What is the probability that a complementary male and female sexual reproductive system would form independently by accident in the earliest sexual animals? Nature would have to get this exactly right in the first pair, otherwise they die with no offspring. You only get one shot at it. The rules of my proposed game apply. If nature fails to pull off this one giant leap in one shot, say goodbye to millions of years of evolution. Of course, to do this in one leap would be contrary to evolution.