Evolution strikes me as a decent explanation for our perceptions of right and wrong. It makes evolutionary sense to cooperate with your fellow man, at least, it did for humans, and ants, and bees. The organism that went on killing sprees and pissed on everyone's food wasn't terribly likely to reproduce before being killed himself. The organism that helped a brotha out was more likely, and is arguably still more likely, to have kids.
We value altruism because altruism is a mark of evolutionary fitness.
Yes, some acts of altruism prevent reproduction- for example, throwing yourself in front of a bus to save an old man. But I reckon things like that are kind of like sickle cell anemia, in that they continue to exist as accidents of evolution because of their association with marks of evolutionary fitness (resistance to malaria in the case of Sickle Cell Anemia, propensity for societal functionality in the case of altruism).
So yeah, objective morality seems kind of unlikely.