Introduction

From a palaeobiological perspective seasonality offers both a positive and a negative dimension. On the positive side is the fact that hominid evolution coincides with a marked increase in seasonal variation in climatic and environmental conditions, and hence is an obvious selective pressure. Furthermore, if seasonality is taken to be the occurrence of alternating periods of rich and poor resource availability, for whatever reason, then seasonality can be seen as a factor leading to more intense selection, and hence the focus for evolutionary change. On the negative side, though, is the difficulty of obtaining seasonal information from fossils and palaeoenvironmental contexts, such that it is difficult to link general patterns of seasonality to the adaptive responses that may develop. In this chapter I shall attempt to pursue a course that elaborates some of these positive elements without glossing too superficially over the negative ones. In particular, I want to look at some of the general mechanisms and environmental conditions that may have led to evolutionary responses on the part of hominids to seasonal conditions. By necessity this will focus more on aspects of foraging behaviour than on detailed physiological evidence.

Climatic change, seasonality and hominid origins

There is a general consensus that the Hominidae (taken here to refer to the sister clade either of the other African apes generally or more specifically of Pan) have their origins in the late Miocene in Africa. Interpretations of molecular systematics in terms of a clock suggest a divergence from chimpanzees and/or gorillas at about 7.5 Ma (Holmes et al., 1989).