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.

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.

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 17^th century. (And, there weren’t that many of them.)

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.

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.

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)