So, for decades scientists have wondered: Who exactly was Lucy? Was she lumbering and land-bound, like us modern humans? Or did she retain some of the ancient climbing abilities that made her ancestors — and our own — champions of the treetops?
A new study suggests she was a little of both: Though her lower limbs were adapted for bipedalism, she had exceptionally strong arm bones that allowed her to haul herself up branches, researchers reported Wednesday in the journal PLoS One.
“This is what makes Lucy so fascinating,” said lead author Christopher Ruff, a biological anthropologist at Johns Hopkins School of Medicine. “She had crossed a lot of thresholds on the path to becoming human, but not all of them.”
The research is the latest installment in a decades-old debate on how much time Lucy spent in trees. Just a few months ago, a team of researchers led by John Kappelman and Richard Ketcham — two co-authors on the new study — published a paper arguing that Lucy died after falling from a tree. (That conclusion proved fairly contentious, even by paleoanthropology's standards.)
It's no easy task to reconstruct the lifestyle of a hominid who's been dead for 3.2 million years. Fossils are bone turned to rock, making them impenetrable to the kinds of CT scans that were available for the first several decades after Lucy was discovered. Scientists can draw conclusions about Lucy's habits based on the shape of her skeleton — noting, for example, that her legs bent at the knee (an adaptation for bipedal walking) and her arms are much longer than those of a modern human (which would make her well suited for life in the trees).
But Ruff noted that those more chimp-like traits could be evidence of primitive retention — ancestral traits that stay in the skeleton even though they're no longer used.
“If she evolved from a more arboreal ancestor she may just not have had the time yet to evolve a shorter upper limb,” he said. “We have to look at traits that changed during her life depending on how she used that part of her skeleton — that’s real evidence of what someone was actually doing.”
Luckily, bones develop “strength characteristics” in response to heavy use; you can tell what arm a professional tennis player served with just by looking at a scan of her skeleton. And scientists at the University of Texas had advanced micro CT scanners capable of finding these characteristics in fossilized bone. So, during a 2008 tour of the U.S., Lucy made a pit stop at the UT lab in Austin to be scanned.
The UT researchers produced cross sections of Lucy's right and left humeri (upper arm bones) and her left femur (thigh bone). They then compared the relative strength of those bones to samples from a range of other primate species. Lucy's arms were not as strong as those of chimps, but they were substantially stronger than those of us puny modern humans.
“And we know she wasn't playing tennis,” Ruff quipped.
He argues that daily tree climbing is the most likely explanation for the strength of Lucy's arms: “You don’t develop strong limb bones like that unless you do it a lot,” Ruff said. Perhaps Lucy and her relatives ventured into trees at night to prevent attacks from predators as they slept.
“Hominins had slowly developed adaptations for walking on the ground, but for millions of years we were still using the trees in a significant way,” he added. “Really, it was only with evolution of Homo the genus” — about half a million years after Lucy — “that we became fully committed to the ground.”
Carol Ward, a paleoanthropologist and professor of anatomy at the University of Missouri, said the scans of Lucy's skeleton were well done and agreed that her arms were clearly strong. But she's more skeptical of Ruff's conclusions about the importance of her tree climbing.
“The important part of science is asking the right questions of our data,” she said. " . . . The kind of question that will really tell you about the forces that shaped our evolution is, what was natural selection asking hominins to do? What were the behaviors that were so important that animals that didn't do them well left fewer surviving babies and grandbabies?”
“The answer to that question for Lucy is very clearly moving well on the ground,” she concluded.
Ward noted that Lucy, like modern humans, lacks an opposable big toe that lets other apes grasp tree branches with their feet — an important tool for true climbers. And there may be alternative explanations for Lucy's upper body strength that scientists haven't considered yet.
The debate over Lucy's tree-climbing habits may seem esoteric, but it's an important one. Our ancestors' adoption of bipedalism is one of the most important developments in the history of humanity; it freed up our hands for hunting and gathering, made us long-distance travelers, and saved energy for other activities. If evolutionary biologists want to understand why this transition happened, they need to understand when it happened — and Lucy's stunningly complete skeleton is ideal for a case study.
The micro CT scans conducted in Austin offered other insights into Lucy's life history. Mechanical analyses suggest that her gait was much less efficient than that of modern humans — she wouldn't have been able to do the kind of long distance traveling that would lead her immediate successors out of Africa.
Additionally, her bones suggest she had a relatively small brain for her body size and relatively large muscles. That seems to support other research arguing that there is an evolutionary trade-off between brawn and brains; human noggins could only get bigger if we stopped expending so much energy on big muscles.
“It's kind of a nice evolutionary story there,” Ruff said.
Lucy is full of these stories. At 3.2 million years old, her species represents the rough halfway point in human evolution (we split from our last common ancestor with chimpanzees about 6 million years ago). With her arms in the past and her feet in the future, she is a creature on the cusp of becoming human. In a matter of a few hundred thousand years, Homo habilis would emerge — the first member of the genus that would someday include all of us.
“There's no topic that fascinates us more than ourselves,” Ward said. “Where did we come from? How did we get here. Why are we the way we are?”
More than 40 years after her discovery, Lucy is still helping to answer those questions.