Actually, if you beleive the story, we also got that big flood, and so while you had a bit of a setback, all of these people were obviously really fruitful. Maybe they had sextuples every 15 months (which is how some jewish sages explain how 69 hebrews went down to Egypt and 600,000 males of fighting age and all of their women and kids and the elderly [a couple of million in total?], came out 210 years later.
“I've done everything the Bible says — even the stuff that contradicts the other stuff!"— Ned Flanders