Final answer:
The Leakeys' critical finding about Homo habilis was its brain size and humanlike features that suggested dexterity for tool use, challenging earlier beliefs about the first toolmakers.
Step-by-step explanation:
One important clue the Leakeys found about Homo habilis, also known as Handy Man, relates to its use of stone tools and features indicating a humanlike way of life. Mary and Louis Leakey, along with their colleagues, studied various traits to classify species within the genus Homo. They noted Homo habilis had a brain size over 600 cc, less protrusive jaws, and a cranial shape more humanlike than earlier hominins. This species also had hands capable of dexterity necessary for tool use. Crucially, while Homo habilis was once thought to be the first toolmaker, later discoveries revealed that some australopithecines may have used tools earlier, broadening our understanding of early hominin behavior.