New evidence that Lucy, our most famous ancestor, had superstrong arms – Washington Post

 In Science

A model of Lucy, one of the most famous human ancestors. (Michael Stravato/Associated Press)

In Ethiopia, she is known as “Dinkinesh” — Amharic for “you are marvelous.” It’s an apt name for one of the most complete ancient hominid skeletons ever found, an assemblage of fossilized bones that has given scientists unprecedented insight into the history of humanity.

You probably know her as Lucy.

Discovered in 1974, wedged into a gully in Ethiopia’s Awash Valley, the delicate, diminutive skeleton is both uncannily familiar and alluringly strange. In some ways, the 3.2-million-year-old Australopithecus was a lot like us; her hips, feet and long legs were clearly made for walking. But she also had long arms and dexterous curved fingers, much like modern apes that still swing from the trees.

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.”

The fossils that make up Lucy’s skeleton. (John Kappelman/University of Texas at Austin)

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.

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