Climatic influences on the evolution of early Homo?

The nature of the human fossil record is less than ideal for the generation of precise correlations between environmental variables and patterns of evolution in specific lineages. Nonetheless, a critical look at what can and cannot be said from individual fossil morphology and the correlation of specific environmental proxies with specific hominin fossils may lead to a greater understanding of the degree of certainty with which we should embrace environmental hypotheses for the evolution of Homo. Climate shifts have been implicated in both the origin of the genus and its dispersal from Africa. Here, I consider three areas in which a climatic influence has been posited to explain evolutionary shifts in the genus Homo: the origin and dispersal of the genus from Africa; geography, climate and body size in early Homo, and the influence of climate-induced sea level rise on morphological isolation in H. erectus. Each of the data sets is far from ideal, and interpretations of each of the data sets are fraught with issues of equifinality. Of the three hypotheses discussed, the clearest link is seen between latitudinal variation (and presumably temperature) and body size in H. erectus. Similarly, climate-induced sea level change seems a reasonable isolating mechanism to explain the pattern of cranial variation in later Asian H. erectus, but the distribution could also reflect incompletely sampled clinal variation. Alternatively, only equivocal support is found for the influence of climate on the differentiation of H. erectus from H. habilis (as proxied by body/brain size scaling), and therefore the dispersal of the genus Homo cannot be as clearly linked to changes in body size and shape as it has been in the past. These preliminary data suggest that an emphasis on understanding local adaptation before looking at global (and specific) level change is critical to elucidating the importance of climatic factors on the evolution of the genus Homo.