Lucy and the proof of bipedalism

On the morning of 24 November 1974, Donald Johanson of Case Western Reserve University and his student Tom Gray were surveying a dry gully in the Hadar fossil beds when Gray spotted a small bone fragment: the proximal end of a left femur. Looking around, they found more fragments — part of a skull, a lower jaw, ribs, vertebrae, the pelvis. Over the following three weeks, Johanson’s team recovered 52 fragments representing approximately 40% of a single skeleton — an extraordinary completeness for a hominin fossil of that age. The skeleton belonged to a female of small body size; she was named “Lucy” after the Beatles song “Lucy in the Sky with Diamonds” that was playing on a tape recorder in the expedition camp on the evening of the discovery.

Analysis of Lucy’s bones revealed the combination that made her scientifically revolutionary: the anatomy of her knee joint — the angle at which the femur meets the tibia — was identical to that of modern humans and entirely distinct from the knee anatomy of chimpanzees; her hip bones and the base of her spinal column showed adaptations to upright walking; her foot bones showed an arch and non-grasping big toe. Lucy walked upright, bipedally, on two legs, just as modern humans do. But her brain (estimated at approximately 400-500 cubic centimetres) was no larger than a chimpanzee’s, and her arm bones were long and curved, suggesting she also climbed trees. The significance was immediate and vast: the prevailing model of human evolution in 1974 held that bipedal walking and brain enlargement had co-evolved together, as a “package.” Lucy demolished that model. Bipedalism came first, by approximately 3 million years. The large brain came later.

Johanson named the species Australopithecus afarensis in 1978, recognising it as distinct from Dart’s South African Australopithecus africanus. The species is now known from dozens of specimens across East Africa, ranging in age from approximately 3.9 to 2.9 million years, and is widely accepted as a direct ancestor of the Homo lineage — the species from which humans, through a series of still-debated intermediate forms, ultimately descended. Lucy herself is housed at the National Museum of Ethiopia in Addis Ababa; a high-quality cast is on permanent display, while the original fragile bones are only occasionally exhibited. The Cleveland Museum of Natural History, where Johanson was based, holds a cast that has been displayed in numerous international exhibitions.