Monday, August 12, 2013

We share an ancestor who probably lived no more than 640 years ago

This post is a revised version of a previous post. The original post has been or will be deleted, with the comments preserved. Typically this is done with posts that attract many visits at the time they are published, and whose topics become particularly relevant or need to be re-addressed at a later date.

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We all evolved from one single-celled organism that lived billions of years ago. I don’t see why this is so hard for some people to believe, given that all of us also developed from a single fertilized cell in just 9 months.

However, our most recent common ancestor is not that first single-celled organism, nor is it the first Homo sapiens, or even the first Cro-Magnon.

The majority of the people who read this blog probably share a common ancestor who lived no more than 640 years ago. Genealogical records often reveal interesting connections - the figure below has been cropped from a larger one from Pinterest.


You and I, whoever you are, have each two parents. Each of our parents have (or had) two parents, who themselves had two parents. And so on.

If we keep going back in time, and assume that you and I do not share a common ancestor, there will be a point where the theoretical world population would have to be impossibly large.

Assuming a new generation coming up every 20 years, and going backwards in time, we get a theoretical population chart like the one below. The theoretical population grows in an exponential, or geometric, fashion.


As we move back in time the bars go up in size. Beyond a certain point their sizes go up so fast that you have to segment the chart. Otherwise the bars on the left side of the chart disappear in comparison to the ones on the right side (as several did on the chart above). Below is the section of the chart going back to the year 1371.


The year 1371 is a mere 640 years ago. And what is the theoretical population in that year if we assume that you and I have no common ancestors? The answer is: more than 8.5 billion people. We know that is not true.

Admittedly this is a somewhat simplistic view of this phenomenon, used here primarily to make a point. For example, it is possible that a population of humans became isolated 15 thousand years ago, remained isolated to the present day, and that one of their descendants just happened to be around reading this blog today.

Perhaps the most widely cited article discussing this idea is this one by Joseph T. Chang, published in the journal Advances in Applied Probability. For a more accessible introduction to the idea, see this article by Joe Kissell.

Estimates vary based on the portion of the population considered. There are also assumptions that have to be made based on migration and mating patterns, as well as the time for each generation to emerge and the stability of that number over time.

Still, most people alive today share a common ancestor who lived a lot more recently than they think. In most cases that common ancestor probably lived less than 640 years ago.

And who was that common ancestor? That person was probably a man who, due to a high perceived social status, had many consorts, who gave birth to many children. Someone like Genghis Khan.

15 comments:

dearieme said...

Allow a nitpick: "due to a high perceived social status" - is there some secret social status that is not perceived?

Uncle Genghis would cut your head off for that.

Deborah Alessi said...

Deborah Alessi and her husband has set-up a non-profitable firm, Face Forward Agency.



Deborah Alessi

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Ned Kock said...

Good point dearieme.

Ned Kock said...

Deborah, are all the reconstructive services to women who suffered abuse pro bono? Where to the funds come from?

excelcompanioncare.com said...

Very interesting post. keep posting

Ugur Gundogmus said...

Hi Ned.

I think you should remove comments that are obviously spam. There are so many even in your old posts.

I know what these people trying to do (getting links to a specific site), but they are wrong.

Ned Kock said...

I agree Ugur. With old posts, however, that would be a full time job. With new ones, I remove spam but tend to err on the side of not removing if they have a positive motivation behind them. Deborah’s comment above is one of these cases, or so it seems.

Ned Kock said...

The comment below apparently was not posted.

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Anonymous has left a new comment on your post "We share an ancestor who probably lived no more th...":

"And who was that common ancestor? That person was probably a man who, due to a high perceived social status, had many consorts, who gave birth to many children. Someone like Genghis Khan."

Did he have many consorts due to high perceived social status, or was he a rapist? I guess that's the relevance of the Face Forward link.

"Face Forward works with community leaders around the nation to identify victims of violence who are legitimately working toward recovery and who carry the physical evidence of past abuse."

Sounds like a good cause, but strange wording. How does someone illegitimately work toward recovery?

Ned Kock said...

Hi Anon. You raise a good point, but it is not very likely. High status in societies where polygamy was either encouraged or tolerated was often maintained via strategic relationships; a warlord taking as wives daughters of powerful allies. This is probably what allowed the Spanish conquistadors to avoid certain death; in addition to cavalry, superior armor, and Toledo swords. In these types of cases, mistreatment of the women would be suicidal.

Nick said...

GK was both a rapist and had high social status. Higher than Mick Jagger. To be our common ancestor, it is unlikely that he did not take advantage of relationships that were not strategic. Think also the many conquered people who were subjected to whatever whims he may have had.

Ned Kock said...

The rapist ancestor theory has been around for a while – i.e., the theory that our most successful ancestors were rapists.

I don’t believe in it. Rape has always been seen as a despicable act that would lower the social status of those who committed it.

The historical record suggests that it was very uncommon in hunter-gatherer societies – e.g., the American plains’ tribes.

It might have been more common in certain societies during certain periods, as a result of group forms of behavioral “disease”; a big one being slavery.

Ned Kock said...

In warrior societies (e.g., the American plains’ tribes), rape was often perceived as a particularly low form of behavior. The reason is that it was associated with cowardly behavior, which in warrior societies was strongly associated with low status among men.

One serious problem in warrior societies was adultery though. High status warriors appear to have been very desirable mates. On the other hand, their lives would typically be short.

We should be careful not to take as historical reality what we see in movies. In epic action movies it always works well to portray a leader as a tyrant who commits all kinds of despicable acts, including rape. This makes them good villains.

This general formula is something that works well in all kinds of action movies, by the way. The ones starring these guys follow it in a very predictable way: Charles Bronson, Jean-Claude Van Damme, Sylvester Stallone, and Jason Statham.

Matthew @ My Little Eye Surgery said...

I'd love to see a huge family tree about my family.

juliane said...

The world population was not as big as it is now in the past. So I guess our youngest common ancestor is max 560 years away.

The world population was about 400 million in 1451 and this is about the same size the theoretical population of uncommon ancestors is.

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