ScottyB See my revised comment, no one actually knows what event triggered the cataclysm 50,000 to 70,000 years ago, it is assumed to have been a volcanic eruption. IT WAS MY ERROR TO SUGGEST THE YELLOWSTONE SUPER VOLCANO AS A POSSIBLE CULPRIT, I WAS THINKING THAT IT HAPPENED ON THE RIGHT TIME SCALE, I CHECKED MY REFERENCES AND I WAS WRONG, I STAND CORRECTED BY SCOTTYB. There have been very severe eruptions in recent geologic history, perhaps not tens on the scale, but still very very big.

Again this is a book report with some other "scientific data" thrown in and some acknowledged paraphrasing, people should read the book and do their own online research. Every university on the planet has information posted on all of these subjects.
Jim