Bone studies show human ancestor walked on two legs

By: Sam Ohmer

Posted: 3/13/08

It's a well-known fact that bones can tell great stories: stories of crimes, stories of age, stories of travel and change.

Christopher Ruff of the Hopkins School of Medicine also uses bones to tell a story - the story of our evolutionary past. He studies the fossils of hominids, our closest human-like ancestors, especially the species Homo erectus.

Ruff is interested in the origins of bipedalism, the ability to walk on two legs, which is seen in modern humans but few other species.

Bipedalism "is one of the most important - some would say, the most important - characteristic defining modern humans and their ancestors, early hominins, back to the split from African apes. So, being able to recognize when and how that occurred in the fossil record is very important," Ruff said.

Evidence suggests that Homo erectus is an especially important hominid because it was the first to leave Africa and spread to the rest of the Old World. They looked quite similar to modern humans, though with a significantly smaller brain.

It is likely that Homo erectus was socially similar to modern humans as well, being hunter-gatherers and actually using tools such as hand axes to do their work. It has been suggested that Homo erectus could communicate in a more sophisticated manner than modern apes, even though they probably lacked the capacity for true speech.

Ruff has studied the upper leg and arm bones of two complete Homo erectus fossils to learn more about how they might have moved. Previously, skeletal studies of the two remains have included external length measurements.

Ruff has found something even more interesting by studying not just the exterior dimensions, but also bone width and strength by using radiological scans, much like a doctor might order.

Because bipedal, or two-footed, animals exert a lot of force on their legs by carrying most of their weight on them, the shape and strength of their leg bones are very different from those of quadrupeds, or four-footed animals, which distribute their weight more equally among all four legs.

This is the basic idea behind Ruff's study, in which he has examined the femur and humerus of both complete Homo erectus skeletons, one a fully grown adult and the other still an adolescent. In addition to the hominid skeletons, Ruff has also studied the bones of adult humans and adult chimpanzees as a basis of comparison.

If Homo erectus skeletal properties are more similar to chimp bone properties, the data would seem to support Homo erectus walking on two legs and two arms, much like a chimpanzee or a gorilla, while more human-like results would support the traditional upright Homo erectus model.

Ruff has found that the leg and arm bones of Homo erectus are much more similar to human bones than to the bones of chimpanzees. "This indicates that [Homo erectus] had a locomotor repertoire that was indistinguishable from that of modern humans," Ruff said.

This hypothesis has been around for quite some time, of course and has been generally accepted, but Ruff's experiment is important in that it confirms the assumption that Homo erectus walked upright with truly scientific data and not just an extrapolation from other human and hominid specimens' behaviors.

Knowing that Homo erectus could indeed walk upright allows for the idea that Homo erectus could have traveled quite long distances, which explains why it was Homo erectus and not earlier hominids that left Africa.

It has also been found that there are gender-linked skeletal differences in modern humans, a trend somewhat mimicked in Ruff's Homo erectus samples. This finding implies, tentatively, that perhaps even 1.7 million years ago, there were sexual dimorphisms, or pronounced differences in the anatomy of members of the same species across gender lines.

Ruff will continue his research into humanity's ancestors. Currently, he is under way in his investigation of "another virtually contemporaneous early hominid, this time a Homo habilis specimen, dated to 1.8 million years ago. This specimen shows a very different pattern - much more chimp-like - demonstrating that there was significant locomotor variability among even early Homo."

There is still quite a lot left to learn about our most recent ancestors: their biology, their behavior and their lives.