Evolution and original sin
Original sin is an important doctrine that points out, among other things, that we are not quite right. As St. Paul says, “I can will what is right, but I cannot do it. For I do not do the good I want, but the evil I do not want is what I do” (Romans 7:8-9). Our passions are at war within us. This is our own fault, not God’s. It indicates something broken within us, which all the therapy and perfect upbringing in the world cannot fix. Only God’s grace can heal this wound.
But, if we believe that our bodies were created by a process of evolution (and I do believe this), of course we are not perfect. Evolution does not make perfection, it makes the next version of a reproducing population of organisms better than the last version in some specific way. Evolution has molded our bodily passions, on the whole, to be in line with our own best interests. But evolution cannot do this perfectly and so we experience being in a state of original sin.