Nearly 500 years before Columbus, American societies across the continent were on the move, emerging with new achievements in technology, architecture and social organization. Of course, no one in America then knew or cared that the Christian era was in its thousandth year, nor would there have been much anxiety over the changing of a date if they had. Time, to the extent it was marked at all on astronomical calendars, was measured in growing, harvesting and hunting seasons. Still, people were aware of changes.
Most scientists feel weather played the dominant role.
It was the warmest period seen in the heart of the continent in perhaps 2000 years. Climate change, rather than computer clocks, would create turmoil early in the new American millennium. It was as warm then as the middle decades of this century, which made it pretty optimal for agriculture and expansion into new territories. In today's New Mexico and Colorado, the Anasazi and related cultures grew corn in river bottoms and built dwellings more than five stories tall with up to 800 rooms. Such buildings wouldn't be erected on the continent again until workers completed the first high rise apartment building in New York City nearly 900 years later.