about it this way. Say I have three children. My children each have
three children. Their children, on average, have three children, and so
forth. In a dozen generations that's over half a million direct
In two dozen generations, it's almost 300 billion.
those aren't all different people. They all interbreed. But it's clear
that in only a few hundred years, everyone who is remotely part of the
gene pool is descended from me many times over - perhaps millions of
times over. If anyone from this gene pool interbreeds with another, they
can then similarly be the millions of times over ancestor of the common
gene pool. It only takes very rare interbreeding, nothing remarkable,
for everyone to be descended from the same person.