Final answer:
Big brains and advanced tool use were seen as key human characteristics by early evolutionists. Climate change and the need for adaptable intelligence are believed to have driven the rapid encephalization in human evolution. Distinctive features of early Homo species like H. habilis have informed classifications within the genus Homo.
Step-by-step explanation:
Early evolutionists postulated that big brains and advanced tool use were defining characteristics of humanity. Tool use demonstrated manual dexterity and intellectual capacity beyond that of other primates. Over the evolutionary period, especially between 800,000 and 100,000 years ago, there was a significant increase in human brain size, from about 600 cm³ to 1400 cm³ in Homo sapiens. This period also saw rapid climate change, believed by many scientists to drive encephalization—a term for the increase in brain size—due to the need to adapt to an unpredictable environment. Paralleling the biological evolution of the brain was the development of culture and technology, allowing modern humans and their ancestors to adapt to various environments globally.
Prominent paleoanthropologists like Mary Leakey, Louis Leakey, and John Napier, as well as primatologist Phillip Tobias, provided criteria for classifying species in the genus Homo, which included a brain size over 600 cc, a round, globular skull, and reduced prognathism. H. habilis, a species with a brain size of around 661-700 cc, capable hands for tool-making, and fully adapted feet for walking, was key in understanding human evolution. These physical characteristics, in combination with changes in jaw and face structure, were critical in distinguishing Homo species from earlier hominins.
The study of human evolution continues to evolve as new discoveries challenge previous understandings, like that of H. floresiensis, a small-brained species that was still capable of using tools and controlling fire, suggesting that encephalization, while important, is not the sole indicator of cognitive complexity.