Based on what you can see with your own eyes. Every pair of people on the face of the earth would need to have a lot of children to make the population multiply by 7 in just 20 years. That includes babies all the way up to 100-year-old people having babies. And you couldn't have any deaths during that time.

This isn't from someone telling me anything. It just doesn't add up.

Math. And having an actual brain.