Final answer:
Bipedalism evolved long before the enlargement of the brain in human evolution. The increase in brain size occurred rapidly later on, between 800,000 and 100,000 years ago, largely influenced by environmental changes and leading to the development of Homo sapiens.
Step-by-step explanation:
The trait that evolved long before other human-like traits such as big brains was bipedalism, which refers to walking on two legs. This adaptation allowed our ancestors to move more efficiently across the landscape and likely made it easier to spot potential predators. It occurred in the genus Australopithecus, which lived between 2.5 and 4 million years ago and had brains about one-third the size of modern humans. The significant increase in brain size, from about 600 cm³ to about 1400 cm³, took place relatively rapidly between 800,000 and 100,000 years ago, during the evolution of Homo sapiens. This period of brain expansion correlated with rapid climate change and the development of culture and technology.
Among the first species in the genus Homo, Homo habilis exhibited a larger brain size than its australopithecine predecessors, with a brain volume around 661-700 cc and dexterous hands capable of making tools. As the environment became more unpredictable, bigger and more complex brains helped our ancestors survive. The evolutionary push for a larger brain also touched on the need for more advanced problem-solving skills and the ability to create more sophisticated tools for better hunting and survival tactics.