What was the crucial change that set us on the path to becoming human? Our big brains, with their capacity for language and making complex tools, set us apart from other animals. Our ability to walk upright, freeing our hands for other purposes, also distinguishes us from other apes. In Darwin's day, and for many decades afterwards, too few fossils had been found to solve the mystery of which came first. To many people, it seemed logical that our earliest ancestors must have been smarter than the average ape.
The first sign that there might be a different road to humanness came in the 1920s, when Raymond Dart described the fossil skull known as the Taung child. The angle at which the child's spine had joined its skull suggested to Dart that it had walked upright, though its brain was not much bigger than that of a chimpanzee.
The question was not fully resolved for many years. In 1973, when Don Johanson found a surprisingly human-looking fossil knee at Hadar in Ethiopia that tuned out to be more than 3 million years old, it was the one of the most compelling pieces of evidence yet found that our ancestors first stood up, and did not get smart until much later. The knee was much older than the earliest known stone tools.
The following year Johanson's team discovered another fossil that is still a landmark in the story of human origins. "Lucy" -- named for the Beatles song "Lucy in the Sky with Diamonds," which was played in the camp when the scientists celebrated their discovery -- was the most complete hominid skeleton that had been found up to that time. Although Lucy's skull was incomplete, enough of it remained to show that she had a small, apelike brain, and other skulls of her species found at the same site confirmed it.
Some experts argue that Lucy was in some ways more adapted to walking upright than a modern human, whose pelvis has to be a compromise between bipedal locomotion and the ability to give birth to large brained babies. Others point out that her arms were longer than a modern human's, and the bones of her fingers were curved -- features seen in tree-dwelling primates. How much she used her climbing abilities, or whether they were simply evolutionary leftovers from arboreal ancestors, is a matter of debate.
Although her hip and knee joints were less specialized for an upright posture than our own, Lucy was clearly capable of walking bipedally, although running like a modern human was probably beyond her. Her funnel shaped ribcage and broad pelvis indicate that she probably had a rather large belly, like a modern ape, reflecting an adaptation to a relatively low-quality, high-bulk diet. The thick waist this gave her would have hindered her flexibility, and her high shoulders and the shape of her torso suggest it would have been difficult for her to swing her arms as we do we running.
Because her skeleton was so complete, Lucy gave us an unprecedented picture of her kind. In 1974, Lucy showed that human ancestors were up and walking around long before the earliest stone tools were made or brains got bigger, and subsequent fossil finds of much earlier bipedal hominids have confirmed that conclusion. Bipedalism, it seems, was the first step towards becoming human.