How many ancestors do I have? (world, population, physical, genes)

[jtur88]

I'm puzzled by something.

I just read on the NatGeo website that:

An international group of geneticists studying Y-chromosome data have found that nearly 8 percent of the men living in the region of the former Mongol empire carry y-chromosomes that are nearly identical. That translates to 0.5 percent of the male population in the world, or roughly 16 million descendants living today.

But wait. Genghis Khan was born about 700 years before I was. Let's assume about 25 generations have passed since then. That would mean that I have about 16-million ancestors who were living in the times of Genghis Khan. Assuming that each of those ancestors had an average of two surviving descendants per generation, 16-million would be the average number of descendants, from any person living in the 13th century.

Given the fact that the world's population has increased at least ten-fold since then, the average number of descendants from any person was well above replacement, and would need to be more like 160-million. What's wrong with this calculation?

[EnricoV]

What's wrong with your calculation is, you'll pretty soon reach a point where you have more ancestors than there were people living on the earth. What you don't have, is 16-million unique ancestors. You are likely descended from the same people over, and over, and over, and over, and over.

[jtur88]

Originally Posted by EnricoV

What's wrong with your calculation is, you'll pretty soon reach a point where you have more ancestors than there were people living on the earth. What you don't have, is 16-million unique ancestors. You are likely descended from the same people over, and over, and over, and over, and over.

Right, I understand that obvious fact. But does it also work the other way? After 700 years, will I have a large number of 'unique' descendants (like Genghis Khan's 16-million), or will there only be a small number of descendants who are in my lineage over and over again? Since each person will share his ancestry with me and 16-million other tangled ancestry lines, that leaves a lot of room for my genes to be in a lot of people. As soon as there are enough generations to assure multiple overlaps, everyone is descended at least once from everyone.

It would seem that Genghis, by having so many first-generation offspring, simply magnified (or accelerated) the "over and over" part of the equation, without significantly raising the number of his gene-carrying individuals 25 generations later who are in that pool, which is already saturated to the point of assured overlap.

[jimboburnsy]

Originally Posted by jtur88

I'm puzzled by something.

I just read on the NatGeo website that:

An international group of geneticists studying Y-chromosome data have found that nearly 8 percent of the men living in the region of the former Mongol empire carry y-chromosomes that are nearly identical. That translates to 0.5 percent of the male population in the world, or roughly 16 million descendants living today.

But wait. Genghis Khan was born about 700 years before I was. Let's assume about 25 generations have passed since then. That would mean that I have about 16-million ancestors who were living in the times of Genghis Khan. Assuming that each of those ancestors had an average of two surviving descendants per generation, 16-million would be the average number of descendants, from any person living in the 13th century.

Given the fact that the world's population has increased at least ten-fold since then, the average number of descendants from any person was well above replacement, and would need to be more like 160-million. What's wrong with this calculation?

Here's the link for anyone curious... Genghis Khan a Prolific Lover, DNA Data Implies

Are you accounting for female offspring? If Genghis Khan were related to you through a maternal lineage you would have no Y-chromosomal similarity (unless there was some double dipping) yet you would be *virtually* as closely related to him as any of the 16 million males who do. In actuality Khan's descendants probably number at least 10 times that amount and possibly a lot more (I'm not sure how to construct an equation to estimate 25 generations with compounding likelihood of the progenitor's Y chromosome extinction within each line of each generation).

Is that what you're getting at or am I 180 degrees off?

[jtur88]

I guess what I'm getting at is, that somewhere 25 generations ago, there were millions of completely unremarkable people, each of whom now has 16-million (or so) living descendants. Including at least one and probably thousands of my ancestors, one of whom might have been Genghis Khan himself. So there is nothing special about Genghis Khan profligacy with his seed, it had the same residual effect as anyone else's.

The Italians believe that every single Italian is descended from at least one pope, and they are probably right.

[jimboburnsy]

Originally Posted by jtur88

I guess what I'm getting at is, that somewhere 25 generations ago, there were millions of completely unremarkable people, each of whom now has 16-million (or so) living descendants. Including at least one and probably thousands of my ancestors, one of whom might have been Genghis Khan himself. So there is nothing special about Genghis Khan profligacy with his seed, it had the same residual effect as anyone else's.

The Italians believe that every single Italian is descended from at least one pope, and they are probably right.

Well, the remarkable thing about those people was probably their physical hardiness. We're all descended from people with robust immune systems who were able to live to reproductive age without the benefit of vaccination, refrigeration, sanitary plumbing, etc. etc. and who had the ability to birth multiple children on a dirt floor without medical oversight or intervention. There is probably another bottleneck in our future to select for the most robust individuals.

In the long run, I guess survivability is better than a giant scratch across the history books.