To our knowledge it has been at a steady level throughout human history. Kant was mentioned in this thread, Henry Cavendish was also thought to be autistic. From Wikipedia:

> Cavendish was taciturn and solitary and regarded by many as eccentric. He communicated with his female servants only by notes. By one account, Cavendish had a back staircase added to his house to avoid encountering his housekeeper, because he was especially shy of women.

There are other threads that go back further as well, and at least in certain cultures there's a fairly broad overlap between shamanism, autism, and related neurological conditions. I've seen some studies tracking certain genetic mutations back and some at least are thought to have come about when humans started becoming human. I haven't been able to find them again but there are quite a few studies that can be found via Google Scholar on the subject.