Lucy, that starlet among ancient human relatives, may have shared the stage with a hominin very different from herself, a newly discovered fossil suggests.

Out of the Ethiopian desert, researchers have unearthed a rare, 3.4-million-year-old partial foot that resembles those belonging to Ardipithecus ramidus, a species thought to have roamed East Africa a million years before Lucy and other members of her species, Australopithecus afarensis.

The findings, published in Thursday’s edition of the journal Nature, provide the first good evidence that another bipedal human relative was still climbing trees at the same time that Lucy and her kind had their feet planted on the ground.

Foot bones are seldom found intact because they’re usually too delicate to survive in harsh environments, said Bruce Latimer, a paleoanthropologist at Case Western Reserve University in Cleveland who worked on the study. This makes the new fossil, made up of eight bones from the front part of a right foot, a valuable find — particularly since it has several toes intact, allowing scientists to get a better sense of how the foot operated as a whole.

Lucy’s foot shares many fundamental qualities with those of modern humans. Our big toes are large and parallel with the other four, and all are able to bend and push off the ground, making Homo sapiens an excellent walker.

This new fossil, however, has more apelike features and closely resembles that of A. ramidus, whose most famous member is the specimen known as Ardi. These creatures had some adaptations for walking on the ground, but they also sported a foot with a grasping big toe that wasn’t very convenient for walking but was excellent for climbing trees. On the ground, Ardi would have shifted her weight to her outside toes, making for an awkward gait in comparison with Lucy.

And, perhaps most surprisingly, the metatarsal bone of the fourth toe was longer than in the first toe — a feature that hadn’t been observed in Ardipithecus before.

“When I first saw this, I was shocked by it,” Latimer said. Though other hominin species are known to have coexisted at other times, the discovery reveals an unexpected diversity among hominins during Lucy’s time, roughly 3 million years before anatomically modern humans first emerged. The fact that different hominin species occupied different niches — one in the trees and one on the ground — may have accelerated Lucy’s adaptation to walking rather than climbing, Latimer said.

Still, more bones are needed to get a better sense of how this creature traveled, scientists said.

“You want the rest of the foot,” said Daniel Lieberman, a paleoanthropologist at Harvard University who was not involved in the study. “We also need to do research and understand what these variations mean for performance. ... How did these animals walk?”

For now, the scientists can’t say for sure that this foot belonged to an A. ramidus because species classifications are determined by looking at skulls, not feet.

But perhaps that’s just a matter of time. The geology of the dig site where the foot was found makes it ideal for fossil hunting, said study coauthor Beverly Saylor, a Case Western Reserve geologist.

Sediment first flowed into the Afar region of Ethiopia, burying bones deep in the earth. But as tectonic forces in the region pushed the land back upward, those layers eroded away, leaving the bones under a thin layer that often just needs to be scraped away.

Sooner or later, in that relative abundance of bones, more feet — and perhaps even skulls — should crop up, the researchers said.

“You have to go back and find more fossils,” Latimer said. “It’d be beautiful to find a skull or a jaw or a knee or a hip. Any of these things will tell us more about this animal.”

amina.khan@latimes.com