Evidence shows that the earliest Americans arrived about 23,000 years ago from Siberia and perhaps other places. Both American continents were populated. Complex societies developed; the best known are the Mayans, Incas, and Aztecs. Evidence suggests that the bulk of the population underwent the Neolithic revolution (started using agriculture). This widespread transition may have occurred around 1800 BC. In summary as of 1491, here is what we know. New scholarship shows that the earliest Americans were a thriving and diverse population with a tumult of languages, trading patterns, and cultures. They had a population of 10s of millions.