Smithsonian.com

The eight bones of the new fossil foot discovered in Ethiopia. © The Cleveland Museum of Natural History

Lucy wasn’t alone. A new fossil foot unearthed in East Africa comes from an unknown hominid species that lived at the same time and in the same region as Lucy‘s species, Australopithecus afarensis. Lucy and her neighbors were both capable of walking upright on two legs, researchers say. But while Lucy spent most of her time on the ground, the newly discovered species was more adept at moving around in the trees.

“This find alters our understanding of the evolution of bipedalism because it shows that there was more diversity than previously recognized in the ways that early [hominids] moved around their environments,” says Brian Richmond, a paleoanthropologist at Smithsonian’s Human Origins Program and George Washington University.

The new discovery—eight bones from the front part of a right foot—comes from Ethiopia’s Woranso-Mille site and dates to 3.4 million years ago. This coincides with the period when Australopithecus afarensis lived in this part of Africa, about 3.0 million to 3.9 million years ago. The analysis of the bones was led by Yohannes Haile-Selassie, curator of physical anthropology at the Cleveland Museum of Natural History, and Bruce Latimer, a physical anthropologist at Case Western Reserve University in Ohio.

The researchers determined that the foot possesses features indicative of bipedal walking, such as certain joints seen in modern humans that allow the toes to push off the ground and propel the foot forward during upright walking. But the foot also appears apelike: Its opposable, grasping big toe suggests the unnamed species was a good tree climber and probably spent much less time on the ground than Lucy and later hominids, the researchers reported last week in Nature.

Haile-Selassie and his colleagues can’t give the species a name based on the scant fossil evidence. But the foot does resemble an even earlier hominid, the 4.4-million-year-old Ardipithecus ramidus, which also had an opposable big toe. Perhaps some species of Ardipithecus survived until this time.

Regardless of who the foot belonged to, it seems two types of hominids were around during this relatively early period in human evolution—and it means the evolution of bipedalism was probably more complicated than scientists suspected. For decades, the question has been what factor led the ancestors of hominids to walk upright. Now anthropologists also have to consider what factor(s) led to the origin of different styles of bipedalism.