Flipping the bird at the Ten Commandments
Lady Atheist has a fascinating post up about eagle behavior. These big birds are remarkably attentive to their offspring, providing not just food but extras like twigs to play with. They form permanent mating pairs (as humans are normatively supposed to do) and work for each others' well-being as well as for that of their offspring. And they do all this without any Ten Commandments or threats of Hell-fire.
It's fairly obvious that most of these behaviors contribute to the eagles' reproductive success (that is, their likelihood of producing offspring that survive to produce offspring of their own). Even the twig-supplying behavior, for example, helps the eaglets get used to manipulating twigs, which will help them later when they're building their own nests -- playfulness in young animals comes from an instinct to fiddle with the environment and see how it works, which builds skills useful for dealing with it later. So genes which promote all these behaviors would be more likely to be passed on and become fixed in the gene pool than rival alleles which don't, because the bearers of those rival alleles would be less likely to produce offspring that survived.
This is highly relevant to where human morality really comes from, of course. Human morality is basically an instinctive inhibition or revulsion against certain kinds of behavior which, if they were widespread and accepted, would be extremely disruptive to social groups. It's almost impossible to imagine a stable society in which murder, rape, theft, etc. were considered normal and acceptable behavior and happened routinely. Over time, as human social groups grew more complex and we became more dependent on their stability, individuals born without those inhibitions (the ones most likely to be "sociopathic rapists, killers, and thieves") would have been poor survivors, likely to be killed by neighbors who felt threatened by their behavior. That is, genetic profiles which omitted such inhibitions would have tended to get weeded out of the population.
Religionists like to claim that morality is rooted in religious taboos and that humans would not be moral without those taboos. Objective data show that this is nonsense -- there are basic moral inhibitions that exist in all known human societies, regardless of religion (though in primitive societies they sometimes apply only to behavior toward members of the in-group, not toward outsiders), and atheists are no less moral than religious people -- likely more so, in fact, given the statistical under-representation of atheists in prison. If anything, religious pseudo-morality has largely served to legitimize behavior which normally moral people would find too revolting to engage in -- shunning of gay family members, mass killing of people who believe the "wrong" religion, etc. Even most modern religious people are repulsed by the more disgusting parts of the Bible such as the sacrifice of Abraham or the offering of Lot's daughters, rejecting them as moral guidelines on one grounds or another -- showing that they, too, have a higher innate basis of morality which they use to judge the Bible, and which therefore does not come from the Bible.
Behavior in many animal species resembles human compassionate or moral behavior because it's the product of the same evolutionary forces. Where it differs, most likely the selective pressures are different. Eagles, for example, probably have little or no instinct for compassionate or "moral" behavior toward other eagles other than their own mates or offspring; they don't depend on tight-knit social groups like many other species (such as humans) do, so there are no selective pressures operating to generate rules for behavior toward unrelated but socially-connected individuals.
So with these selective pressures working for millions of years, why aren't we all perfectly moral? For the same reason we don't have perfect eyesight or perfect eye-hand coordination, despite relentless selective pressure to refine those things. Natural selection can work only on the genetic variability available, which in turn is limited by the range of random mutation which happens. And just as occasionally people are born with a missing body part or color-blindness, so some rare defective individuals are apparently born without moral inhibitions -- sociopaths, people who sometimes go on to become serial killers or other destructive types. Evolution, unlike what one would expect from the religionists' imaginary divine creator, produces imperfect results.
[Image at top found via Lady Atheist -- how could I resist?]