Final answer:
Bipedalism evolved in hominins before the development of large brains. Australopithecus was one of the first bipedal hominids with smaller brains, while Homo erectus showed a larger brain capacity and efficient bipedalism preceding significant encephalization in later Homo species.
Step-by-step explanation:
Evolution of Bipedalism and Brain Size in Hominins
Bipedalism evolved before large brains in hominins. The first hominids, such as Australopithecus, emerged about 3.9 million years ago and were bipedal with brains about one-third the size of modern humans. Walking on two feet, or bipedalism, is a key adaptation that occurred early in human evolutionary history. Fossil evidence suggests hominins like Australopithecus were already walking upright, which is a hallmark of the genus Homo. This adaptation provided numerous advantages such as the ability to use hands for tool-making and carrying objects, improved energy efficiency, and thermoregulation. As hominins continued to evolve, their brain size increased significantly.
One of the successors to Australopithecus was Homo erectus, which had a brain three-fourths the size of modern humans and was also bipedal. This species made more advanced tools and is noted for its efficient bipedal locomotion. However, the significant increase in brain size, or encephalization, primarily occurred later in the genus Homo, with several hypotheses proposed to explain this, including the benefits of advanced cognitive abilities for socializing, problem-solving, and tool usage.