The evolution-autism argument doesn't work, because evolution occurs to improve a species' survivability. Meaning, species evolve to be better adapted to their changing environments. People developing autism doesn't increase their survival rates, (actually, if you are speaking in purely evolutionary terms, you only care about living long enough to reproduce and pass on your genes). Having autism decreases your chance of passing on your genes to the next generation, so it's not a change that can be explained by a species evolving.
So far, the "cause" of autism has not been found. But Yote is right on that pretty much everyone is certain that it has environmental roots, be it from chemicals/additives/hormones in the food we eat, or the air we breathe, or that sinks in through our skin. Just what exactly is not understood yet.