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Saturday, April 3, 2010

Lucy walked upright

Although hominids tracing all the way back to Ardipithecus ramidus (Ardi), an ancestor of ours that lived about 4.4 million years ago, were known to be bipedal, it wasn't clear exactly how those creatures actually moved about. Did early hominids stride about in an upright fashion like modern humans, or used a more shuffling, bent-legged posture like modern apes?

David Raichlen of the University of Arizona and his colleagues studied some 3.6 million year old trackways found in Laetoli, Tanzania to find out. The footprints were made by an Australopithecus afarensis, the same species as ‘Lucy’. Like Ardi, which lived almost a million years earlier, Lucy’s family members spent time in trees as well as on the ground. It would be well over a million years before humans left the trees for good.

Raichlen and his team built sand trackways in Raichlen’s motion capture lab and filmed volunteers walking either upright or in a crouched, chimp-like gait. They then compared three-dimensional models of the new footprints to those found in Laetoli. They were surprised to find that the ancient footprints matched those of humans walking in a modern upright position.

Photo credit: Randy Haas, University of Arizona

Images show aerial and lateral images:
(A) a human footprint made walking normally with an extended knee and hip gait
(B) a human footprint made walking with bent-knees and hips to mimic a chimpanzee
(C) a scan of a cast from the Laetoli fossil trackway.
Notice that the toe is much deeper than the heel in the middle print and that the Laetoli scan looks more like the human print.

Our upright manner of locomotion is known to be highly energy efficient. Apparently, it is so much so that our ancestors adopted it even while spending much of their time in trees.