In Matt Ridley's book it was almost as if he was saying that humans are like a race of clones. That the most 'different' humans on the earth, genetically, are far more similiar then any two members of most other species. This suggests a bottleneck., like what happened to Leapards. I hope I wasnt misunderstanding the chapter he spoke about this, because I thought it was neat. From this book on the link.
http://www.amazon.com/gp/product/0060932902/?tag=brite-21
Below is my guess on how the rapid divergence from apes happened.
The climate changed
Less trees were around.
Less trees to climb.
Proto humans became bipedal as a result.
Our diet changed to incorporate scavenging dead animals.
Being bipedal allowed us to see farther on grassy plains. Which gave us a really good advantage in scavenging the dead.
It also freed up two limbs to poke into swollen antelope bellies, and get to the bugs or brains inside, or whatever it was we ate back then.
We lost our hair as an adaptation to scavenging. Just like the vultures lost the feathers around their necks and heads. We also develop sweat glands since we are in the sun a lot more.
Changing our diet to decaying meats exposes us to a host of new micro-organisms and larger parasites.
This new soup of virus, bacteria and parasite that we are now exposed to speeds up our divergence from apes at break neck speed, since not only do these new diseases and parasites kill us, but some of them actually rewrite our code.