Re: Did the Rift Valley separate the apes from the humans?

Excellent question Matt.
	The formation of the Great Rift Valley is believed by many to have had 
a significant impact on human evolution.  Climate change is a major 
mechanism of evolution.  The formation of the Rift Valley is believed to 
have caused a rain shadow producing the conditions you mentioned. For the 
hominines (human related species) in this area, one hypothesis states, food 
sources would become farther apart, neccessitating a more energy-efficient 
form of locomotion, i.e. bipedalism, one of the hallmarks of hominine 
evolution.  This hypothesis, (as well as most hypotheses) is in dispute.
	Until recently a good deal of Anthropologists believed there was an 
east/west divide separating hominines and apes.  Before 1995, the fossils 
of early hominines could be found exclusively in eastern and southern 
Africa, and so it was assumed that the dense forests of central and west 
Africa were unsuitable for hominine habitation.  In 1995, however, 
Anthropologists discovered an Australopithecus mandible in Chad, 2500 
kilometers west of the Rift Valley.  It is now believed that hominines were 
widespread throughout all of Africa.
	The earliest inarguably hominine fossils were found in Lothagam in  
northern Kenya 5.6 million years ago and in Tabarin in Central Kenya 5 mya.
Since this is east of the Great Rift Valley, it supports your hypothesis, 
however, correlation is not causation.  There was also a global climate 
change underway at this time. 
	In closing, it is possible that the Rift Valley may have been a 
significant factor in the evolution of our ancestors, but there is not 
enough conclusive evidence in the fossil record to prove this hypothesis.
	For a much more in depth discussion of human origins, check out Roger 
Lewin's "Principles of Human Evolution" text book. Much of the information 
in this answer was gleaned from its pages.