There is a gene–NOTCH2NL–that is found only in humans (and Denisovans and Neanderthals, once upon a time). It’s actually part of an ancient family of genes, but this particular version is only found in humans–and, more, we have multiple copies of it.
What this gene does is slow down the development of stem cells into neurons. Why does this matter? This delay actually causes more stem cells to turn into neurons, meaning without NOTCH2NL, our brains wouldn’t have anywhere near as many neurons and thus wouldn’t be anywhere near as big.
This gene is found on chromosome 1, in the location 1q21.1. As the original article in Cell notes, additional copies of this region have been found in people with autism. In other words, it’s possible that at least some autistics have even more copies of NOTCH2NL, resulting in even more neurogenesis. More neurons could push the brain toward greater positive feedback, which seems to be a main feature of autism regardless of various potential causes.
What this implies is that the very process that made us humans–the proliferation of NOTCH2NL (after it evolved)–could be behind the emergence of autism. In other words, some autistics may be more human than human.