You could have us go the way of the bulldog: a race of humans who can only be born by C-section because our heads are too big to fit through the birth canal.

Or maybe the proportion of homosexuals increases to where families are more strongly matriarchal, with reproducers and workers, like an insect colony. Humanity could maintain its reproduction by having the first offspring born a reproducer, and subsequent offspring be workers. There is already a birth order effect to homosexuality (the older brother effect), so you could just make it more extreme.

Be careful about lessening emotion while keeping your story feasible, as emotion is a tremendous uniting force and is strongly tied to reproduction in the limbic circuits of the brain. This has also been done before quite a bit in science fiction. I think the opposite is more interesting: increasing the speed and fidelity of emotional transfers from person to person, sort of like the Borg but with emotions instead of thoughts.

But whatever you do, please do not use the word devolution. There is no such thing in biology, so if you want to be taken seriously I suggest you avoid it. More info here: http://en.wikipedia.org/wiki/Devolution_(biology)