After a major cooling five to six million years ago, when changes in earth's orbit affected global climate, subtropical Africa's warm, moist environment was subject to oscillating cycles of cool, dry and warm, wet weather. Forested areas which proliferated in warm cycles, receded in cool cycles, allowing grasslands to take over until the climate shifted and woodlands returned. The cycles were relatively brief and mild, and had little effect on the evolution of animal species.(Patrerns in Prehistory)