Is orbital forcing of the local environment the forgotten cause of human evolution?

Orbital Forcing has two major influences on African climate: first it modulates global climate and second it directly influences the local climate. A lot of work has focused on orbital forcing of global climate for example glacial-interglacial cycles. Indeed the interaction between the different orbital parameters and the climate system are implicit in key global climate transitions; such as the onset of Northern Hemisphere Glaciation (3.2-2.5 Ma), alteration of the Walker circulation (~2.0 Ma) and the Mid-Pleistocene Revolution (~1.0-0.6 Ma). Less attention has been paid to the dominant role precession (cyclicity of 23 kyrs and 19 kyrs) has on the local African climate. For example over the last 8 million years eccentricity modulated precession has varied the insolation on the 21st March at the equator from minimum of 390 to a maximum 490 W/m2, a variation of nearly 25 percent. Within the East African Rift valley the long and short rainy seasons are strongly controlled by April-March and October-November insolation, respectively. Hence without any other influences orbital variations can have a huge effect on the moisture availability of East Africa. Because orbital parameters are cyclic the rate of change derivative contains two periods of very rapid change. 60 percent of all the variation within a precessional cycle is concentrated into two 2 kyrs periods. If this is then combined with the localised threshold effect of the Rift Valley on rainfall, then large-scale localised climate variations can occur on the time-scale of an individual. We suggest that eccentricity modulated precession provides the regularly rapid large environmental changes in Eastern Africa which may have had a strong influence on human evolution. This mechanism may provide a repetitive forcing essential for brain expansion and behavourial flexibility implicit in the Variability Hypothesis.