Mitochondrial Eve and Y Chromosome Adam

In the last post in this series, we had arrived, at last, at the origins of our very own species, Homo sapiens. Based on the oldest-known remains in the fossil record, we know that anatomically modern humans were present in Africa at about 200,000 years ago. From this starting point, our species was poised to expand its range into Asia and Europe, starting around 100 KYA (i.e 100 kiloyears ago, a shorthand for 100,000 years ago), and in significant measure from about 50 KYA. In doing so, we would follow, and later encounter, other Homo species that had left Africa before us. Such groups include the Neanderthals and Denisovans, as well as Homo erectus, which, as we have seen, also left Africa and was distributed widely in Asia, including the populations in Indonesia that would form the basis for Eugene Dubois’ seminal discoveries. Neanderthals and Denisovans share a common ancestral population at about 400 KYA, though it is not yet clear if their common ancestral population left Africa or if their lineages separated in Africa and both groups migrated out independently. As for Homo erectus, fossil remains establish that it was widely distributed in Asia as far back as 1.8 million years ago.

It is around this time that we reach a point in human evolution that many evangelicals have at least heard about—the last common female ancestor of all modern humans, popularly known as “mitochondrial Eve”, and the male equivalent—our last common male ancestor, popularly known as “Y-chromosome Adam”.

Mitochondrial Eve and Y-chromosome Adam: common ancestors, but not unique ancestors

Wait just a second, you say—isn’t the evidence strong that modern humans descend from a large population (and as such, is a topic of significant theological consideration)? How is it, then, that all humans can share a single woman and single man as common ancestors? The short answer is that all humans do share a single man and single woman as common ancestors—but that these ancestors are not our unique, or sole, ancestors. Rather, they both come from that large population – the evidence for which (and the theological questions it raises) we will discuss in upcoming posts.

Understanding how humans can have single maternal and paternal ancestors within a genetically diverse population requires us to take a brief excursion into genetics, and specifically how certain forms of DNA are inherited. Our mitochondria have their own small chromosome as a remnant of their time as free-living bacteria. In humans, mitochondria are passed down only (with rare exceptions) from mother to child: sperm do not donate mitochondria to the fertilized egg. As a result, mitochondrial DNA is inherited through the maternal lineage only, in contrast to regular chromosomal DNA, which is inherited through both maternal and paternal lineages. The maternal-specific pattern of inheritance for mitochondrial DNA lends itself to certain mitochondrial variants “taking over” a population, which we can illustrate using a large family tree, or pedigree. (A note about pedigree symbols: circles represent females; squares represent males; a horizontal bar connecting them represents a mating; and a vertical bar from a mating is connected to the offspring of that mating.)

In the pedigree below, we see a large extended family that shows the inheritance of three mitochondrial variants (labeled with different colors). In order to keep the pedigree compact enough to show, the dashed lines indicate matings that connect to one another by wrapping around from one side to the other. As we can see, the red “Mito 3” variant has taken over, or “swept” this population. All individuals in the most recent generations of this family share the woman at the top right as a common ancestor for their mitochondria:

Using the same pedigree, let’s now trace some hypothetical Y-chromosome variants. Y chromosomes, obviously, are passed only from father to son, giving a paternal-specific pattern of inheritance. This pattern, like the maternal-specific pattern for mitochondrial inheritance, can also lead to certain variants easily sweeping a population. Let’s suppose that this same family also has three Y-chromosome variants in the oldest generations:

In this case, the “Y chromosome 1” variant sweeps the population, and everyone in the most recent generations has the man highlighted in yellow as their most recent, common male ancestor.

Now that we have identified a common female and male ancestor of the most recent generations in this pedigree, we can illustrate that they are not their unique ancestors. Both the mitochondrial “eve” and Y-chromosome “adam” of this family come from a larger population—and we can easily show this by looking at variation present on regular chromosomal DNA—the kind passed down through both maternal and paternal lineages.

Let’s return to the exact same pedigree, but now illustrate variation on regular chromosomal DNA with different colors. It is now much harder for this variation to sweep a population in short order, because this variation can be passed on by both males and females: