Since our discipline is fundamentally concerned with what makes us human, defining our own genus takes on special significance for anthropologists. The genus is the next level up from species in the classification system originally devised by Carolus Linnaeus. In the 1758 publication Systema Naturae, Linnaeus assigned humans the genus name Homo, meaning “person.” Under this classification scheme, Linnaeus included several ape species, as well as wild children and mythical humans such as cave-dwelling troglodytes. In the present-day classification, the apes and monster people have long been removed, and our species, Homo sapiens, remains as its only living representative. But ever since scientists have acknowledged the existence of extinct species of humans, they have debated which of these display sufficient “humanness” to merit classification in our genus.
When grouping species into a common genus, biologists will consider criteria such as physical characteristics (morphology), evidence of recent common ancestry, and adaptive strategy (use of the environment). However, there is disagreement about which of those criteria should be prioritized, as well as how specific fossils should be interpreted in light of the criteria.
There is general agreement that species classified as Homo should share characteristics broadly similar to our species. These include the following:
- a relatively large brain size, indicating a high degree of intelligence;
- a smaller and flatter face;
- smaller jaws and teeth; and
- increased reliance on culture, particularly the use of stone tools, to exploit a greater diversity of environments (adaptive zone).
Some researchers would include larger overall body size and limb proportions (longer legs/shorter arms) in this list. There is also an apparent decline in sexual dimorphism (body-size differences between males and females). While these criteria seem relatively clear-cut, evaluating them in the fossil record has proved more difficult, particularly for the earliest members of the genus. There are several reasons for this. First, many fossil specimens dating to this time period are incomplete and poorly preserved, making them difficult to evaluate. Second, early Homo fossils appear quite variable in brain size, facial features, and teeth and body size, and there is not yet consensus about how to best make sense of this diversity. Finally, there is growing evidence that the evolution of the genus Homo proceeded in a mosaic pattern: in other words, these characteristics did not appear all at once in a single species; rather, they were patchily distributed in different species from different regions and time periods. Consequently, different researchers have come up with conflicting classification schemes depending on which criteria they think are most important.
In this chapter, we will take several pathways toward examining the origin and evolution of the genus Homo. First, we will explore the environmental conditions of the Pleistocene epoch in which the genus Homo evolved. Next we will examine the fossil evidence for the two principal species traditionally identified as early Homo: Homo habilis and Homo erectus. Then we will use data from fossils and archaeological sites to reconstruct the behavior of early members of Homo, including tool manufacture, subsistence practices, migratory patterns, and social structure. Finally, we will consider these together in an attempt to characterize the key adaptive strategies of early Homo and how they put our early ancestors on the trajectory that led to our own species, Homo sapiens.
