you kjnow what will cause a population crash? A supervolcano on the likes of Toba, where some 10,000 are estimated to have survived—that was some 70K years ago. We have been bunnies since.
Population of the world at the start of Sumerian civilization circa 5000 BC was about 10 million.
Population of the world at the time of cyrus the great who founded the Achaemenid Persian empire was about 50 million (and he ruled over 23 million!)