This is scientific American 60 second science.

I'm Christopher Intagiyata.

Men inherit their white chromosomes from their fathers,

and they get an almost exact copy

other than a few mutations, meaning very little changes about

the y-chromosome from generation to generation.

You may just have a hundred people with the same y-chromosome because they are all descended

from one man.

Marcus Feldman, a population geneticist at Stanford.

They all have this great, great, great, great, great, great grandfather.

And every male relative of that individual who is the first degree relative has the same

Y chromosome. Researchers have thus used Y chromosome data, along with mitochondrial DNA,

which is passed down only by our mothers to investigate aspects of ancient populations.

For example, one recent study found a huge drop in Y chromosome diversity

five to seven thousand years ago.

At the same time, mitochondrial DNA diversity continued to grow,

implying a possible crash in the male population, with 17 women to every man.

But... It seemed to us that the 17 to 1 sex ratio was just too extreme to be real.

So Feldman and his team used computational models to investigate other ideas and they found that bloody fights between genetically homogenous clans could have produced the same results.

Essentially we're saying that the clans that are fighting one another are built around having the same Y chromosome.

Imagine a single deadly battle, knocking out a whole clan of men with the same Y chromosomes.

Then repeat that unpleasant scenario a few times.

...