In a column in the New York Times, Jonathan Sacks, the chief rabbi of the UK's United Hebrew Congregations, makes an argument for religiosity, by claiming that humans are "moral animals" who are hardwired for empathy and altruism:
If this is so, we are in a position to understand why religion helped us survive in the past — and why we will need it in the future. It strengthens and speeds up the slow track. It reconfigures our neural pathways, turning altruism into instinct, through the rituals we perform, the texts we read and the prayers we pray. It remains the most powerful community builder the world has known. Religion binds individuals into groups through habits of altruism, creating relationships of trust strong enough to defeat destructive emotions. Far from refuting religion, the Neo-Darwinists have helped us understand why it matters.
I agree with Sacks' general premise, if not necessarily his conclusion. Altruism—the willingness to sacrifice one's individual interests for the good of the group—is almost certainly a trait that confers a broad evolutionary advantage in a context where survival of the individual is so heavily dependent on survival of the group. This is especially true when one considers that the driving force of evolution is not survival of the individual, but of the genetic code, code that is shared closely with the rest of one's tribe. If there is an altruism gene, then sacrificing oneself on behalf of one's cousins might assure that this gene lives on.
Likewise, the sheer prevalence of religion in human societies strongly suggests that this behavior is at least rooted in a trait that provided early humans an adaptive advantage. Maybe it's closely connected to altruism. Social cohesion is necessary for one group to compete against another, and religion is certainly an effective tool toward that.
That said, I don't think that Sack's conclusion—that society cannot do without religion—is as obvious as he thinks. Both altruism and religiosity can and do exist without the other. Many people act morally without religion, and many an immoral act has been committed in the name of one god or another. But mostly, I find Sacks' whole evolutionary biology argument a rather odd one for a religious leader to make, as it appears to devalue the significance of faith.
For to say that we are moral animals (and I'd agree that we are) is to admit that we are just animals nonetheless: Animals that crave sweets and sex and spirituality and whatever else has proved advantageous over millions of years of natural selection. And while that's a process that doesn't necessarily preclude a god, it certainly doesn't require one any more than an act of altruism requires a religious faith.