Two people reproduced 7 billion people.

[John V]

Six words: each line is a separate generation.

For example, the second line is nine. That's 3+3+3, the three surviving children of each of the three initial couples. The parents are not included in that number. We're accounting for death.

Three children per couple seems reasonable, but let's lower it. Set the growth to just 10%. Now you hit 7 billion in the 293rd generation. If a generation is 20 years, that's 5,860 years, and we're not even building in the effect of the early long life spans.

It just doesn't take that long.

Are you taking into account divine murder, infertility, disease and plagues, death in infancy and childhood (kind of a big deal in the past!) the limited life expectancy before the modern era, war, and the fact that some people just don't want to have kids? Not to mention homosexuality, or the fact that people just don't pair up in neat little rows like you're expecting, nor do they spit out the requisite number of kids?

Yes, I'd say that 10% takes those things into account. Are you taking into account younger start to childbearing and lack of modern contraception?