Early researchers hypothesized that brain enlargement was the first hallmark of the hominin lineage. Beginning in the mid 1800's until the early 1900's, almost all known fossil hominins had relatively large brains. The large brain bypothesis was falsified after the discovery of early hominin fossils exhibiting ape-sized brains and bipedally-adapted morphology.
In 1924, Raymond Dart identified the fist australopith fossil, known as the Taung Child, from South Africa. This specimen belonged to the species Au. africanus and had a relatively small brain similar to the size of a modern chimpanzees. The inferior placement of the foramen magnum, Dart argued, suggested that the Taung Child was bipedal. Dar's hypothesis that bipedalism evolved before larger brains ran counter to the scientific consensus at thetime. Because of his small sample size and the fragmentary remains, debate about the timing of bipedalims and brain size continued for the next 50 years.
Everything changed in 1974 when Donald Johanson found the nearly complete fossilized skeleton of Lucy, a member of the species Au. afarensis dating to 3.2 Ma. Lucy was unique at that time because she was one of the first fossils to exhibit both small relative brain size and the highly derived features characteristic of bipedalism. As other contemporaneous and older fossils (perhaps as old at 7 Ma) are found, scientists continue to revise the bipedalism timeline. Today, the evidence undoubtedly demonstrates that bipedalism was one of the first hallmarks of the hominin lineage, and may have led to many more advances. For example, one advantage of bipedalism is that the hands are freed, which allowed for the production of more technologically advanced stone tools. In turn, the production of more complex tools may have led to a higher protein diet that affected brain size8-10,27-29.
