One of the challenges for discussing evolution within evangelical Christian circles is that there is widespread confusion about how evolution actually works. In this (intermittent) series, I discuss aspects of evolution that are commonly misunderstood in the Christian community. In this post, we tackle the issue of why “Mitochondrial Eve” and “Y-chromosome Adam” are not an ancestral couple from whom all humans descend, as claimed by the Old-Earth Creationist organization Reasons to Believe.
It is reasonably well known among evangelical Christians that all living humans trace their mitochondrial DNA back to a single woman (a so-called “mitochondrial Eve”) and that all living males similarly trace their Y-chromosome DNA back to a single male (a so-called “Y-chromosome Adam”). These individuals are commonly assumed by evangelicals to be the Biblical Adam and Eve, the first humans alive and the progenitors of the entire human race. While most young-earth and old-earth creationist organizations make this claim, perhaps one of the best-known organizations to do so is the old-earth creationist / anti-evolution organization Reasons to Believe, who have produced numerous articles, podcasts, and even entire books on the subject.
In contrast to this common evangelical understanding, the scientific picture is rather different. Mitochondrial Eve, though the most recent common matrilineal ancestor of all humans, was but one of a large population living about 180,000 years ago. So too for Y-chromosome Adam: he was also a member of a large population, and he lived about 50,000 years ago. As has been discussed several times here at BioLogos, there are multiple lines of evidence that indicate the human population has never been below around 10,000 members at any time in its history: we branched off as a large population to form our own species.
When presented with the evidence for human population sizes over our evolutionary history, a common point of confusion for evangelicals is how this evidence fits with Mitochondrial Eve. How can we all come from one woman (and one man) but also come from a large population of 10,000 individuals? Aren’t these two observations in conflict?
The answer is no, these lines of evidence fit together. Humans do come from a large population, and all present-day humans do inherit mitochondrial and Y-chromosome DNA from specific individuals in the past. The reason for the apparent discrepancy lies in how mitochondrial and Y-chromosome DNA are inherited, as we shall see below.
Mitochondria are organelles responsible for energy conversion, and they contain their own small, circular chromosome that they replicate apart from regular chromosomes in the cell nucleus. Mitochondria are not passed on to progeny through sperm, but only through the egg: as such, mitochondrial DNA is passed on solely through the maternal line. Consider a small pedigree (family tree) below. Circles represent females, males are represented with squares. In this family, one grandmother (the woman at the top right of the pedigree) has passed on her mitochondrial DNA to her sons and daughter, but only her daughter passes it on to the next generation. All individuals who have this grandmother’s mitochondrial DNA are shown in blue:
Conversely, if we examine Y-chromosome inheritance in this same family, we would see that (obviously) women cannot pass it on to their children. Here, the red lines show all males who have descended from a grandfather of the family (the male at the top left of the pedigree):
Now we are ready to examine how these types of DNA are inherited in a larger group, and compare their modes of inheritance with regular chromosomal DNA. While it is not possible to draw out a pedigree for a population of 10,000 individuals, let’s examine a smaller group to see how a specific mitochondrial sequence can “take over” a population of organisms (note that this effect applies to other organisms besides humans that use an XX – XY system of sex chromosomes).
In the family tree below, three mitochondrial DNA variants are present in the first generation (the top row of the pedigree) and a represented with different colors (green, blue and red). Tracing the inheritance of these mitochondrial DNA versions through the family tree shows that all living members of this population (the bottom two rows) have inherited the red version only. The blue and green versions eventually hit a dead end where they were not passed on (either through females who did not have children, or males). As such, all living individuals can trace their mitochondrial DNA back to this group’s “mitochondrial Eve," the woman at the top right of the tree with the “Mito 3” variant.
Let’s now examine Y-chromosome inheritance patterns in the exact same family tree. Suppose there are three Y chromosome variants present in the first generations:
Here we can see that the current population has inherited its Y-chromosome DNA from one individual as well (variant 1, the red lines) and that the other Y-chromosome variants (blue and green) hit dead ends through males that did not reproduce or men who only had daughters. All living members of the population trace their Y chromosome DNA back to an individual (filled in with yellow) who lived two generations after their most recent matrilineal common ancestor (the woman at the top right).
Now we are ready to examine regular chromosomal inheritance in this same family tree. Genetic variation on chromosomes other than the Y can be passed through either gender without problem, and individuals can have two variants at a time (one on the chromosome inherited from mom, the other on the chromosome inherited from dad). These key differences (compared to how mitochondrial DNA and Y chromosomes are inherited) produce a very different effect. In this same family, numerous variants (represented by the different colors) have been transmitted to the present generation without loss:
Notice the middle couple in the first generation in the pedigree. This man’s Y chromosome did not make it to the present day, and similarly his wife’s mitochondrial DNA did not make it either (scroll up to see this if you need to refresh your memory). So, they contributed nothing to the current generation, right? Not at all: both of them have passed on regular chromosomal variation to the present day (traced as blue and black lines).
In other words, it would be incorrect to examine this population, determine (correctly) that they share common mitochondrial DNA and Y-chromosome ancestors, and then go on to conclude that these two individuals were an ancestral pair that started this entire family. We know that this group descends from a larger population, because genetic variation in the present population is too large to explain as coming from one pair (there are five colors, or genetic variants in this population, and the max any one pair could carry is four, with two each).
While this example examines a small family, the same principles apply to larger groups: mitochondrial and Y-chromosome lineages, though interesting, cannot be used to estimate population sizes over time. For that type of work, regular chromosomal variation should be examined. Present day human genetic variation indicates that though we all share a common mitochondrial DNA and Y-chromosome source, these individuals came from a population of at least 10,000 individuals, and that they lived over 100,000 years apart. If you are interested in examining the evidence for human population sizes, Darrel Falk and I have discussed it previously.
In summary, anti-evolutionary groups, such as Reasons to Believe, that claim that the evidence for Mitochondrial Eve and Y-chromosome Adam supports an ancestral couple for the entire human race are not interpreting the data correctly. They have failed to account for the unique pattern of inheritance these types of DNA have in populations.