Lucy, it turns out, had company — another prehuman that also walked but spent more of its time in trees.
Until now, there was no proof of another human relative living around the same time as the species made famous by the Lucy skeleton. But a fossil discovery reveals there was another creature around 3 million years ago and it gives new insight into the evolution of a key human trait — walking on two legs.
The creature came to light when an international team of researchers unearthed a partial foot in eastern Africa. Like Lucy, it walked upright, but had a grasping foot that it used to climb tree branches. Scientists said it's now clear that various human relatives experimented with upright walking.
"This is just another window into solving the problem of how we got from a primitive foot to the modern human foot," said Bruce Latimer of Cleveland's Case Western Reserve University, who helped discover the fossil remains.
Various hominin species have co-existed throughout human evolutionary history, but this is the first sign of another during Lucy's time.
So what was this tree-climbing and ground-dwelling creature? Scientists don't yet know because no skull or teeth have been recovered to make a determination. But it's clear the foot did not come from Lucy's species, Australopithecus afarensis.
It's rare to find prehuman feet because bones are fragile and don't preserve well. So American and Ethiopian scientists led by the Cleveland Museum of Natural History were excited when they excavated eight foot and toe bones in 2009 in the remote Afar region of Ethiopia, 30 miles north of where Lucy was discovered in 1974.
By analyzing the bone structure and dating the surrounding dirt, the team concluded the fragments came from the right forefoot of a human relative that lived 3.4 million years ago. While Lucy had humanlike feet, this creature was less advanced.
The discovery was detailed in Thursday's issue of the journal Nature. The authors did not name the new species because they know so little about it.
"This find is the first good evidence that there was a second, different species lineage" at that time, said Tim White, director of the Human Evolution Research Center at the University of California, Berkeley, who had no role in the discovery.
The ability to walk upright is a key feature that separates humans from other great apes. That a different human relative ambled around the same period as Lucy suggests upright walking evolved more than once, scientists said.
Bipedalism "was a complicated affair and not just a 'one-off' occurrence," William Harcourt-Smith of the American Museum of Natural History in New York, who was not part of the study, said in an email.
Over the past millions of years, the human foot has changed to give us a springy step. We have a stout big toe that lines up with the other toes. We also have a stable heel and an arch that distributes our weight when we walk, run or jump.
The new specimen's foot resembled that of Ardi, short for Ardipithecus ramidus, a species that lived a million years earlier than Lucy in what is now Ethiopia. But scientists don't know whether it is a descendant or close relative. Like Ardi, its big toe is set apart from the rest of its foot, allowing it to grip tree branches, and it had no arch. There are signs in the bones and joints that it walked on two legs — at least some of the time. Instead of pushing off from the big toe like modern human, it took off from the outside of its feet.
Scientists said it's hard to glean what its stride was like without knowing the shape of its ankles, knees and hips. But it likely was not very efficient and moved around awkwardly. Without a foot arch, it also could not travel as far as Lucy.
While the 3-foot-6-inch Lucy spent some time in the forest, her vastly different feet meant that she was better adapted and more comfortable wandering around open fields than the newly discovered creature was.
Lucy discoverer Donald Johanson called the new find "one of those fascinating evolutionary experiments" that tried walking but never fully committed.
It "didn't seem to want to make up its mind whether it wants to live in the trees or on the ground," said Johanson, founding director of the Institute of Human Origins at Arizona State University.
What are the chances the two interacted? If they met, scientists said they probably did not socialize, given their different lifestyles.
"They went on with their own lives," Johanson said.