According to new research, hominid species appeared in Africa in response to pulses in climate:
"If you look at the new species of hominid that evolved, 80% of those, or 13 out of 15, appeared during these pulsed climate periods. It suggests new human species evolved when the climate was highly variable. We don't know if it's the wet period, the dry period or the transition that triggers this, but we can say that when the climate is highly variable, you get a big change in species."
This is something that most palaeoanthropologists have thought true for some time and drives the "forest/fringe" hypothesis of hominid origins.