I just yesterday finished reading Krakatoa: The Day the World Exploded and today I find this article about recently completed DNA research that shows humans may have nearly gone extinct about 70,000 years ago.
In the Krakatoa book was discussion of the largest known (from physical evidence) volcanic eruption in the history of humans: the Toba volcano in Indonesia, approximately 73,000 years ago.
The Toba explosion was so big as to make the explosion of Krakatoa look like a firecracker. To compare, the Mount St. Helens eruption spewed 1 cubic kilometer of rock and debris into the air. Krakatoa's eruption was about 20 cubic kilometers. Toba was 2,800 cubic kilometers making it the largest by far.
Like other volcanic eruptions, the material that flew high into the air with the Toba eruption began to drift over the world, eventually encircling it. Sunlight was drastically reduced, temperatures plummeted, plant life began to die off, and so did the animal life that depended on it -- including humans. One study, mentioned in the article, indicates as few as 2,000 humans were left alive, bringing our species perilously close to the edge of extinction.
This is fascinating in its own way; but it is even more fascinating to think of the repercussions of such an eruption were it to happen today. Global warming would come screeching to a halt, and the world would be dropped into another year without summer (like 1816, thought to have been caused by the massive eruption of Mount Tambora) that could last several years wiping out millions of humans and other animals.