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.
October 28th 2011
“Mitochondrial Eve, though the most recent common matrilineal ancestor of
Reply to this commentall 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. “
Yes that would be the hominins from Herto or something like them. These are the first hominins that are reasonably modern in appearance. They date to between 170 and 180 ky BP.
October 28th 2011
Nice article, but by now a headline of ‘RTB mangles science’ is about as unsurprising as ‘dog bites man’ or ‘Pope is a Catholic’. Anybody who takes even a cursory glance at the way they present scientific research, and compares the claims they make with the actual scientific articles they cite, will know that they long ago gave up any pretence to scientific credibility. I would rank Hugh Ross as probably the single largest source of bogus information, specious factoids, and misquoted proof texts that I have encountered. Most of the prominent YEC leaders are paragons of scientific scholarship and probity in comparison.
Reply to this commentOctober 28th 2011
The astounding thing is that, once upon a time, when he dealt only with astrophysics, he was a shining light in Old Earth Creation. While it is true that he never accepted evolution, that was only a small aspect of his overall message and was always subjugated to his attempts to get people to understand the evidence for and old universe. When he came to the University of Tennessee in the late 1980s, his entire message was astrophysical in nature.
Reply to this commentThe problem is that he has attempted to branch out into an area of which he knows nothing. Worse, because he does not know the first thing about evolution, he has hired someone to do the genetics pieces who is quite willing to twist the information to suit the purposes of RTB, and Dr. Ross doesn’t know better.
October 28th 2011
i>The problem is that he has attempted to branch out into an area of which he knows nothing. Worse, because he does not know the first thing about evolution, he has hired someone to do the genetics pieces who is quite willing to twist the information to suit the purposes of RTB, and Dr. Ross doesn’t know better.
October 29th 2011
Steve, what do you mean by the key concept being anointing?
Reply to this commentOctober 29th 2011
Steve was probably referring to the article written by Giberson and Stephens, The Evangelical Rejection of Reason, which you can read here:
Reply to this commenthttp://www.nytimes.com/2011/10/18/opinion/the-evangelical-rejection-of-reason.html?_r=2
October 29th 2011
Summarized in this HuffPo piece. I haven’t read the book yet.
October 29th 2011
Dennis,
Reply to this commentWhen criticizing or commenting on somebody else’s articles or opinion, it is proper to give a reference to their work. You haven’t. For those interested:
http://www.reasons.org/were-they-real-scientific-case-adam-and-eve
I’m curious about an article that is referred to by RTB:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1766376/
It’s not clear from the article exactly how large they would have estimated the founder population of sheep to be, but they admit that it was larger than the actual number (which was two). I would be curious if this would have some relevance to the estimate of the founder population of the human species.
November 1st 2011
October 29th 2011
Steve,
Reply to this commentGiven how quickly you misinterpreted my questions at your own blog, I doubt whatever interpretations you give of other people whose views you object to.
October 29th 2011
RTB also has an interesting, relevant article here:
Reply to this commenthttp://www.reasons.org/human-origins/adam-and-eve/orangutan-genetic-diversity-sheds-light-humanitys-origin
October 29th 2011
Mathesson’s comment may be referring to this from the NY Time article:
October 29th 2011
Hi Hornspiel, I did not claim that BioLogos in general missed that point. I referred to recent critiques, and was thinking of Mark Mann’s recent post which focused in on a distinction between “secular knowledge” and Christian faith. Mann suggests that Giberson and Stephens make such a distinction, but I think he may be misinterpreting them. I suspect that Giberson and Stephens are pointing to the overwhelming evangelical tendency to make such (false) distinctions. And that tendency helps to undergird the anointing of particular evangelical “experts” who are empowered to feed evangelicals a steady and ruinous diet of pseudoscholarship.
October 29th 2011
Thank you Steve for your reply. After rereading Mann’s post of Oct 26 I see what you mean. There is only one sentence in Giberson and Stephens’ NT Times piece that even uses the words secular and integrated:
Reply to this commentOctober 29th 2011
There is something screwy with the editor here. It chopped of the end of my comment above.All I wanted to say to finish is:
October 29th 2011
Okay, guys, perhaps we need to have been more clear. You are correct. The point of Karl and Randal’s piece in the NY Times was not centered on the theme that Mark Mann is developing in his current series. Mark was not doing a critique of the article.
October 29th 2011
Darrel, thanks for the response. I think that Mark’s theme is a very important one, and I reiterate my enthusiastic support for his thesis. The choice of a jumping-off point was a bit unfortunate, but perhaps when/if it becomes clear that Karl and Randall are on the same page on all of this, we can get the focus back onto the dangerous secular/religious dichotomy that Mark seeks to break down. (If I’m reading him right.)
Reply to this commentOctober 29th 2011
From the second RTB article:
“The orangutan species Pongo abelii and Pongo pygmaeus are found on Sumatra and Borneo, respectively. Conservationists have counted around 7,000 to 7,500 P. abelii individuals on Sumatra and around 40,000 to 50,000 P. pygmaeus on Borneo. Both species are endangered, P. abelii critically so. Evolutionary biologists believe that these two species diverged from a common ancestor around 1 million years ago.
As part of the orangutan genome project, the researchers spot-sequenced the genomes of ten orangutans, five from each island, and compared them. To their surprise, they discovered that the orangutans on Sumatra displayed a much greater genetic diversity than did the apes of Borneo. This result is counter-intuitive because generally larger populations display greater diversity
This unexpected result means that geneticists have a limited ability to relate population size to genetic variation within a population. According William Amos, a geneticist at the University of Cambridge, “We don’t fully understand the relationship between genetic diversity and population size.”2
Reply to this commentOctober 29th 2011
October 29th 2011
And from the first RTB article:
“Even though the genetic data traces humanity’s origin back to a single woman and man, evolutionary biologists are quick to assert that mitochondrial Eve and Y-chromosomal Adam were not the first humans. Rather, according to them, many “Eves” and “Adams” existed.7 Accordingly, mitochondrial Eve and Y-chromosomal Adam were the lucky ones whose genetic material just happened to survive. The genetic lines of the other first humans were lost over time.
While this explanation is not out of the realm of possibility, it is highly contrived. It would work if only a few of the first humans reproduced, or were allowed to reproduce. If the data is simply taken at face value, the biblical model is the more parsimonious explanation.”
Reply to this commentOctober 29th 2011
Bilbo,
October 29th 2011
That error is so basic that it should be considered profoundly embarrassing to the person who wrote it. It’s almost rude to post it and to name the author. That’s how bad the error is, and that’s how important it is to encourage RTB to change.
Reply to this commentOctober 29th 2011
Bilbo, that quote from RTB is a good one to compare with my post above: Fuz is claiming that it is “highly contrived” that “the genetic lines of the other first humans were lost over time.” Yet in the post above I show how easy it is to “lose” a mitochondrial or Y-chromosome lineage and have others take over a population. It is not “highly contrived” in any way - it is a natural consequence of how these special types of genetic material are inherited, and every geneticist knows that. It’s not rocket science.
October 29th 2011
Back in the late 1980s, when the mtDNA analyses were coming out in favor of the “out-of-Africa” replacement model of modern human origins, Milford Wolpoff pointed out that researchers were not accounting for stochastic loss of mtDNA lineages and that the trees and resultant dates were too simplistic. As David Maddison later showed (http://sysbio.oxfordjournals.org/content/40/3/355.extract) that was, indeed the case.
Reply to this commentOctober 29th 2011
Darrel’s questions:
1) “So here’s my question: in what way do you think Dennis (and virtually all geneticists, actually) thinks that he has just shown that the position that Fuz outlines here is no longer tenable?”
I assume you mean Fuz’s position that Mitochondrial Eve and Y-chromosome Adam are more simply explained by a 2-person origin than by thousands of Eves and Adams (or their offspring, or their offspring’s offspring, etc.) failing to reproduce females or males at some time, and thus failing to pass on their mitochondria or Y-chromosomes. I think Dennis thinks that he has shown that this is indeed possible, and that since the human population has never fallen below 10,000 people, then this must indeed be the case.
“Which statements in Fuz’s article are clearly (in the mind of almost all geneticists) inconsistent with what Dennis has outlined so clearly?”
That the human population has been lower than 10,000 people.
I would point to the data of the smaller population of orangutans having a larger variation in their genomes than the larger population of orangutans, followed by, ” According William Amos, a geneticist at the University of Cambridge, “We don’t fully understand the relationship between genetic diversity and population size.”
I would also point to the genetic study that estimated that the number of founder sheep on the island was larger than two, when in fact it was two.
October 29th 2011
Oh Bilbo, you are so fortunate that you chose not to major in genetics or worse yet to take a class from me. :) Here was your answer: “I think Dennis thinks he has shown that this is indeed possible, that since the human population has ...etc”
October 29th 2011
October 29th 2011
From the article that Dennis so kindly linked to:
In 1957 a male and female yearling were placed onto Haute Island (an island in the Kerguelen Archipelago). These two sheep were taken from a captive population in France. By the beginning of the 1970s, the number had grown to 100 individuals and peaked at 700 sheep in 1977. Since that time the population has fluctuated in a cyclical manner between 250 and 700 members. Given that the population began with only two individuals (the founder effect), has experienced cyclical changes in the population size, and was isolated on an island, the researchers expected very low genetic diversity (measured as heterozygosity).
Using mathematical models, the heterozygosity of a population can be computed at any point in time from the heterozygosity of the ancestral population (which was known for the original mouflon pair) and the original population size. What the researchers discovered, however, when they measured this quantity directly for the sheep on Haute Island was that it exceeded the predictions made by the models by up to a factor of 4. In other words, the models underestimated the genetic diversity of the actual population.
Reply to this commentOctober 29th 2011
I hope I’m not posting answers to the test.
October 29th 2011
I’ll tune in tomorrow for my grade.
Reply to this commentOctober 29th 2011
I dont know if this will help. Mitochondrial Eve is the ancestor of all living women. She lived 50,000 years ago. But if we went back 10,000 years, and checked to see when the last common ancestor of all the women who were living 10,000 years ago lived, it would not be the same woman. It would be someone who lived much earlier. In fact our version of Eve would be one of the descendants of the prior Eve. But over the next 10,000 years, as some lines go extinct (new lines cannot form) the timing of the last common ancestor becomes more and more modern.
Another way to put this is that it is entirely possible that a woman alive today, (like Beaglelady for example) could end up being the Mitochondrial Eve of a future human population, especially if we happen to undergo another major bottleneck.
Population size will affect the timing, but not whether or not the phenomenon occurs.
Reply to this commentOctober 30th 2011
Darrel: “Bilbo, all you have done is restated the article’s conclusions. You have not demonstrated your knowledge of how those conclusions were reached.”
Reply to this commentOK, I’ll try to explain Dennis’s explanation: The mitochondrial DNA of a woman is passed on only to her female offspring. So if she has no female offspring, her mitochondrial DNA won’t be passed on. She may have male offspring, in which case her genomic DNA will be passed on. Thus, in a population of 10,000 people, assuming half were women, and assuming that each woman had different mitochondrial DNA, it is possible that eventually only one of those women’s mitochondrial DNA will survive in her progenitors, while the mitochondrial DNA of all the other women will cease to exist, since at various places in the geneologies they might have failed to produce female offspring. Dennis gave no indication of how probable such a result would be.
Meanwhile, the Y-chromosome DNA is passed on from the male to his male offspring. If he has no male offspring, his Y-chromosome DNA will not be passed on, even if he has female offspring. Thus, in a population of 10,000 people, assuming half were men, and assuming that each man had different Y-chromosome DNA, it is possible that eventually only one of those men’s Y-chromosome DNA will surive in his progenitors, while the Y-chromosom DNA of all the other men will cease to exist, since at various places in the geneologies they might have failed to produce male offspring. Again, Dennis gave no indication of how probable such a result would be.
October 30th 2011
Bilbo:
October 30th 2011
Bilbo, the MtDNA is passed on to both her male and female offspring but her male offspring do not pass it on to their children. However, her female offspring pass it on to their children.
Reply to this commentNovember 1st 2011
div>“Again, Dennis gave no indication of how probable such a result would be.”
Reply to this commentNovember 1st 2011
October 30th 2011
Fuz’s point was that a simpler explanation for the fact that there is only one type of mitochondrial DNA and Y-chromosome DNA in the human population is that originally there were only two human beings, one female and one male, each with their respective mitochondrial and Y-chromosome DNA. Given such a scenario, the probability that we would all have the same mitochondrial or Y-chromosomal DNA is 100%.
Reply to this commentWhat would be interesting is to know what the probability is that we would all have the same mitochondrial or Y-chromosomal DNA, given an original human population of 10,000. If the probability is still relatively high, then Fuz’s argument doesn’t have much cogency. If, however, the probability is very low, then Fuz’s argument would be rather strong.
Fuz would still need to explain why our genetic diversity shows that the human population has never dropped below 10,000. From the evidence that he already gave, it sounds like his answer is that genetics is not that exact a science, especially over very long time periods.
October 30th 2011
Darrel,
Reply to this commentRegardless of whether or not you flunk me, I’m curious what your response is to my extra credit answer, since you would, in effect, be answering Fuz’s points, not mine.
October 30th 2011
Before we move forward in answering Bilbo’s questions, I think we should address some of the misconceptions that he has understandably taken on board. Here is an important one that I see.
November 1st 2011
testing…
Reply to this commentNovember 1st 2011
br>
November 1st 2011
*evolved over time within the existing human population
Reply to this comment