A team of scientists has announced the discovery of a 3.4 million-year-old partial foot from the Woranso-Mille area of the Afar region of Ethiopia. The fossil foot did not belong to a member of "Lucy's" species, Australopithecus afarensis, the famous early human ancestor. The discovery of the Lucy hominin was significant because the skeleton shows evidence of small skull capacity akin to that of apes and of bipedal upright walk akin to that of humans, providing further evidence supporting the view that bipedalism preceded increase in brain size in human evolution. Research on this new specimen indicates that more than one species of early human ancestor existed between 3 and 4 million years ago with different methods of locomotion.
"Her species co-existed with close relatives who were more adept at climbing trees, like 'Ardi's' species, Ardipithecus ramidus, which lived 4.4 million years ago."
The partial foot is the first evidence for the presence of at least two pre-human species with different modes of locomotion contemporaneously living in eastern Africa around 3.4 million years ago. While the big toe of the foot in Lucy's species was aligned with the other four toes for human-like bipedal walking, the Burtele foot has an opposable big toe like the earlier Ardi.
"This discovery was quite shocking," said co-author and project co-leader Dr. Bruce Latimer of Case Western Reserve University. "These fossil elements represent bones we've never seen before. While the grasping big toe could move from side to side, there was no expansion on top of the joint that would allow for expanded range of movement required for pushing off the ground for upright walking. This individual would have likely had a somewhat awkward gait when on the ground."
The partial foot has not yet been assigned to a species due to the lack of associated skull and dental elements.
"Argon-argon radioactive dating showed the fossils were in a sandstone layer younger than about 3.46 million years old," said co- author Dr. Beverly Saylor of Case Western Reserve University. "Nearby fossils of fish, crocodiles and turtles, and physical and chemical characteristics of sediments show the environment was a mosaic of river and delta channels adjacent to an open woodland of trees and bushes," said Saylor. "This fits with the fossil, which strongly suggests a hominin adapted to living in trees, at the same time 'Lucy' was living on land."
The analysis will be published in the March 29, 2012 issue of the journal Nature.
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