From Time:

Whatever started it, Gavrilets notes, humans’ transition to monogamy was much more radical than the sexual revolution of the 1960s — even though it went in the opposite direction. “Not many people realize that the most important sexual revolution for our species probably happened several million years ago,” Gavrilets says. “This revolution was accomplished by the masses of lower-ranked males, along with females, directly against the elite of alphas, so the term revolution is even more appropriate.”

So, maybe when the 2001 monolith arrived in the Olduvai Gorge, what it really did was not inspire the proto-human apes to bash each other's heads in with tools (that was already pretty obvious), but to take that special girl ape out for a nice steak.