An under-appreciated fact: the average child is born with perhaps a hundred new mutations. Most of these will have no noticeable effect, a few will cause some harm, and only very rarely will one actually prove helpful. The net effect is a small decrease in fitness.
This means that children will, on average, be less fit than the average fitness of their parents, e.g. the children of average-fitness Alice and average-fitness Bob will have below-average fitness (on average).
Assuming their below-average-fitness children mate with their peers, Alice & Bob’s grandchildren will have even lower fitness. This cycle continues until their descendants are too unfit to successfully reproduce and/or protect their territory from the competition. The end.
Gnon gave us mutations so we could adapt to a changing environment, but in exchange for this gift he demanded sacrifice: the gene-pool requires constant pruning. Gnon was happy to help with this: he gave us predators to remove the genes of the slowest and least-aware, diseases to weed-out those with the poorest immune systems, warfare to cull the weakest and least dexterous. He tested us with famine and drought, parasites and pestilence: always favoring those whose genes had taken the least damage from mutation. He gave us instincts which counseled our women to mate with only the fittest males. Combined, all these things kept the constant dysgenic effects of mutation at bay. And Gnon saw that it was good.
Meanwhile, man saw that it was bad. We became furious when our children were slain, and built walls to keep out predators or even hunted them to extinction. We reproached Gnon for taking our weakest with his diseases and parasites, and learned how to keep those at bay. We figured out how to store food for lean times, and dig wells to escape drought. To keep our weakest males from falling in battle, we resolved to send only the strong, and designed weapons to kill them indiscriminately (Gnon is still scratching his head over this). In our competition with each other we were forced to take up monogamy, which meant that a lot of less-fit male genes were retained in the gene pool.
And Gnon saw that it was bad. In our determination to reject every eugenic measure he offered us, our genes went into a dysgenic death-spiral. The higher our mutation levels grew, the poorer was our health, and the more-erratic our behavior. Our birthrates plummeted to far below sustainable levels. We lost the discipline that had allowed us to build our civilization, allowing our people to revert to their instinctual mating patterns (which were no longer viable in the world we had created). Gnon decided that we were too retarded to exist, and began implementing his backup-plan: replacement by still-vigorous populations.
I am skeptical that we can talk Gnon into changing his mind about us at this point. I doubt that it would be politically viable to reintroduce predators onto our playgrounds, allow diseases free-rein to spread, or the like. Even less so implementing a eugenics program aimed at keeping the less-fit from reproducing. Our only hope may lie in advancing the science of embryo-selection. Of course, this would then have to be widely-adopted, and we would have to start having children again: both of which seem more than a little problematic.