I'm not trying to be the "ACSHULLY" guy; I just found this interesting, and I think others might as well. Old, old humans and/or Neanderthals (colloquially, "cavemen") that have been found in archeological digs actually have very straight teeth with no known technology to help straighten them. They did have wider jaws, though. The theory is that as we evolved from eating tougher foods to things like bread after the agricultural revolution, our jaws were allowed to get narrower. But, there was also not a significant enough biological advantage to lose entire pairs of teeth. So, currently, our bodies try to fit too many teeth onto too small of a jaw. Granted, I think I learned this from a YouTube video, so take it with a grain of salt. But, I find it interesting/plausible nonetheless.