The traditional view of how human beings came to have Original Sin is that they inherited it biologically from their parents, and that this could eventually be traced back to our original parents, Adam and Eve. Most geneticists now claim that there never were just two people in the human population, but always at least several thousand. If this is indeed the case, it would require a revision of some sort in our understanding of how Original Sin was transmitted.
I've been reading C. John Collins' book, Did Adam and Eve Really Exist?. He discusses an alternative model proposed by Derek Kidner in his (1967) commentary on Genesis. Kidner wrote:
"It is at least conceivable that ...[Adam and Eve were] the first human pair as God's vice regents (Gn. 1:27,28)...God may now have conferred his image on Adam's collaterals, to bring them into the same realm of being. Adam's 'federal' headship of humanity extended, if that was the case, outwards to his contemporaries as well as onwards to his offspring, and his disobedience disinherited both alike." (p.124)
Collins finds this acceptable, with the proviso that
"...one would need to imagine Adam as chieftain, or 'king,' whose task it is not simply to rule a people but more importantly to represent them (the basic idea of a king in the Bible)." (p.125)
So one plausible solution to the genetics problem is that in the original human population, consisting of several thousand people, Adam would have been the chieftain or ruler, representing the rest of humanity. When he falls, so does the rest of humanity. To our modern democratic minds this might seem unfair. But it would have made sense to those living under a monarchy. And if Christianity is true, we do indeed live under a monarchy. The good news is that even though our first king fell, our second king did not. And as Paul put it (I Corinthians 15:22):
"For as in Adam all die, so in Christ all will be made alive."