Mar 08 2013
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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