If there was an alternate dimension where all the humans were somewhere on the autism spectrum, how would it operate? One can only guess, but its fun to think about these things.
First off, 'progress' would be severely stifled. The first humans would probably live solitary lives, foraging, hunting and dying virtually almost like animals because one of the crucial aspects which makes us humans- being social- would be severely hampered. Higher-functioning humans might form small, isolated groups held together by shaky alliances. The lower functioning humans would probably die out, being unable to form the beneficial relationships with other humans which improve survival chances, and not having the neccesary intelligence to make use of tools or make their own clothes. In fact, it would be odd for such a species of helpless animals to have evolved in the first place. The lower-functioning autistic humans will definately die out, being outcompeted by higher functioning humans, or possibly evolved to have greater strength, to have fur, and/or to have claws to cover the weakness brought on by lack of intelligence.
So what now of the remaining population of higher-functioning autistic humans? They may well continue to evolve to become increasingly socially-adjusted (by our standards) until they became quite similar to us. In fact, it would be difficult for humans not to evolve in this direction. Being able to form social groups is a huge advantage.
But lets say they did not evolve in that direction. Such a species may be doomed to live almost like animals for the rest of their lives. Any new technologies or discoveries made by humans will be taken to the grave due to poor communication skills. Agriculture, a sedentary lifestyle, and eventually civilisation, will never take place because the humans are unable to work cooperatively. The human species will subsist as isolated pockets of hunter-gatherers for all time, until they became extinct.