Final answer:
Early evolutionary biologists suspected that bipedalism was the first significant evolutionary trait in human evolution, a belief supported by evidence from early hominids like Australopithecus and Homo habilis.
Step-by-step explanation:
Early evolutionary biologists believed that bipedalism, or the ability to walk on two legs, was likely the first character to evolve in human evolution. This belief was due to the discovery of australopithecines, which were bipedal hominids that existed before the genus Homo. Australopithecus was crucial in paving the way for future hominids, including the first Homo species, which most likely was Homo habilis. Other criteria used by anthropologists to classify a species under the genus Homo, beyond bipedalism, include an increased brain size and the capacity for tool-making, as evidenced by the dexterity of hands and the repositioning of the thumb. These changes were essential for the survival and adaptation of early humans in various environments, driven by natural selection as proposed by Charles Darwin, a mechanism that remains central to evolutionary biology today.