According to this book review by New Scientist of The Brain and the Meaning of Life (Paul Thagard):
For instance, animals that lack social bonds are unhealthier than those who enjoy close relationships, so pursuing goals in love makes life meaningful because it satisfies vital biological needs. By connecting moral questions to objective facts about the universal structure of brains, Thagard hopes to pave the way for a morality that is rooted in science rather than religion.
[Link: "For morality and theology, read biology"]
So the only rational basis for morality is “vital biological needs”. I think a big problem with this (apart from the fact that it assumes the non-existence of a higher authority) is that it assumes that the ends justifies the means. So if I need to lie, steal, cheat or murder, that’s ok (i.e. morally good), as long as my purpose is to preserve or promote the health and safety of myself or my offspring.