Scientists have gained new insights into an extinct South African creature with an intriguing mix of human and apelike traits, and apparently an unusual way of walking. But they still haven't pinned down where it fits on our evolutionary family tree.
It will take more fossil discoveries to sort that out.
The human branch of the evolutionary tree called Homo is thought to have arisen from a group of species called australopithecines. The newly studied species is a member of this group, and so its similarities to humans are enticing for tackling the riddle of how Homo appeared.
It's called Australopithecus sediba, which means "southern ape, wellspring." It lived some 2 million years ago, and it both climbed in trees and walked upright. Its remains were discovered in 2008 when the 9-year-old son of a paleoanthropologist accidentally came across a bone in South Africa.
A 2011 analysis of some of A. sediba's bones showed a combination of human and more apelike traits, like a snapshot of evolution in action. That theme continues in six papers published online yesterday by the journal Science, which complete the initial examination of two partial skeletons and an isolated shinbone.
Jeremy DeSilva of Boston University, lead author of one of the papers, said the fossils reveal an unexpected "mosaic of anatomies." "I didn't think you could have this combination, that hand with that pelvis with that foot. . . . And yet, there it is," he said.
For example, the ribs show the creature's upper trunk resembled an ape's, while the lower part looked more like a human.
One study found a mix of human and apelike traits in leg bones, and concluded that A. sediba walked like no other known animal. Its heel was narrow like an ape's, which would seem to prevent walking upright, but the more humanlike knee, pelvis and hip show A. sediba did just that, DeSilva said.
People walking strike the ground with the heel first. That would be disastrous for A. sediba's narrow heel bone, so instead the creature struck the ground first with the outside of the foot, DeSilva and co-authors propose. The foot would react by rolling inward, which is called pronation.
In people, chronic pronation can cause pain in the foot, knees, hip and back, said DeSilva, who tried out the ancient creature's gait: "I've been walking around campus this way, and it hurts."
But the bones of A. sediba show features that evidently prevented those pain problems, he said.