On an unrelated note, it's possible the entire world population at around 400BC only numbered in the low tens of millions; it's currently around eight billion. Seems that each time the human population starts a growth spurt, disease, famine, volcanic eruptions or war appear and take out large chunks of humanity.
Over time we've had the Toba younger eruption, the Deccan traps, the Chicxulub impact, the Great famine, the Justinian plague, the Black Death, the Spanish 'Flu, two world wars, the discovery of nuclear weaponry and we're still here. And volcanologists are now warning the Yellowstone is making ominous rumblings.