Now it seems that Lucy shared eastern Africa with another prehuman species, one that may have spent more time in trees than on the ground.

A 3.4-million-year-old fossil foot found in Ethiopia appears to settle the long-disputed question of whether there was only a single line of hominins — species more closely related to humans than to chimpanzees — between 4 million and 3 million years ago. The fossil record for that period had been virtually limited to the species Australopithecus afarensis, made famous by the 3.2-million-year-old Lucy skeleton.

Of perhaps more importance, scientists report in the journal Nature, published today, the newfound foot not only belonged to a different species but had evolved a distinctive mode of locomotion, which scientists described as “equivocal.” It clung to the trees and never adapted to terrestrial mobility outright.

The Lucy species had long before evolved almost humanlike upright walking, bipedality, as attested by the Laetoli footprints in Tanzania from as early as 3.7 million years ago. This other species was still built for climbing trees and grasping limbs. It was capable of walking, though less efficiently and probably at an awkward gait.

At a pivotal period in prehuman evolution, the discoverers concluded, two lines of hominins practiced contrasting locomotion behavior. Their feet, mostly, told the tale: The divergent, opposable big toe, long digits and other bones of the newfound species did not match the feet of afarensis. Lucy’s foot had a strong arch and the big toe was lined up with the other four digits, much like the feet of modern humans and all critical for effective bipedality, while retaining some agility for climbing trees.

Yohannes Haile-Selassie, a paleoanthropologist at the Cleveland Museum of Natural History in Ohio, and his colleagues said the species the foot belonged to remains undetermined, for lack of any cranial or dental remains associated with the specimen. But they said the foot was strikingly similar to the earlier hominin Ardipithecus ramidus, nicknamed Ardi, which lived 4.4 million years ago, also in what is now Ethiopia.

Ardi’s foot also had a divergent big toe, similar to those of apes and gorillas, for tree climbing, although Ardi was an occasional upright walker.

Daniel Lieberman, a human evolutionary biologist at Harvard who was not involved in the research, wrote in a commentary for the journal that the hominin foot “is a valuable addition to the fossil record as it extends the existence of Ardipithecus-like feet by a million years.”

This and other recent discoveries, Lieberman said, indicate “that there was more diversity in hominin locomotion than we had previously thought, and not all of it took place on the ground.”

Donald Johanson, the discoverer of the original afarensis specimen Lucy, admired this new member of the rarefied fossil kingdom.

“It’s a lovely little foot to have,” he said, agreeing that its similarity to the Ardipithecus mode of locomotion suggested the existence of “two parallel lineages in this long time period.”

Johanson, the founding director of the Institute of Human Origins at Arizona State University, discovered the Lucy skeleton in 1974, only 30 miles from the site of this latest find. In February 2009, at a place in the central Afar region known as Burtele, a member of Haile-Selassie’s team, Stephanie Melillo, spotted the first bone fragment eroding out of sandstone.

One author of the report, Bruce M. Latimer of Case Western Reserve, said the findings clearly showed that the adaptation to bipedality, although considered one of the decisive transitions in early human evolution, was not a single, isolated event. One group, the Lucy species, relinquished the arboreal habitat and became functionally committed, long-distance walkers. For reasons unknown, another group, represented by the Burtele foot, maintained a climbing foot and stayed at least part time in the trees.

In hindsight, Latimer said, “it is apparent which group succeeded.” Homo erectus appears to have been the first to walk on a fully modern foot.