The world's most famous hominin fossil has provided us with an answer to one of the big questions about our evolution: When did our ancestors come down from the trees? It seems the shift was underway (but incomplete) 3.18 million years ago, when the individual known as Lucy died.
Lucy was an Australopithecus afarensis, discovered in 1974 near Hadar, Ethiopia. She became famous as the first member of her species where we have more than a single bone, and has been the subject of intense study ever since. Despite decades of research, advances in technology are bringing to light new findings about Lucy and her species. The latest indicates that she was less of a tree dweller than chimpanzees, but not yet fully adapted to walking on the ground.
Professor Christopher Ruff of Johns Hopkins University scanned Lucy's remaining bones using X-ray microtomography, allowing for the creation of 3D models of her humerus (upper arm bone) and femur (upper leg bone) that are more accurate than anything we have had before.
Humans have much stronger leg bones than arms, reflecting the fact that we mostly get around on two legs. Chimpanzees, on the other hand, are far more evenly balanced, since they need strong arms to climb trees.
The midsection, or diaphysis, of long bones is shaped by the mechanical strains it experiences. This is not a matter of evolutionary legacy, but depends on the lifestyle of the individual whose bone is being measured. Fossil evidence from some of our more recent ancestors, such as Homo erectus, show their legs bones were within the ranges of modern humans, but similar studies have not been done on australopithecenes.
We don't have all of Lucy's bones, but the collection is unusually complete. John Kappelman/University of Texas at Austin