Humans descended from apes that lived in trees, but researchers have been battling over whether the earliest humans remained agile climbers when they started walking upright on the ground. A new study concludes that early humans may not have been very good at tree-climbing. If correct, the results suggest that our ancestors traded in their arboreal adaptations to become fully human.
Researchers estimate that the chimpanzee and human lineages split about 5 million to 7 million years ago. Yet experts are divided about what happened next to the first hominins, as members of the human subfamily are called. One group argues--based on evidence that early members of the human line lived in woodland environments and had curved fingers and toes that were good for climbing--that the first hominins spent some time in the trees even as they adapted to their new ground-dwelling lifestyle. The other side counters that our distant ancestors' arms, legs, and feet were more humanlike than apelike and poorly adapted for climbing. At issue is whether bipedalism evolved rapidly or gradually and whether early hominins were adapted to life on the ground and in the trees.
To try to get a leg up on the issue, Jeremy DeSilva, an anthropologist at Worcester State College in Massachusetts, filmed wild chimpanzees as they scaled trees in Uganda's Kibale National Park. He found that when pushing off from a tree branch, chimps flex their ankles (raise their foot) about 45 degrees from the normal resting position. Modern humans, on the other hand, flex their ankles a maximum of 15 to 20 degrees when walking and suffer serious injury if their ankles are bent much further.
DeSilva then compared two primary bones of the ankle joint--the tibia in the lower leg and the talus in the foot--in great apes and fossil hominins ranging from 4.12 million to 1.53 million years old. DeSilva found that all of the hominin ankle joints resembled those of modern humans rather than those of apes, suggesting that this joint took on its current configuration early in human evolution. In a report published online this week in the Proceedings of the National Academy of Sciences, DeSilva concludes that if early hominins were engaging in any substantial amount of tree-climbing, they certainly were not doing the kind of ankle-flexing that chimpanzees do today.
DeSilva doubts that early hominins could have been good at walking and climbing trees at the same time. That means full bipedalism would have evolved relatively rapidly as tree-climbing ability declined. But he concedes that his study will not end the debate. It is possible that the common ancestor of chimps and humans climbed trees without flexing its ankle as much as chimps do today, and thus early hominins might have been tree climbers, too.
Other researchers agree that the study, although important, isn't definitive. "This is a very valuable and provocative study, but there's a lot more to this story than meets the ankle," says William Jungers, an anthropologist at Stony Brook University Medical Center in New York state. For example, Jungers says, some modern humans are very good at climbing trees without flexing their ankles 45 degrees. And David Begun, a paleoanthropologist at the University of Toronto in Canada, says that even if the early hominins weren't gymnasts, some limited climbing ability "may well have been crucial for their survival, either by providing relative safety from predators or perhaps even access to food."