The view has been expressed by a number of geologists that the magnificent fossil-bearing deposits at Olduvai can be interpreted without reference to the possibility of Pleistocene climatic changes. This does not seem to be compatible with the observed facts. The changing sequence of deposits exposed in the gorge, when taken in conjunction with their geographic setting, seems to be incapable of satisfactory explanation without reference to major fluctuations of climate during the time that they were being formed. Even if there was no other supporting evidence elsewhere in East Africa, the Olduvai sequence suggests most strongly that there were prolonged periods when the climate was wetter than it is today, and others when it was fully as dry, or even drier.

Before summarising the evidence for climatic change in Pleistocene times in East Africa, it is necessary to define clearly what we mean by the terms ‘pluvial’ and ‘interpluvial’. These two words are used to refer to major changes of climate which were comparable to the glacials and interglacials of the Pleistocene in Europe and North America. Just as the term ‘glacial’ does not imply that it snowed all day and every day, so the word ‘pluvial’ does not mean that it rained every day. In Europe the words ‘glacial period’ imply a time during which generally colder conditions than those of the same region at the present day, persisted over a prolonged period.