My apologies to Zakkie, but having just read Sarah's theory, I feel the need to add ours as I think they belong together.
{Sarah's mini-rant, at long lost}
Sarah's personal theory on autism:
I personally believe that autism has always existed. Someone mentioned ying and yang, I think autism is a perfect example. There has to be someone that sees things in one way, someone else in another. In the history of the world people that see things in one way tend to group together and form a society. As time and events morph society, different ways of thinking raise to the needs of said society.
I'm not in any way suggesting that we owe auties the renaissance, the age of enlightenment and the space age... well, actually, I think that's exactly what I'm saying.
How important would have it been for an Egyptian architect to have a "way with numbers and spatial concepts"? Pretty important, I'd say...
What about the other special skills and interests that auties tend to have? Music? Art? Writing? Math? Science? Those all have higher times of emphasis throughout the history of the world.
And likeways (the ying to this yang, so to speak) is that other times have come up as well... times when others' strengths are idealized by their society. Long speeches that warm hearts and bring tears to a crowds eyes is a strength rather than a drone on emotional supplies. When having a long list of friends means that you will live a better life because of the connections that you are able to keep and maintain. So perhaps these times were not huge growth times for the furthering of civilization, but they have their merits.
The problem with any of these "ways of thinking ebb and flows" is that at some point each rise will crumble. The cold war ended and the great need for science and math nerds has slowed down and now the "most important" thing a kid can learn at school is to socialize properly. However, in this time that is lacking in the science and math types (I know I'm being grossly stereotypical, apologies) there will soon come rise to the art and music types (hopefully for me) and probably the literary creatives as well. But the math/science nerds will eventually rise again, when our society calls them up once more.
For every rise to one there is a fall.

There you have it, my own lofty ideals on autism and the spectrum of humanity.
Right, wrong or regurgitated. At least it’s out there.
My son & I have similar but slightly different thoughts.
WFM & son's theory:-
It would seem to me, that (non fatal, or deleterious) genetic variations causing such things as AS, are part of the grand Genetic design to ensure the survival of the species.
For example many people with AS are known to have social problems (for example will avoid social gatherings) and also have problems with members of the opposite sex.
Although this is somewhat debilitating for a person with AS, and may appear to in fact be a flaw to NT`s consider this:
If a highly communicable disease was to break out with a high mortality rate, then AS`s would be more likely to survive than NT`s due to behavioral differences (lack of socialisation, and physical contact). This would undoubtedly ensure the survival of the species, in extreme circumstances.
This behavioral diversity has in fact been observed in other species, for example rabbits:
There was an man-made outbreak of Myxomatosis, among rabbits, designed to control the species. Now rabbits, as we all know live in burrows in the ground. However, a very small proportion are born with a genetic `abnormality` which causes them to behave differently. In the case of the rabbits, this small proportion, did not live in burrows, but instead lived above the ground, and did not socialise with the other rabbits.
This would at first appear to be a genetic disadvantage, since small mammals living on exposed terrain, would be more likely to be picked off by predators.
Thus when Myxomatosis was introduced, it rapidly spread through the rabbits living in the burrows, and killing them.
The rabbits living above the ground however, having not been in contact with the ones in the burrows survived. The rabbit population for many years after, consisted mainly of those living above the ground, with a small percentage making burrows. After a long period of time, the population distribution swung the other way, and again, most live under the ground, with a small percentage of `abnormal` rabbits living above the ground.
I think Darwin was almost right, but it is not survival of the fittest, but survival of the diverse.
Many people believe that the great inventors, throughout history exhibited AS traits, and this added diversity has lead to the advancement of the species as a whole, albeit artistically/technologically.
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If we hypothosise that AS's have been on the earth since the beginning and it is known that 'odd' people did exist
was it 'the outcast' who invented the wheel?
It is now commonly believed, that many of the 'great inventors' and 'builders', etc, were AS or had AS traits, so why not him / her?
Assuming this is the case, man would not have evolved technologically as quickly as we have without the AS's. I believe that it is the 'thinkers'/ 'odd ones' who have moved us forward. (some may well say backward !!).
The 'herd instinct' does not necessarily ensure survival.