Even counting the story of Noah's flood, they are saying the nearly 6.8 billion people on the planet came from only 8 people, about 4500 years ago.
The number is mathematically possible and would require an exponential population growth rate of only about 0.46%. For comparison, the current growth rate is around 1.2%, down from it's peak of 2.19% in 1963. At 1.2% growth, 8 people would, in 4500 years, become 162,261,595,301,000,000,000,000 (162 sextillion) people.
However, just because the math says it's possible, doesn't mean it is. Before the mid-1700's (roughly the beginning of modern medicine and technology) population growth was nearly flat due to extremely high death rates and short life spans. People had to breed like rabbits to keep the death rate from surpassing the birth rate. For every ten children a woman had, maybe two would survive long enough to have their own children-- at the best of times. That doesn't even account for the numerous plagues (the Black Death is only the most famous one of literally hundreds of major plagues) or wars, famines, droughts, etc, etc, etc.
And then there's the inbreeding bit. Do some research on inbreeding in cheetahs, for example. Or watch this vid: http://www.youtube.com/watch?v=rIlWKp44T50
Oh, and that doesn't even bring into question the DNA evidence of ancestry dating back hundreds of thousands of years to the first modern humans without even going into the Evolution bit.
