Evolution of early Homo: An integrated biological perspective

Background Until recently, the evolution of the genus Homo has been interpreted in the context of the onset of African aridity and the expansion of open grasslands. Homo erectus was considered to be a bona fide member of the genus Homo, but opinions diverged on the generic status of earlier, more fragmentary fossils traditionally attributed to Homo habilis and Homo rudolfensis. Arguments about generic status of these taxa rested on inferred similarities and differences in adaptive plateau. However, there was near-universal agreement that the open-country suite of features inferred for Homo erectus had evolved together and provided the adaptations for dispersal beyond Africa. These features foreshadowed those of more recent Homo sapiens and included large, linear bodies, elongated legs, large brain sizes, reduced sexual dimorphism, increased carnivory, and unique life history traits (e.g., extended ontogeny and longevity) as well as toolmaking and increased social cooperation. Hominin evolution from 3.0 to 1.5 Ma. (Species) Currently known species temporal ranges for Pa, Paranthropus aethiopicus; Pb, P. boisei; Pr, P. robustus; A afr, Australopithecus africanus; Ag, A. garhi; As, A. sediba; H sp. , early Homo >2.1 million years ago (Ma); 1470 group and 1813 group representing a new interpretation of the traditionally recognized H. habilis and H. rudolfensis; and He, H. erectus. He (D) indicates H. erectus from Dmanisi. (Behavior) Icons indicate from the bottom the first appearance of stone tools (the Oldowan technology) at ~2.6 Ma, the dispersal of Homo to Eurasia at ~1.85 Ma, and the appearance of the Acheulean technology at ~1.76 Ma. The number of contemporaneous hominin taxa during this period reflects different strategies of adaptation to habitat variability. The cultural milestones do not correlate with the known first appearances of any of the currently recognized Homo taxa. Advances Over the past decade, new fossil discoveries and new lines of interpretation have substantially altered this interpretation. New environmental data sets suggest that Homo evolved against a background of long periods of habitat unpredictability that were superimposed on the underlying aridity trend. New fossils support the presence of multiple groups of early Homo that overlap in body, brain, and tooth size and challenge the traditional interpretation of H. habilis and H. rudolfensis as representing small and large morphs, respectively. Because of a fragmentary and distorted type specimen for H. habilis two informal morphs are proposed, the 1813 group and the 1470 group, that are distinguished on the basis of facial anatomy but do not contain the same constituent fossils as the more formally designated species of early Homo. Furthermore, traits once thought to define early Homo, particularly H. erectus, did not arise as a single package. Some features once considered characteristic of Homo are found in Australopithecus (e.g., long hind limbs), whereas others do not occur until much later in time (e.g., narrow pelves and extended ontogeny). When integrated with our understanding of the biology of living humans and other mammals, the fossil and archaeological record of early Homo suggests that key factors to the success and expansion of the genus rested on dietary flexibility in unpredictable environments, which, along with cooperative breeding and flexibility in development, allowed range expansion and reduced mortality risks. Outlook Although more fossils and archaeological finds will continue to enhance our understanding of the evolution of early Homo, the comparative biology of mammals (including humans) will continue to provide valuable frameworks for the interpretation of existing material. This comparative context enables us to formulate and test robust models of the relationships between energetics, life history, brain and body size, diet, mortality, and resource variability and thereby yield a deeper understanding of human evolution. Pleistocene people and environments In the past few decades, hundreds of hominin fossils have been recovered from well-dated sites in East Africa. In addition, early representatives from far outside Africa have been found in Asia and Europe. Recently, discoveries at Malapa in South Africa and at Dmanisi in western Asia have brought important new fossils and archaeological residues to light. Analyses of local stratigraphy, windblown dust, sea and lake cores, and stable isotopic analyses have improved the reconstruction of ancient environments inhabited by early humans. Antón et al. review recent evidence and arguments about the evolution of early Homo, arguing that habitat instability and fragmentation imposed an important selective force. Science, this issue p. 10.1126/science.1236828 Integration of evidence over the past decade has revised understandings about the major adaptations underlying the origin and early evolution of the genus Homo. Many features associated with Homo sapiens, including our large linear bodies, elongated hind limbs, large energy-expensive brains, reduced sexual dimorphism, increased carnivory, and unique life history traits, were once thought to have evolved near the origin of the genus in response to heightened aridity and open habitats in Africa. However, recent analyses of fossil, archaeological, and environmental data indicate that such traits did not arise as a single package. Instead, some arose substantially earlier and some later than previously thought. From ~2.5 to 1.5 million years ago, three lineages of early Homo evolved in a context of habitat instability and fragmentation on seasonal, intergenerational, and evolutionary time scales. These contexts gave a selective advantage to traits, such as dietary flexibility and larger body size, that facilitated survival in shifting environments.