Our Common Ancestors, part I

“To the extent that ancestry is considered in genealogical rather than genetic terms, our findings suggest a remarkable proposition: no matter the languages we speak or the colour of our skin, we share ancestors who planted rice on the banks of the Yangtze, who first domesticated horses on the steppes of the Ukraine, who hunted giant sloths in the forests of North and South America, and who laboured to build the Great Pyramid of Khufu.” – Rohde, Olson, and Chang, “Modeling the recent common ancestry of all living humans” quoted in Swamidass 2019.

The quote above was a comment on a study the authors performed on universal genealogical (not genetic) ancestors. Understanding this begins with the realization that each of us has vastly more genealogical ancestors than genetic ones. Genealogical ancestry refers to every single person whom you can claim as a biological ancestor. Generally, that mean two parents, four grandparents, and so forth, doubling with each generation you go back. In just a few generations, the number of ancestors you have becomes enormous. However, most of those people have not contributed any DNA to you. This is because the DNA molecule, although made up of millions of base pairs, is not divisible just anywhere. During reproduction, it divides into discrete chunks, which are passed on as units. (The number of chunks involved is somewhat larger than the number of chromosomes you have, but not immensely so.) This means that just a few generations back, it’s impossible for most of your genealogical ancestors to have contributed any DNA to you. This is even more pronounced if you limit your search to certain portions of your DNA. Mitochondrial DNA, for example, only passes from mother to child, meaning that only one person in each generation is your mitochondrial ancestor.

Genealogical ancestors, as I mentioned above, double in each generation. So after 40 generations, or about 1,000 years if you consider a generation to be 25 years, you would have over one trillion ancestors. This is, obviously, far more people than have ever lived, so equally obviously, a large majority of your ancestors that far back will be people you are related to along more than one line. I have the same number of ancestors you do, so no matter how far different we seem to be, it’s very likely that within just a few hundred years, we start having ancestors in common.

In 1999, Joseph Chang published the results of a computer simulation in which he calculated that everybody alive on planet Earth was related through an ancestor who lived roughly 700 years ago. Chang used a very simple model, which didn’t take into account that some populations are more isolated than others. A more careful study in 2004 by Rohde, Olson, and Chang refined the date of the most recent universal genetic ancestor (MRUGA) to about 2,000 years ago, and this finding seems to be holding on pretty well. Even more interesting is the identical ancestors point (IAP). It should be apparent to anybody thinking about it that the further back in time you go, the more ancestors we have in common. The IAP is the point in time where everybody alive on the planet either has no descendants in the present day, or is an ancestor of everybody alive. That point comes roughly twice as far back as the MRUGA, or about 4,000 years.

This finding drives the last nail into any biological argument in favor of racism. Every single one of us is related to every other one of us so recently that any idea of race is simply absurd. It’s long been observed that human biological variation does not cluster into identifiable races, and genealogical ancestry reveals why that’s the case; no human population has been isolated anywhere near long enough for separate races to have evolved.

This finding also led computational biologist S. Joshua Swamidass to an even more interesting finding, with theological implications. But that’s a subject for another post.

 

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