The Shrinking Tree

Or, Why Charlemagne is Not My Ancestor

By Eilene Lyon

My recent post, The Instant Tree, sparked a discussion with Zoe Krainik from Hollywood Genes which I thought worthy of expanding on.

Zoe provided a link to this article that seems to suggest that each European today is descended from everyone living in Europe during the reign of Charlemagne (800 – 814 CE) who left descendants – including Charlemagne himself, of course. A reader could extrapolate that to any American who is of 100% European ancestry.

This absurd conclusion is the result of what I call the “pedigree-chart fallacy.” Take a look at a pedigree chart and it appears that the number of your ancestors doubles with each successive generation. This exponential expansion, if you go back 30 generations, gives you more ancestors in that generation than the number of people living on the planet at the time. Obviously impossible.

Pedigree Chart
A pedigree chart gives the mistaken impression that the number of your ancestors doubles each generation you go back.

But there is no exponential expansion to your tree after relatively few generations. The reason is something called “pedigree collapse” – a term coined by Robert C. Gunderson (darn, I thought I was being original).

In my tree it begins with my great-grandparents. Guy Halse married his second cousin, Mabel Cutting. As a result, Thomas J. Painter and Mary Williams show up as my ancestors twice. Likewise, John Painter and Susannah Stratton show up three times.

(Courtesy of N. Ingram – modified)

As you move back more generations, there will be an increasing number of these cousin marriages in your tree, whether the participants were aware of the connection or not. First cousins or fifteenth, they all have the impact of shrinking your pool of ancestors.

Rather than a tree branching off two-by-two into infinity, the branches begin merging into upside-down trunks. These trunks point to various geographic locations – places where many, many generations of your ancestors resided. Let’s face it. People simply weren’t all that mobile before the 17th century. (And, there weren’t that many of them.)

Satellite image of Iceland (Wikimedia Commons)

One notable example illustrates this concept – modern-day Iceland. This is a case of hundreds of thousands of descendants all from a small pool of pioneer ancestors. The same is true for all of us. We just usually come from many more small pools.

You may have a pool from Exeter, England; one from a village in Poland; another from Sicily. Rather than being individuals with an astronomical number of ancestors, spanning the globe, we each have an astronomical number of cousins and way fewer great-grandparents than you suppose. The same couples keep turning up on your family tree, over and over.

It’s true that we are all related, you just have to go back much, much further – 60,000 years further. The science of genetics has traced all modern humans back to a tribe in eastern Africa who began migrating around the planet about that time (some geneticists come up with slightly different estimates). The ultimate “small pool” of ancestors.

Utah13 004
This image is an optical illusion: a tiny stock pond masquerading as a larger body of water. (E. Lyon 2013)

The book and film “Journey of Man” gives an excellent overview of how the research was done and analyzed. You’ll find it fascinating!

But wait! Human beings have been on earth for 2 million years. How can we all come from this one group just 60 millennia ago? Extinction, baby!

Homo sapiens haven’t been around all that long – and might not be for much longer. Other hominid species have gone extinct for a variety of reasons: ice ages, famines, natural disasters, clumsiness, etc.

Some, such as Neanderthals, didn’t so much go extinct as they interbred and became absorbed into other species of hominids. Some people living today have Neanderthal genes.

Comparison of Homo neanderthalensis and Homo sapiens. (Wikimedia Commons)

We should never forget that Homo sapiens do not have a lock on permanent existence on planet earth, any more than any other species. Witness how fast extinction is occurring today. Our species is also at risk. And we are all one big family, so let’s get our act together, Cousins!

Perhaps a bit off-topic, but this great TEDx talk provides some perspective to our place in the animal kingdom.

Feature image: Atherton Gardens, Kauai, Hawaii. (E. Lyon 2013)

36 thoughts on “The Shrinking Tree

Add yours

  1. The math of these charts escapes me. But the idea (fact) that the world really is one big family tree is heartening. If only we understood the implicit obligation we have to making sure this crib is kept in good order. Because you’re right, we ain’t extinction proof, even if some of us behave that way.

    Liked by 1 person

    1. I think a significant faction really can’t see beyond their own puny lifespan. I tend to go geologic, which helps dispel despair. When we’re done wiping out most of life on earth, ourselves included, new species will arise and the planet will move on.

      Liked by 1 person

      1. My husband pointed out a vacant property that had had the parking lot resealed not long ago. There are plants sprouting from the asphalt all over the place. The plants really rule the earth, you know.

        Liked by 2 people

  2. Great post Eileen! I have explained the same thing countless times here in Italy. However, locals refuse to believe that they have cousins (lots of them) in their tree. Because the Val di Non was isolated for so many years, marriage prospects were often limited to those in one’s village and the pickings were slim. A half day’s horse ride to find a bride was often just out of the question. The result? Many fortified genetic traits such as twins in every family tree, predominately green eyes, distorted aortic valves, etc. One village here finally instituted a law that said you could not marry anyone in the same village because so many people had birth defects resulting in marrying third coursins for hundreds of years.

    Liked by 2 people

  3. I suppose I realize that all of mankind is a bunch of cousins. It makes sense in the abstract, but in the particular I sometimes wonder about some of these cousins of mine. How can they be so cruel– and stupid? But then I remember the kind and smart ones, so I feel better about family.

    Liked by 1 person

  4. “Witness how fast extinction is occurring today. Our species is also at risk. And we are all one big family, so let’s get our act together, Cousins!”

    Well put and so true! The permafrost is melting away like wildfire and scientists aren’t telling the public enough. In the near future, the weather is going to get increasing insane, erratic, and extreme. What that means for crops…

    (I go way back in ancestry… i keep some of my cousins in large aquariums.) 🙂

    Liked by 2 people

  5. Nice blog – I have been prompted to self-refer to a “cousin” for your post – because it has similar motivations, and some similar conclusions. A specific local motivation was references to Robert the Bruce, rather than Charlemagne. Another was the fashion for DNA testing. Finally, I had an idea for a compromise which we can all get our heads around. I wish I had known the phrase “pedigree collapse”… I’ve added it along with a link to your blog in my notes – thanks.

    Liked by 1 person

    1. Most of my tree doesn’t go back very far, because records get pretty sketchy. I focus on the people who are known to be my genetic kin rather than playing fanciful games of trying to link to royalty and such. Glad you found my explanation somewhat understandable.🙂

      Liked by 2 people

  6. I love how you explain this. Within 4 generations on my tree I have first cousins marrying and sisters marrying brothers. It makes for lots of interesting double relationships.

    Liked by 1 person

  7. Great post! I remember reading somewhere–maybe in National Geographic–that at one point in our distant past, due to glaciation and drought, the entire tribe of Homo Sapiens was reduced to about 10,000 individuals in East Africa. So, we almost went extinct once already.

    Liked by 2 people

  8. Fascinating, Eilene (I mean, Cousin)! This is new to me–the pedigree collapse. Make so much sense, but I never thought about it, and yes, all those family trees are always branching, when sometimes they should be “trunking.”

    Liked by 1 person

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