One of the first things that paleoanthropologists noticed about the fossil primate they dubbed "Lucy" was that she could have walked upright, on two legs. This ability, bipedalism, placed the 3.2-million-year-old human relative at the evolutionary cusp between humans and our precursors.
But a question remained: Did she, also, climb trees regularly, like the nonhuman primates? The answer would help determine just how humanlike Lucy and her species, Australopithecus afarensis, was and could even yield clues about the conditions that led to our own evolution.
Now, scientists say they have incontrovertible evidence that Lucy was up in the trees – a lot. And the clues, they say, lie in her bones.
"This study puts a sturdy nail in the coffin of the notion that our early Australopithecus ancestors no longer climbed trees as part of their normal behavior," Brian Richmond, curator of human origins at American Museum of Natural History in New York who was not involved in the new study of Lucy's bones, writes in an email to The Christian Science Monitor. "If anyone was still sitting on the fence about this debate, the fence just fell over."
Most previous evidence fueling the debate over whether Lucy was a tree climber focused on whether her skeleton allowed such motion. For example, the A. afarensis shoulder socket is structured more like that of modern nonhuman apes, who are adept tree climbers, than modern humans. And, on the other side, Lucy's foot may have had an arch, which would have made her much more well-adapted for life on the ground than in the trees – like modern humans.
But, as any parents of risk-taking children know, Homo sapiens can climb trees too.
So Christopher Ruff, a paleoanatomist at Johns Hopkins University School of Medicine in Baltimore, and colleagues at the University of Texas at Austin decided to look for evidence of tree-climbing behavior actually being performed.
The characteristics of bones aren't entirely determined by genetics, Dr. Ruff explains in a phone interview with the Monitor. Some of the strength of an animal's bone can be altered by how it is used in life. For example, he says, scientists have found that the bones in professional tennis players' racket arms tend to be stronger than those in their non-racket arm.
So Ruff and his colleagues studied the scans of the internal architecture of Lucy's arm and leg bones, and compared them to modern great apes and humans.
In modern humans, who spend very little time in the trees, the arm bones are relatively weak compared with leg bones. But in chimpanzees, who swing, snack, sleep, and seek shelter in trees, that ratio looks very different.
"We don't think that Lucy was playing tennis, so if she had strong upper limb bones, there's really just one explanation for it: She was probably using them to pull herself up," Ruff says. "And it turns out she did."
Lucy's bone strength suggested she was both frequently walking on both legs on the ground and clambering around in the trees, Ruff and his colleagues report in a paper published Wednesday in the journal PLOS ONE.
"The evidence is convincing but the conclusions are not really new," David Begun, a paleoanthropologist at the University of Toronto who was not involved in the research, writes in an email to the Monitor. "Since the 1980s, there has been a debate about Australopithecus locomotion and the degree to which it included arboreality, with most researchers including myself concluding that they were much more arboreal than Homo erectus or later Homo (but maybe not Homo habilis). It is nice to have CT scan data and cross-sectional data but it only confirms most previous analyses."
But Carol Ward, a paleoanatomist at the University of Missouri who also was not involved in the study, disagrees.
"The evidence that they present is compelling to suggest that she had bigger, stronger upper limbs than we do," she tells the Monitor in a phone interview. But, Dr. Ward says Lucy and her relatives, lacking a grasping foot, were poorly suited for adept tree-climbing.
Through natural selection "they gave up most of the things that make you good at climbing," she says, so there must have been strong selection toward bipedality and against tree-climbing.
Why the strong arms then? It could be some other behavior, Ward says, perhaps throwing things became advantageous for these bipedal primates, for example.
"We know apes climb trees, she seems more apelike, so a natural hypothesis is that that's because she climbed trees some too. It's a perfectly viable hypothesis," Ward says, "But I think we as scientists need to keep in mind that there may be other reasons that Lucy and her relatives changed when they evolved into our genus Homo."
But, Dr. Richmond points out, "living life permanently on the ground comes with serious risks because of the exposure to many large predators." So evolving to live a less apelike life outside the trees would require other means of protection.
Randall Susman, chair of the department of anatomical sciences at Stony Brook School of Medicine in New York who was not involved in the research, agrees. "If I'm going to come to the ground, I'd want to retain the possibility to get up a tree if I had to," so retaining a few tree-climbing traits while Lucy and her relatives were adapting to life on the ground would have helped them survive.
Not only that, Dr. Susman says, no nonhuman primate the size of Lucy (at three-and-a-half feet tall) that lives today lives entirely on the ground. Most nest in the trees, munch on fruit in the trees, and escape predators in the tees. "That animal would have been living in trees using every living primate model that we have today," he says.
Life in the trees is clearly very important to humans' closest relatives, the other primates, so scientists want to understand what changed in our lineage.
"At some point, living on the ground required a shift in social strategy, weaponry, and use of defenses like fire to keep our ancestors safe from predation," Richmond says. So scientists see the stage at which our ancestors abandoned any last arboreal adaptations as a key "shift towards 'becoming human'."