Mar 08 2013

Father of Us All

Each time a new genetic analysis dates the time to the most recent common ancestor (MRCA) of humans, we get headlines proclaiming that the “mother,” “father,” or “ancestor” of all humanity has been discovered – commonly referred to as “Eve” or “Adam” when the analysis involved mitochondrial or Y-chromosome DNA respectively. I find the reports to usually be at least a bit misleading, which is not unexpected given that the topic is fairly complex.

This is happening again with the recent headlines, “Father of all humankind is 340,000 years old.”

What they are talking about is a new study looking at the Y-chromosome of a particular African American male whose family submitted his DNA for ancestry analysis. It turned out to be a very rare type, and in fact represents an ancient Y-chromosome ancestry. Understanding what this means, however, requires a bit of background.

Humans are a very large population with a complex history, and it is difficult to trace our complex ancestry back in time and make meaningful statements about common ancestors. It’s even harder to have an intuitive grasp of how genes flow through populations over long periods of time.

Imagine a tree of your own ancestors. You have two parents, four grandparents, eight great-grandparents, etc. This is a geometric progression, doubling the number of ancestors every generation. If you go back 30 generations, to the middle ages, you have over 1 billion ancestors, which is greater than the number of people living at that time. This apparent paradox is resolved by realizing that as you go back generations, ancestors are counted increasingly numerous times. You can probably trace back to each 10th century ancestor through hundreds of pathways.

Another layer of complexity comes from the fact that our genes are a mixture of genes from our parents, who in turn each have a mixture of genes from their parents. It therefore becomes tricky to trace ancestry by looking at the complex assortment of all of the genes.

Another way to trace ancestry is through a single gene – really you are tracing the ancestry of that gene, rather than of any person or population. Autosomal genes (those that are found on chromosomes other than the X or Y sex chromosomes) are still tricky to trace because they don’t have a linear ancestry, because of the random mixing of genes from parents.

Geneticists therefore like to focus on two types of DNA. Mitochondrial DNA (mtDNA) is transferred only (well, mostly, but it can be treated as only) from mothers, and so you can use mtDNA to trace a linear matriarchal line. Y-chromosome DNA, likely, is only transferred from father to son, and so can be used to trace a linear patriarchal lineage.

In either case, as you go back in time from one specific mtDNA or Y-chromosome gene there will be fewer and fewer ancestors with that gene until you get to one person with that particular version of the gene. This person is theme deemed the “mitochondrial Eve” or the “Y-chromosome Adam.”

The mitochondrial Eve dates back to about 200,000 years ago. The new study discovers a more ancient form of Y-chromosome then was previously known, and this version dates back to 340,000 year ago. (Obviously the genetic Eve and Adam never hooked up.)

In neither case does it mean that this individual is the sole ancestor to all humanity, or that they were a literal couple, or even that there was a population bottleneck at this time.

The dating comes mostly from looking at known mutation rates and then calculating how much time it would have taken for the mutational differences that we see to have emerged.

The single gene MRCA is also very different than the genealogical MRCA – which is closer to what most people likely think of as a common ancestor. The genealogical MRCA cannot be calculated directly from genetic analysis. Rather, computer models, and historical models of population isolation, migrations, and mixing need to be taken into consideration. Genealogical MRCA can also change over time as populations mix.

There are several estimates of the genealogical MRCA, but these are all only a few thousand years ago – about 3-5 thousand year. That is much closer than the genetic Adam and Eve. This is partly based on the assumption that historically isolated populations, like some South American tribes or the Australian Aborigines, now have genes mixed in from 16th and 17th century explorers. This is how the MRCA can move closer in time, as we mix our genes we gain the same common ancestors.

The date for the genealogical MRCA can also be move farther into the past if we confirm a truly isolated population with no mixing of outside genes.

There is another related concept to all of this called the identical ancestors point. This is the point beyond which all humans (or members of a species) essentially have the same ancestors. It is no longer meaningful to talk about separate populations or genealogies. This is usually a little further in the past than the genealogical MRCA, in the case of humans, about 5-15 thousand years ago. From this point back, all the way to the slime out of which our ancestors crawled – we all share the same ancestors.

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11 responses so far

11 Responses to “Father of Us All”

  1. PharmD28on 08 Mar 2013 at 3:16 pm

    well, I cannot say I really follow it all, but I certainly have a greater sense for how much more complex the issue is….

  2. BillyJoe7on 09 Mar 2013 at 4:56 am

    The elephant in the room.

    Steven Novella: “The mitochondrial Eve dates back to about 200,000 years ago. The new study discovers a more ancient form of Y-chromosome then was previously known, and this version dates back to 340,000 year ago. (Obviously the genetic Eve and Adam never hooked up.)”

    And that, therefore, unless the error bars of both estimates is at least 140,000 years, there was no Adam and Eve and that, therefore, JC, if he existed, died for a mere metaphor, thereby invalidating all of the Christian religions.

    |:

  3. MarcusGPon 09 Mar 2013 at 8:31 am

    Not only the Christian religions, but all the Abrahamic religions. Could push it further and say all creation myths.

    Not that that is the point, eh?

  4. almostmedicineryon 09 Mar 2013 at 8:40 am

    “And that, therefore, unless the error bars of both estimates is at least 140,000 years, there was no Adam and Eve and that, therefore, JC, if he existed, died for a mere metaphor, thereby invalidating all of the Christian religions.”

    Because that’s what gave it away, wasn’t it?

    Frankly, if we expect Genesis to be right, then the farthest ancestor we could trace anyone to (with this method), would be Noah. So if Genesis was right, we’d be confusing Noah with Adam.

  5. almostmedicineryon 09 Mar 2013 at 8:46 am

    Correction, the farthest, male ancestor. Noah’s sons’ wives might have had a common traceable female ancestor. If it was true.

  6. norrisLon 11 Mar 2013 at 12:11 am

    I did once calculate the number of offspring from a single couple whose first child was born in 1000AD, allowing 3 surviving children per generation to each produce 3 further offspring, and allowing 3 generations per century. So by 1100AD there were 27 surviving offspring, and so on. If anyone else wants to calculate that through to 2000AD, go ahead and let me know what you come up with.
    The final number at 2000AD is probably greater than the current population of the world.

  7. The Other John Mcon 13 Mar 2013 at 8:07 am

    I always thought it was interesting people don’t ever seem to trace their geneologies back very far, until I realized that we have 2 parents, 4 grandparents, 8 great-gps, 16 great-great-gps, then 32, then 64, then on and on and on.

    Makes it real tough to back for more than a few generations just as a practical matter. And by then, you are just as related to a guy on the street then to some of your distant great great greats, so it’s tough to even care at that point. This also makes the historical attempts at “preserving bloodlines” by kings a spectacularly silly idea, forever doomed to failure. If only they did a little math.

  8. Bill Openthalton 13 Mar 2013 at 10:19 am

    John –

    The “kings” did manage to breed for a number of disorders such as haemophilia, prognathism, … :)

  9. Jared Olsenon 14 Mar 2013 at 6:10 am

    Not sure what you mean by

    “This apparent paradox is resolved by realizing that as you go back generations, ancestors are counted increasingly numerous times.”

    I’m hopeless with numbers and stats…

  10. Murmuron 14 Mar 2013 at 6:42 am

    Jared, what this means is that the further you go down your history, the more likely it is that your ancestor on one branch of your tree is the same person as on another branch of the tree.

    One of your mother’s great great great great great great grandfathers could very well be your father’s great great great great great great grandfathers too. At six generations you have 64 great^6 grandfathers, and historically people did not travel as far so it is likely ancestors are shared even more closely than this. This is only going back 200 years or so, once you start going back 600, you have over a million grandfathers and pretty soon your grandparents would outnumber the population of the planet if your branches had not intermingled at some stage.

    It is much easier to show graphically, which unfortunately I can’t through this medium.

    Even as close as the Victorian age, cousins marrying cousins was not frowned upon, and in Westeros not too long ago a Targaryen brother would marry his sister without batting an eyelid… or am I mixing reality with fiction again?

  11. Jared Olsenon 15 Mar 2013 at 5:09 am

    Murmur, thanks for the clarification, I get it now…

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