LOS ANGELES - Lucy, that starlet among ancient human relatives, may have shared the stage with a hominin very different from herself, a newly discovered fossil suggests.
Researchers in the Ethiopian desert 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 today'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, giving scientists a better sense of how the whole foot worked.
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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.
But this new fossil 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, 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.
Perhaps most surprising, the metatarsal bone of the fourth toe was longer than in the first toe - a feature that hadn't been observed in Ardipithecus before.
Though other hominin species are known to have coexisted at other times, the discovery reveals an unexpected diversity among hominins during Lucy's time, 3 million years before anatomically modern humans 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.
