The theory of natural selection explains that as a species of any kind evolves, the weaker members of that species will not survive. These weaknesses can be anything from not being able to outrun another species that views you as its next meal, to deciding to leave the oceans without bothering to evolve lungs.
Early man evolved and developed through the ability to adapt to his surroundings. One of the weaknesses of humans however was stupidity. This stupidity was to a greater extent kept in check by natural selection. If for example, a stupid human decided it would be a good idea single handedly attack a sabre toothed tiger, the result would be that the human in question would learn the error of his ways shortly before his death. This was a good thing because it kept the number of stupid people low, and more importantly they didn’t get the chance to add stupidity to the gene pool.
http://www.bbc.co.uk/dna/h2g2/A3164186
[Antingen reglerar vi vem som ska få fortplanta sig, eller sitter vi och väntar tills mänskligheten framavlat en generell IQ som överensstämmer med chimpansernas.]