Most reputable scientists now believe that autism
has existed throughout the history of humankind. Some have speculated that ancient legends about "changelings" are actually stories of children with autism. Celtic mythology is redolent with stories of elves and visitors from "the other side" who steal a human child and leave their own damaged child in its place.
This can include complete dialogues that the Autistic child has heard from a conversation or a movie or radio broadcast. It may not have a meaning to them at the time but they just repeat it. Echolalia can be a little confusing when the child uses it all the time and then gets frustrated when people don't understand what they are trying to communicate.