In recent years, researchers have determined that most cases of autism are not rooted in rare de novo mutations but in very old genes that are shared widely in the general population while being concentrated more in certain families than others. Whatever autism is, it is not a unique product of modern civilization. It is a strange gift from our deep past, passed down through millions of years of evolution.
~ Steve Silberman
(NeuroTribes: The Legacy of Autism & The Future of Neurodiversity)