Before Lucy's discovery, scientists reasoned that the main trait separating humans from the apes was intelligence, anatomically represented by the cranial capacity of the skull. The prevalent hypothesis held that intelligence was a precursor to, if not the impetus for, bipedalism, and anthropologists expected to one day find the remains of a sort of big-brained chimp. Upright, tiny-brained Lucy flew in the face of that line of reasoning. Lucy's discovery forced anthropologists to reexamine the evolutionary pressures that drove bipedalism.
Numerous hypotheses attempt to explain why bipedalism arose at all.
Some anthropologists propose that standing upright helped our ancestors see over tall grasses or appear larger to predators. Some say standing upright helped our ancestors stay cool by reducing the amount of surface area exposed to direct sunlight. Other researchers argue that bipedal locomotion is less energetically costly than walking on all fours, or that running on two legs helped us escape four-legged predators. Kimbel discards the above hypotheses because they all assume hominid evolution took place on the grassy African plains. And for that, says Kimbel, there is just no evidence.