As I remember from the book "Power, Sex, Suicide: Mitochondria and the Meaning of Life" [1], the bottleneck may be not the evolution of the intelligence, and not even of life itself, but specifically multicellular life. There are reasons to believe it only happened once, and for about 1bn years before that life already existed in single-cellular form, which gives an idea on how improbable the step from single to multiple cell organisms is. I am vague on specifics though, but can wholeheartedly recommend a book to anyone interested in evolution and origins of life.
[1] http://www.amazon.com/Power-Sex-Suicide-Mitochondria-Meaning...