Passing On Human Genes is Neither Easy Nor Efficient

Human beings are sometimes described as the apex species of earth, which you would assume from the way we've taken over. But it wasn't easy getting here, and the way we make more humans is real crapshoot, evolutionarily speaking. This is true of all mammals, in comparison to insects or birds, but humans have a harder time and a much higher failure rate than other mammals. For every fertilized egg that makes it to actual birth, two are failures and are discarded along the way, most even before a woman knows she is pregnant. About half of these miscarriages are because the fertilized egg has the wrong number of chromosomes. Another way that human reproduction is so fraught is because of the huge demands a human fetus put on the mother, leading to conditions like gestational diabetes and pre-eclampsia, which can be fatal.

What makes humans so uniquely bad at reproduction among mammals? There are plenty of theories, but the evidence points to the fact that we developed large brains that demand more resources, and to the fact that such a brain, and other features that define us as human, developed when the world's population was so low that mutations weren't selected out early. Read how the human gene system went strangely wonky at the Conversation.