How DID Lucy Walk?

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May 07, 2008

"Lucy" is the three million year old fossil discovered in the Afar region of Ethiopia in 1974 by Don Johanson. For more than three decades this remarkable find, which has told us so much about out origins, has been the subject of controversy. A forty percent complete skeleton, many of her large bones had been preserved through fossilization and from these, and in particular the femur (thigh bone) and pelvis, we know she stood upright and moved about on two legs and it was claimed she walked in the same manner as humans today: a straight legged stride, knees extended, posture upright. But not everyone agrees.

Other researchers noted some critical bones were missing or shaped slightly differently than the same bones in humans today and from this they concluded Lucy walked with bent knees and her upper body bent forward slightly at the hips.

This argument was not settled but rather prolonged by the discovery, in 1978, of a sixty foot long trail of footprints at Laetoli in Tanzania. These foot prints predated Lucy by almost half a million years and were formed in a most fortuitous way: one day nearly 3.5 million years ago a volcano in what is now Tanzania, in East Africa, was erupting and light ash was falling, like snow, over a wide area. The ash was several inches deep when two individuals (and maybe a third) walked on their two legs through the ash, leaving their footprints in the ash. Birds scampered across the ash also, as did a variety of four legged animals.

A very light rain was falling at the time, so the ash was the consistency of moist sand or mud. Foot impressions were left in the ash, which soon set up like concrete and hardened, preserving the footprints. Millions of years of sediments covered the area of the ash fall until uncovered in 1978 by a team led by Mary Leakey. The footprints of interest to us were clearly distinguishable from those of the birds and the four legged animals, and proved there were animals walking upright on two legs nearly half a million years before Lucy.

Some scientists claimed the foot prints were almost indistinguishable from those of modern humans, leaving clear impressions of heel and toe, and thus proving Lucy (and the fellow members of her species, Australopithecus afarensis) walked then as we do today. Others asserted the prints were indistinct and did not establish the characteristics of a fully extended stride, a deep heel strike and a nearly as deep (as when impressed on wet sand or moistened volcanic ash) "toe off", the impression left by the ball of the foot and toes when one, while walking, pushes off and shifts weight to the other leg.

In the meantime, older fossils (but not as complete as Lucy) have been found, with strong indications they walked upright and were not quadrupeds, knuckle walking as do our closest relatives, gorillas and chimps. But HOW Lucy walked then, fully upright and legs extended, or slightly stooped and bent at the knee, is still an open question, awaiting more evidence.