Final answer:
Evidence from hominin fossils with variations in brain size and tool-making capabilities, as well as genetic data, support that multiple Homo species coexisted in Pleistocene eastern Africa. The interbreeding with other populations and varying evolutionary developments among these species enforce the notion of a complex evolutionary process rather than a linear one.
Step-by-step explanation:
Paleoanthropologists suspect that more than one species of the genus Homo coexisted in early Pleistocene eastern Africa due to several reasons. Firstly, the discovery of many new fossils has shown that multiple hominin species were often alive simultaneously, rather than sequentially leading to modern Homo sapiens. This suggests multiple branches of hominin evolution, some of which may not directly contribute to our lineage. Additionally, variations in tool use, brain size, and bone structure indicate the presence of distinct hominin species with varying capabilities and adaptations.
For example, the study of early hominin fossils, such as those of Homo habilis, by paleoanthropologists like Mary and Louis Leakey, reveals a much larger brain size and more dexterous hands compared to australopithecines, suggesting species-specific evolutionary developments. Complicating the picture, the interbreeding of these hominins with other populations such as Neanderthals indicates a dynamic and interactive evolutionary process. Moreover, genetic data from molecular genetics research supports a complex tapestry of hominin evolution, showing various migrations and interactions across continents even as late as 40,000 to 15,000 years ago when human diversity declined, leaving only Homo sapiens.