend a lot of time climbing trees; as a result, they have more bent, rigid wrists, elbows and shoulders, which allow for greater joint angles. The bent limbs lower an animal’s center of mass relative to whatever it’s holding onto, thus improving the ape’s balance as it swings through the trees, Kivell and Schmitt report. It is like when humans walk on ice — we tend to bend our knees and hips because it lowers our center of gravity and makes us more stable, Kivell says.

“In contrast, gorillas are totally different and hold their arms in a column,” Kivell says. The bones of the wrist and the forearm are stacked on top of each other, similar to the column-like leg of an elephant, she adds. Gorillas are more land-based than chimpanzees and bonobos — spending more than 95 percent of their time on the ground — and require less bent limbs that further their balance on the ground.

The researchers thus deduced that if African apes have different styles of knuckle-walking, then that form of locomotion must have evolved at two independent times. If that’s the case, then it is likely humans did not have a knuckle-walking ancestor but instead evolved from a more generic arboreal ape that had not yet evolved a specialized form of walking on the ground. Kivell and Schmitt propose that knuckle-walking arose after the split between apes and the human lineage.

This “is another nail in the coffin for the knuckle-walking theory,” says Robin Crompton, a paleoanthropologist at the University of Liverpool in England. Crompton’s own work looking at the locomotion of orangutans, which live in Asia, also supports the idea that human bipedalism has arboreal roots. Orangutans do a lot of upright walking in trees, and Crompton suspects that human ancestors might have also learned how to walk while in the canopy.

But Brian Richmond, a paleoanthropologist at George Washington University in Washington, D.C., is unconvinced by Kivell and Schmitt’s argument. “Common chimpanzees and bonobos also show some differences in the development of their wrists,” Richmond says, “and no one has argued that these two species evolved knuckle-walking independently.”

Kivell acknowledges that this study is only the beginning. More work needs to be done to establish the similarities and differences between how humans and living apes function and move, Kivell says, and how that compares to the albeit sparse fossil record.

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