When I told my father I was going to Cambridge to give a talk on the question of whether humans were good or bad, he looked at me sternly over his glasses. "You know what the answer is, don't you?" Total depravity and filthy rags he was hoping I would say of our nature – the first is a primary tenet of Calvinist doctrine, and the second is a phrase from Isaiah. I was about to say that we are at our root neither good nor bad, but pulled in contrary directions with the ability to make a decision. So I knew we were in for … a discussion.

From an evolutionary perspective, considering other social species on this earth, it is remarkable that a bunch of unrelated adult males can sit on a plane together for seven hours in the presence of fertile females, with everyone arriving alive and unharmed at the end of it. We could be a lot worse than we are, according to our common notions of right and wrong. We have certainly come a long way towards becoming a co-operative, sympathetic, even loving species.

Granted, this depends on your perspective: if you're a biologist, as I am, you might notice how far we've come. If you're a theologian, perhaps the more salient realisation is how far we haven't. The meeting place between these perspectives is that we are full of conflicting tendencies and inconsistencies in our attitudes and behaviour. So we would do well to ask why this conflict exists, in addition to arguing whether we've done well or poorly in it.

At several points in our evolutionary history, sources of conflict have arisen, leading to moral tension and ambivalence. Perhaps the oldest and most significant is the fact that we as individuals have gained by looking out for ourselves in competition with others, but that we also have depended on our social groups and so gained by supporting and contributing to the stability of those groups. From this ancient situation eventually arose the tug of war between selfishness and altruism that is a common aspect of our moral experience.

We should realise, however, that these often contrary tendencies both evolved in our nature through natural selection based on individual advantage. Even more importantly, though, we should realise that an evolutionary mechanism does not necessarily trickle down into our intentions and motives – caring for each other may have evolved by natural selection, but this does not rule out the possibility of genuine love and kindness.

Furthermore, we can extend our moral consideration far beyond what was beneficial to our ancestors – to humanity as a whole, even to the natural world. This leads to another important source of angst in our moral life: the difference between attitudes and behaviours that would have been advantageous for our ancestors, and those we wish to embrace and promote today. We need not wait for evolutionary adaptation to catch up with our vision of goodness, if ever it would. We can do this on our own, but it requires that familiar battle between what we feel like doing and what we know we ought to do. The former very often comes from our past, our evolutionary heritage, whereas the latter comes from whatever is most important to us.

Many of the evolutionarily savvy among us have chosen one of two roads with regard to describing our moral nature. One is the comforting notion that we are generally prosocial nice folks except for those odd meanies who must be explained as having some strange allele or bad childhood environment. The other common option is a descent into moral scepticism or nihilism where nothing matters anyway because it's all just a product of our evolution. These alternatives together look remarkably like a sour grapes attitude: either we are fundamentally good, or else forget it there's no such thing as good and bad. The main reason for Isaiah's admonition to remember how we fall short, as for most Jewish and Christian moral admonitions come to think of it, is to counteract our tendency to look at ourselves with rose-coloured glasses and become complacent. It looks like we could use a dose of my father's old time religion after all.