There’s a meme out there about “if people evolved from chimps then why aren’t the intermediate stages walking around now?”
I think right now in human history is a good example of how that can happen. Times are so good right now. There’s a pretty wide variation of humans across the planet, and birthrates are okay, and infant mortality is SUPER low. Darwin is basically taking a nap right now.
At some point, this will cease to be the case (heh.) What if the new normal favors some weird subset of humans? Solar radiation changes drastically, and vitamin D becomes an issue. Volcanic or nuclear winter makes food extraordinarily scarce near the equator. Autism ends up being correlated with a natural resistance to some super disease. A wild plague that spreads mainly through municipal sewers, but not through air or water, sparing almost everyone in the country.
As for the original case, the answer is probably that chimps are adapted to eating leaves, and fire made eating meat more efficient, and intestines shrank while brains grew. Hominids with less guts than chimps and less guts than humans wouldn’t be adapted to either, and would lose out to the extremes. So in that case, the autists win.