We are being told that genetic evidence shows that the human species didn't begin with two people, but more likely with several thousand people. And enemies of Christianity hold this up as evidence that the doctrine of original sin is, as Jerry Coyne recently put it, "a metaphor."
Now I'm only a layperson in theology, so I don't know exactly how original sin is defined. I've always thought it referred to our natural inclination to rebel against God and to want to be gods, ourselves. And that is certainly more than a metaphor. It is indisputable fact. If you doubt me, try raising a two year old. Whether humanity began with two people or ten thousand, none of us naturally wants God to be in charge of our lives, and all of us want to be the sole masters of our fate.
One of the reasons that Jesus died, so we Christians believe, is to help us to put to death our natural inclination to rebel against God and to want to be gods. We try to take Jesus' life into ourselves in various ways, through baptism, communion, prayer, fellowship, and attempts at obedience and love. And through attempting to take up our cross daily and deny our natural tendencies to want to be in charge, slowly, imperceptibly, our old nature will die away and a new nature of wanting and living the way that God wants us to live will grow. We do not believe that we will reach a state of perfection in this life time, but only when we finally see Jesus will we be changed in the twinkling of an eye to be like Him.
The issue, then, is why we do have original sin? Most Christians have believed that originally humans were created with a natural inclination to want God to be God, and not themselves. The first pair of humans were thought to have rebelled against God, and lost this natural inclination to obey God, and now we all have a nature that rebels against God. But if humanity began with several thousand people, how do we explain our natures? Did they all disobey at the same time? Or is there some other explanation? It is worth pondering.