Fascinating story. Shows how fragile our collective existence is (was?). Also the link to the National Geographic's genographic website is well worth a look.

I wonder if it happened so slowly, they did not realize it?

Things just getting incrementaly harder, year after year after year, till the village up river was gone and the tribe across the hill had disapeared.

I wonder if they sat around the fire and debated whether things were getting worse or better?

I suspect so.

It's also fascinating to think what it would have been like if humans never evolved.