I think "autism" is such a broad spectrum and seems to consist of many different root causes manifesting with broadly similar 'symptoms', that it is useless to talk about the diagnosis as a single cohesive thing. Some people diagnosed with autism are so retarded that they are unable to understand/express many emotions and have severe difficulty learning things. Others seem to be somewhat normal people who fall into the extreme of "nerdy" personality traits, with normal intelligence but difficulty with social matters. Others still have been suggested to be "hyper-empaths" who are so perceptive of emotions and social matters that they become emotionally-overloaded and short-circuit. This outwardly manifests in symptoms that seem similar to the individuals who genuinely are unable to understand emotions.
Others may be diagnosed with autism because they do not comply with Western cultural and psychological standards. (For example, in many cultures it is discouraged or even considered an extreme taboo to make eye contact with people for various reasons, but apparently this is considered a major 'symptom' of autism. ...Or, you know, the child has been so traumatized by their parents/adults that they try to avoid initiating contact with them at all costs by avoiding eye contact).
I think this is similar to ADHD to some extent as well. There may be evolutionary causes for hunters and herders to have evolved ADHD, but kids are also diagnosed with ADHD simply because they get bored as hell by the Western school system, and instead of adapting the system to the kids, they drug the kids so they comply with the system.
(For reference, here's a study about how ADHD could benefit herders as well. I think there are other articles on this subject also.)
Impulsivity and a short attention span may be the bane of every parent with a hyperactive toddler, but those same traits seem to help Kenyan nomads keep weight on. A gene mutation tied to attention...
I recall reading a study which examined brainwave scans, which determined that a disease as uncontroversial as schizophrenia is actually something like 7 different things that all express somewhat similar symptoms, but have different root causes in the brain. Things like ADHD and autism, which are much more complex and have a broader range of symptoms, likely have many more root causes.