Final answer:
A primary characteristic for classifying a finding in the genus Homo includes a brain size over 600 cc, along with other features such as tool use, a round skull, less protruding jaws, and adaptations for walking upright. These criteria differentiate Homo species from australopithecines, exemplified by the well-studied H. habilis, indicating the complexity in the classification of early human ancestors.
Step-by-step explanation:
A primary characteristic that allows paleoanthropologists to place a particular finding into the genus Homo is a variety of distinct traits that are associated with early human species. Important criteria other than bipedalism include a brain size greater than 600 cc, the presence of a round, globular skull, evidence of tool use, reduced prognathism with smaller jaws and mandibles, humanlike postcranial features, and feet that are fully adapted for walking upright. These features distinguish members of the genus Homo from the earlier australopithecines.
Studies on H. habilis by significant figures like Mary Leakey, Louis Leakey, and John Napier, helped to establish these criteria. H. habilis, often known as "handy man," had a brain capacity of 600 to 750 cubic centimeters and less prognathic jaws compared to the australopithecines. Stone tools found with its remains are indicative of the tool use characteristic important for the classification into Homo.
The diversity within the genus Homo has made classification challenging, but the tools and methods from paleoanthropology provide a framework for scientists to evaluate different traits and place findings accurately within the larger hominin phylogeny, which is the study of evolutionary relatedness among various groups of organisms.