Certainly not an original point, but this is the conclusion I've come to from my readings in philosophy and science (tiny bit of psychology too). Would be interested to hear any counterarguments:
In human evolution, we have developed certain ingrained traits that help us survive as a species. For example, when we see a child in pain, we are evolved to to help. Many other facets of our social life seem to be based on these primitive instincts: for example, we can see that taller men, who would be stronger and more able to fight in primitive societies, are given social advantages (Sad News For the Vertically Challenged: Tall Men Are Happier And Richer, Study Says | Discoblog | Discover Magazine).
Much of this social organisation can be described by the theories of Eusociality and Kin Selection (Eusociality - Wikipedia, the free encyclopedia), basically saying that we co-operate in groups, with the weakest/genetically inferior members of humanity helping superior specimins to breed at the expense of their own chance at breeding. Also see the mechanistic 'sexy sons hypothesis' (Sexy son hypothesis - Wikipedia, the free encyclopedia) which posits that females are attracted to certain qualities in males, which causes a cycle whereby those with most 'attractive' features pass on their genes, ad infinitum.
(as an aside - it's not uncommon in British society to hear people bitterly complain that those with 'real jobs' are paid pittance compared to beautiful 'celebrities'. Eusociality would perfectly account for this seeming injustice)
Now, when we are under the influence of these instincts, we have a tendency to make moral judgments. Perhaps the masked terrorist who kills a hostage is an 'evil' man. Perhaps the banker who rips you off is a 'bloody bastard'. What science tells us is these emotions are caused by chemical and hormonal responses in the body, which have evolved over time to maintain human social structure, ensuring the genes do not die out.
Socially, it is taboo to refer to obvious facts, for example that certain people are less sexually desirable than others. There's quite a good parody of this phenomenon available here: http://www.youtube.com/watch?v=Q2Eqk2Uppe4:
To conclude:
In the wake of modern science, where so much of our behaviour and moral decision-making has been linked to genetics, is there any place for inherent moral judgements? If not, then surely this is the definition of nihilism.