At least prior to the arrival of the Europeans.

Last October, I read the book 1491 by Charles Mann. Mann documents the growing evidence that the Americas had a much larger population, and far more advanced cultures, than we have previously recognized. Upon the arrival of the Europeans, in tropical climates, with all of their diseases, the population of the Americas plummeted. Exact numbers are hard to come by, but an attrition rate of more than 90% would not be unreasonable. And this includes more areas than just the classic cultures we all learned about in school. Specifically, the book documents the Amazon Basin, which by the time the Europeans actually penetrated the dense jungle, there were only small tribes left, totally bereft of their culture. There is evidence that they once had vast farming areas, irrigation, speciallized soil building technology, fish farms, trade ... gone. They are the last of the last, centuries of survivors from a civilization long gone and over-run.

This is not an anti-European exapansion rant. Those things happen; ecosystems clash all the time, including human cultures, which are ecosystems in their own way.

I can't help but think, there is some evidence that we are all descended from only a small group of Homo sapiens; a tribe not unlike this, perhaps. Were there a world-wide pandemic, equal to that that ravaged the Americas, or worse, all hope of homo sapien's further evolution might rest in a deeply isolated tribe.