Mitochondrial Eve, Y-Chromosome Adam, and Reasons to Believe
Today's entry was written by Dennis Venema. You can read more about what BioLogos believes here.
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.
Photo courtesy of Lewis Schofield.
Dennis Venema is Fellow of Biology for The BioLogos Foundation and associate professor of biology at Trinity Western University in Langley, British Columbia. His research is focused on the genetics of pattern formation and signalling.
November 1st 2011
Jimpithecus, thanks for clearing that up for me.
Reply to this commentSteve, yes I misspoke (or miswrote). Our mtDNA and Y-chromosome DNA is not all the same, but can be traced to common ancestors. Yes, the difference in dates (Eve earlier than Adam, I believe) should be a problem for RTB. I would like to see a response from them on that, unless they want to use the same, “Genetics is an inexact science when determing exact history or population sizes.”
But once we correct my imprecision, my point (or Fuz’s point) still stands: If the founder population was two people, then the probability that our mtDNA and Y-chromosome DNA can be traced back to two common ancestors is 100%. And I agree that Fuz still has the problem of explaining why the rest of our genomes does not show the appropriate amount of diversity. But once again, Fuz would say that Genetics is an inexact science when determining population sizes.
Argon, I didn’t fully understand your point, but I think it’s mistaken. 10,000 years from now we should still be able to trace our mtDNA and Y-chromosome DNA back to the same two common ancestors, who either won out in the competition, or who were the first two humans, whichever interpretation is correct.
November 1st 2011
Darrel,
Reply to this commentI’m disappointed that you haven’t responded to Fuz’s arguments. In a thread where you and Dennis were determined to make RTB look bad, you’re not doing so well, so far.
November 2nd 2011
With respect to Fuz’s arguments with regard to Orangutans, it’s true all else being equal, that we expect larger populations to have more genetic variation so it is a surprise that Sumatra orangutans have such a high genetic diversity. However, I think it’s obvious that the small present day population size is relatively recent. If they have dramatically decreased in population size over the last 1000 years, from a larger population, then they would not have lost much of their original diversity yet.
November 1st 2011
Argon,
Reply to this commentI forgot to say that it is not at all clear to me why only one female and one male will win out as common ancestors. It seems at least as likely, if not much more likely, that if there were many different mtDNA and Y-chromosome DNA among the founder population, that we would be able to trace our lineage back to many of them.
November 1st 2011
November 2nd 2011
Bilbo writes: “it is not at all clear to me why only one female and one male will win out as common ancestors. It seems at least as likely, if not much more likely, that if there were many different mtDNA and Y-chromosome DNA among the founder population, that we would be able to trace our lineage back to many of them.”
This means that Dennis and I and others have failed to communicate some very basic concepts to you. Let me try a different tack to see if we can make some progress.
You appear to picture the scenario as involving individuals “winning out.” This is wrong, and that was one of the plain themes of Dennis’ review. I strongly recommend that you re-read it. What Dennis is describing is NOT one individual winning out. Look at the pedigrees. What is “winning out” is not a person but a piece of DNA.
Here’s another way to look at this, adapted from a blog post I recently wrote on the subject. Suppose that one individual in a small population (of, say 1000 breeding individuals) bears a new mutation that confers a reproductive advantage. Let’s call this person Adam. Suppose that mutation, as a result, becomes more common in the population as its advantage is felt, and that 20 generations later, most of the individuals in the population have that form of the gene. And then, perhaps 100 generations later (about 2500 years), that form of the gene is “fixed,” meaning that essentially every individual in the population carries it.
This is how gene forms get fixed. The numbers are arbitrary, and my scenario involves selection instead of drift. But my scenario is the basic outline of how evolution of gene forms happens, whether in fish or finches or sunflowers or people. In my opinion, if someone were then to express skepticism regarding whether that one founding individual could have “won out,” I would conclude that they were missing something very, very basic. And I would encourage them to read more science, more basic genetics, before they read anything written by RTB or the DI. That’s my recommendation to you.
BTW, regarding my scenario above, you might wonder how it can be that the gene in Adam can become fixed without everyone in the population being essentially identical to him. In other words, everyone in the population has Adam as a common ancestor, and yet the population is still diverse. How can this be? As you consider the answer to the question, you will approach a better understanding of how population genetics is misused by Rana and others to create the illusion that the 2-founder hypothesis is reasonable.
Reply to this commentNovember 3rd 2011
Steve - a slight quibble: it’s a shame you call your hypothetical mutation-donor “Adam”, rather than Sydney or Wendy. It seems to me that the heat (and lack of illumination) in this discussion comes from correlating our mitochondrial donor with “Eve” and our Y donor with “Adam”.
Reply to this commentThis gets Creationists looking to map Genesis as history to the palaeolithic, TEs looking to map Genesis as myth to the palaeolithic and everybody talking about biology rather than covenant or salvation history, as if the former were what Genesis cares about most.
It was, after all, the scientists who first confused the issue for lay-people by giving the impression, via the terms “Y-Adam” and “Mitochondrial Eve”, that the findings had anything to do with human origins rather than the merely fortuitous (or adaptive) survival of the few big chunks of DNA that happen to be traceable, from random points in the race’s pre-history.
And maybe the Creationists are right to suspect that some of that confusion was sown deliberately to further a secularist agenda. “You have your fairy-tale orgins stories - but see here - genetics reveals their falsity. Adam is just one chromosome, and Eve just one mitochondrium amongst many.” The field, after all, is hardly free from polemic.
November 2nd 2011
For anyone local to the Vancouver / Lower Mainland area, tomorrow evening (Thursday) Fazale Rana of RTB and I will be having a discussion at 7pm at Regent College (RTB is renting the space, it’s not an official Regent event). This session will be on the evidence for evolution, human evolution, etc. Then, on Saturday we are having a second discussion discussing population genetics / Adam & Eve, etc - this one is at 10th Avenue Alliance Church, also at 7pm. All and sundry are welcome.
Reply to this commentNovember 2nd 2011
Steve,
Reply to this commentIf you want to make the claim that there was some sort of selective advantage for the mtDNA or Y-chromosome DNA that all of us have a descended form of, fine then I get the argument. But then I think it would be up to someone who made that claim to provide some sort of supporting evidence for it.
If you want to say that it was just a matter of genetic drift, fine I get that argument, also. But if it is just a matter of genetic drift, then the likelihood that only one kind of mtDNA and one kind of Y-chromosome DNA became the sole ancestors of every human being is rather small. If you disagree with this, please explain why.
November 2nd 2011
No, Bilbo, there’s no need to explain either of those mechanisms. The point is not whether selection or drift is more or less likely to have caused one form of mtDNA or Y-chromosomal DNA to have been fixed. Both mechanisms are well established, and so basic that I will direct you to some basic readings on the topic if you think that would help. There is no need to provide “supporting evidence” for the ability of selection to drive alleles to fixation. Ditto for drift, which is not at all unlikely to contribute to fixation in small populations. (Of course, both can contribute.) The point is that no knowledgeable biologist would find any of this to be remarkable, and no competent biologist would make the mistakes that RTB makes. Again, if you really do want references for the effects of drift and/or selection, let me know.
November 2nd 2011
Ashe,
Reply to this commentI certainly don’t understand your point. What makes me think that the orangutan example is a problem for population genetics is the response of William Amos, geneticist at the University of Cambridge, who said (regarding the orangutan example),
“We don’t fully understand the relationship between genetic diversity
and population size.”
November 2nd 2011
That quote was already explained by Darrel Falk in comment #65894 did you not understand it?
Reply to this commentNovember 2nd 2011
Steve,
Reply to this commentI better be more precise, or you’ll jump on my imprecision as evidence that I don’t understand the topic. When I said, “...then the likelihood that only one kind of mtDNA and one kind of
Y-chromosome DNA became the sole ancestors of every human being is
rather small….” I meant that only one kind of mtDNA and one kind of Y-chromosome DNA became the sole ancestors of every kind of mtDNA and Y-chromosome DNA that every human being possesses.
November 2nd 2011
Bilbo, I do think it’s pretty clear that you don’t understand the topic. I don’t think that’s a bad thing, and I don’t think it’s an insult for me to tell you this. What we’re trying to accomplish (this is my view, anyway) is to explain some fairly demanding genetic concepts in the presence of multiple misunderstandings. At this point, it’s clear that your grasp of the relevant concepts is weak enough that you aren’t able to see the relevance of the original post (by Dennis) or the meaning of the claim that mito Eve and Y-chr Adam were not contemporaries.
November 2nd 2011
Dennis,
Reply to this commentSince I won’t be in Vancouver, feel free to explain here why you think Fuz’s arguments are wrong. Apparently Darrel is too busy to do so.
November 2nd 2011
Bilbo, have you drawn out the pedigrees like John suggested above? That was an excellent suggestion. That and the post above is all about showing why Rana’s view is not tenable. There is a well-known, high-probability mechanism that accounts for mtDNA and Y chromosome coalescence that maintains high levels of autosomal diversity. We have multiple lines of evidence that indicate a population size of ~10,000 or more as far back as we’d like to go. Do you really think one misunderstood quote on Rana’s part changes all that? Rana’s comments don’t even deal with several independent means for estimating population sizes over time (Linkage Disequilibrium studies, which assume NO mutation, for example).
Reply to this commentNovember 2nd 2011
When are you guys coming to the northeast , I would pay for that, seriously
Reply to this commentNovember 3rd 2011
Theology does not include science comletely, for science is in development always to remove wrong visions from imperfect reasoning. Also science is not able to observe the spirit or the concious that together form the soul. But the fear of Jahveh is the beginning of all wisdom.
This is what Jesus Christ our lord taught me as a servant of him:
Reply to this commentGod created the first man Adam almost one million years ago. In the male fruit of a female man-ape He changed some chromosomal DNA to give man speach. This DNA is not in the X-, Y- or mitochondrial DNA. Also He desided that the human soul had to become indestructible. This is not by changing some DNA, because the holy spirit knits the elemental particles together with bosons for every individual and so the holy spirit makes our soul indestructible.
Then Adam had to get a wife. God planted the speach giving DNA from Adam in the female fruit of another female man-ape and Eve was created.
From then on the human descendants from Adam and Eve spread over the earth.
God is well able to protect people against being whiped out, if needed. It is written that He is well capable of bringing a man to the other side of the sea on a piece of wood.
Mutations in the Y-chromosome and the mitochondrial DNA occured in the evolutionary way..
This teaches us that the coming in existance of the human kind depended on an act of God and could not occur through evolution by chance, because the connection between elemental particles by bosons is not a natural proces, it is done by the holy spirit for every single connection in every single creature.
November 3rd 2011
Ashe: “That quote was already explained by Darrel Falk in comment #65894 did you not understand it?”
Reply to this commentThis is why I hate the “Reply to this comment” system that is used here. I didn’t even though that Darrel had made that comment until you pointed it out to me. Thanks. Unless one has the time (and I certainly do not) to continually go back and find and review all the comments that might be relevant to the discussion, one can miss very important information. If I were in charge here, I would eliminate this system immediately.
November 3rd 2011
Darrel,
Reply to this commentThanks for your comment at #65894, which it took me a while to find, since comments here are not necessarily in numerical order. OK, Fuz was quoting from an article (I think it was Science Daily) that was quoting from Nature. So it was the Science Daily article that originally took the quote out of context. I suspect that Fuz never checked the Nature article. But he should, and I agree with you that he should retract or at least correct his comments on it.
November 3rd 2011
Steve,
Reply to this commentit is clear to me that even though I understand this topic, you do not understand me. I am not surprised. At your own blog, it was clear that you have an inability to understand anyone with whom you have the least disagreement. I see no point in us trying to communicate with each other anymore.
November 3rd 2011
Dennis,
Reply to this commentYou suggest I follow John’s suggestion: “Start with 3 breeding pairs with different mtDNAs and Y chromosomes and
randomly mate a subset of their offspring through several generations.”
It appears to me that the problem with this suggestion is that the results would be based on a very small, and perhaps misleading sample size. For example, I could toss a coin 10 times and get 7 heads. From this I might incorrectly conclude that if I tossed the coin 10,000 times, I would get 7,000 heads. Likewise, starting with 3 breeding pairs, instead of starting with 5,000 breeding pairs, might give a false sense of what the probabilities are for 5,000 breeding pairs.
So let me ask you: Has anyone done actual probability calculations, based on a population size of 10,000 humans, to see what how likely it would be to get down to one mtDNA ancestor and one Y-chr DNA ancestor?
November 4th 2011
Bilbo, I’ve seen you comments but my connection has been difficult. While it’s up…
November 3rd 2011
And finally, to either Dennis or Darrel,
Reply to this commentHave either one of you addressed Fuz’s example of the mouflon sheep on the island? Did I miss that response, also?
November 4th 2011
Reply to this commentNovember 4th 2011
[continued from previous comment]
November 4th 2011
In comment #65993, Bilbo asks, “Have either one of you addressed Fuz’s example of the mouflon sheep on the island? Did I miss that response, also?”
November 23rd 2011
We see this kind of “balanced polymorphism” in humans as well, with the sickle cell anemia/malaria problem. There is selection for the heterozygote because it has the best chance of beating sickle cell anemia and yet not contracting malaria.
Reply to this commentNovember 8th 2011
I’m not a Young Earth Creationist, but I was curious whether any of them had written anything on this topic. Robert Carter has:
Reply to this commenthttp://creation.com/historical-adam-biologos
I don’t know if his views are at all valid, but they were at least interesting.
November 9th 2011
The heart of his argument seems to be figure 2. Genetic drift does tend to result in differences between populations, but human populations are, despite what many people think, extremely similar to each other at the genetic level. One measure of differentiation, called Fst, which measures the amount of ‘total’ genetic variation that is due to between-population differences, is about 10% in humans, i.e. pretty small. So we don’t expect big differences between populations, except where there has been local selection for different genetic variants - e.g. lactase or the Duffy blood group protein.
Reply to this commentDecember 13th 2011
I doubt its as easy as that. There is evidence of extra-terrestrial seeding of life, even cohabiting with humans which science is yet to investigate. Recently there is also some speculation that junk dna is ET in origin. I’m not sure about the reliability of that story. But there are enough smoking guns in the legends of various lands of ET activity in the past. The Dropa people in china is one such example. Indian legends speak of many kinds of humanoids who interbred with humans, viz. the asuras, the devas, the nagas, the apsaras, rakshasas, etc. etc. The lakshman rekha was a psychic boundary drawn by lakshman to protect his brother’s wife seetha, who was desired by the rakshasa named Ravanna. The rakshasa way of marriage was by taking away desirable women by force. These are not myths. There are people who still follow these customs. Unless all these cases are investigated, I don’t think it is going to present a clear picture of our descent.
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