Nature: Genetic Adam and Eve did not live too far apart in time

Mitochondrial Eve finally meets Y-chromosome Adam (sort of).

Announcing two studies that “re-date ‘Y-chromosome Adam’ and ‘mitochondrial Eve,’ Nature reports, “The Book of Genesis puts Adam and Eve together in the Garden of Eden, but geneticists’ version of the duo—the ancestors to whom the Y chromosomes and mitochondrial DNA of today’s humans can be traced—were thought to have lived tens of thousands of years apart. Now, two major studies of modern humans’ Y chromosomes suggest that ‘Y-chromosome Adam’ and ‘mitochondrial Eve’ may have lived around the same time after all.”

A May-December marriage?

“It has been suggested that the date of our last common maternal ancestor could have been three times older than that of our last common paternal ancestor,” ScienceMagazine reports.1 Mitochondrial-DNA calculations have typically estimated the last common maternal ancestor of modern humans lived 150,000 to 240,000 years ago.2 And until now most Y-chromosome calculations have placed the most common male ancestor only 50,000 to 115,000 years ago.2

“Adam” and “Eve” reconciled

Stanford population geneticist Carlos Bustamante, using improved technology to sequence the entire Y-chromosome from 69 men scattered across the globe, discovered almost 9,000 previously unknown genetic variations. Using them his team now calculates that humanity’s most recent common male ancestor (MRCA) lived 120,000 to 156,000 years ago. Bustamante’s lab also reassessed humanity’s maternal ancestry, calculating that the female MRCA lived 99,000 to 148,000 years ago. The authors attribute the lower maternal estimate to their exclusion of the “between species”2 (i.e. chimpanzee) data used in most studies to calibrate the mitochondrial DNA-clock. The dates reconciled, they conclude, “Contrary to previous claims, male lineages do not coalesce significantly more recently than female lineages.”2

The Stanford geneticists hope their genomic discoveries will be useful in mapping the geographical history of humanity’s spread over the earth. However, ScienceDaily says the “reconciliation of the timing of ‘Adam’ and ‘Eve’” could be their greatest contribution. “This has been a conundrum in human genetics for a long time. Previous research has indicated that the male MRCA lived much more recently than the female MRCA. But now our research shows that there's no discrepancy. In fact, if anything, the Y chromosome may be a bit older,”3 Dr. Bustamante explains. “This idea of a very recent common ancestor of all men is not that true.”

Population specific but the same ballpark

An international team led by population geneticist Paolo Francalacci assessed Y-chromosomes from 1,200 men from the Mediterranean island of Sardinia. He reports the most recent common male ancestor of most people lived 180,000 to 200,000 years ago. These numbers differ primarily because they are derived from different samples of men. Back in March, Michael Hammer reported in the American Journal of Human Genetics that a very small sample of men in Africa have a Y chromosome traceable to a common ancestor 237,000 to 581,000 years ago. (See the second illustration to see how a sample like this can produce a “new Y-Adam.”)

Obviously it would be impractical to collect and sequence DNA from every man in the world, and different populations can yield different results. Geneticists are delighted with results like Bustamante’s “worldwide” sample of men and women whose results stay in the same ballpark. Commenting on the significance of the apparent discrepancy between Francalacci’s results and his own—particularly the new maternal MRCA age—Bustamente said that his work and Francalacci’s match up well. He explains, “It’s not so much we’re shifting the mitochondria down—which we are, a bit—but we’re pushing the Y farther back.”

The biblical Adam and Eve: not the same ballpark!

But what are we to make of these large numbers? None are in the same ballpark as 6,000 years. The Bible indicates that about 6,000 years ago God created Adam and Eve—the parents of all humans, modern and otherwise (Genesis 3:20, Acts 17:26). Has science provided a date to improve on the Bible, constraining the time when the original humans could have met? No.

Mitochondial Eve” and “Y-chromosome Adam” have nothing to do with the biblical account of creation. They do not imply the existence of a single original human female and male. They don’t even have anything to do with identifying when men and women supposedly evolved. They instead are a way of estimating how long ago and from where modern humanity’s ancestors spread out. It is vital that Bible-believing Christians understand what this information does and does not mean in order to avoid adopting “Arguments Christians Shouldn’t Use” or compromising biblical truth.

adam-eveThis family tree illustrates the reason mitochondrial and Y chromosome DNA is useful for tracing ancestry. Only males have Y chromosomes, and being unmixed with DNA from the mother, the mutation pattern on the Y chromosome can be matched with similar patterns in other people whose DNA happens to be in the database. In this way, people can be grouped according to their shared common ancestors. Similarly, even though everyone has mitochondrial DNA, it is passed on through the mother without much opportunity for alteration, so its mutation patterns can also be traced. But the effort to determine how long ago people shared a common ancestor is flawed by many unverifiable assumptions. Image credit: saypeople.com

adamThis family tree helps illustrate the way different population samples can generate different results for Y-Adam. If the sampled population does not include the individuals with the bright green Y chromosome (lower right), then the “most recent common ancestor” would be the more recent “Y-Adam,” not the “new Y-Adam” that is pinpointed when the “green genes” are included. Y-chromosome Adam calculations do not imply that only one male was alive at a particular time, only that the sampled population can trace its lineage back to a particular man among many. This statistical principal has nothing to do with the historical biblical Adam. Image: Dr. Barry Starr, Stanford University, genetics.thetech.org

The meaning of the method

“Mitochondial Eve” and “Y-chromosome Adam” are ways of looking at the mutations in a population’s DNA to estimate how long ago its members shared a common ancestor. Over time, chromosomes mutate quite a bit. Most mutations don’t hurt anything. But “point mutations” do produce unique combinations that can be used to trace ancestral relationships and sometimes even geographical origins.

The Y-chromosome is the easiest-to-track portion of men’s genomes since it is only passed on through males without any mixing of parental genes. Mitochondrial DNA, similarly, is the easiest to track in women, as it is carried in a mother’s eggs and can be tracked across generations through daughters.

Maternal and paternal DNA testing is useful for reconstructing specific family trees over several generations, resolving paternity issues, and determining the most likely geographical homeland for a family’s ancestors. However, genes don’t always cooperate, sometimes leading to incorrect conclusions when extrapolating over generations.1 Moreover, technological limitations have until recently limited the accuracy of Y-chromosome sequencing. But even with the newest and greatest technology, both male and female common ancestry estimates are enormously affected by the assumptions on which they are calibrated.

Calibrating the clock

Let’s have a look at the assumptions the researchers in these studies say they made. We’ve already noted that Bustamente’s group tried to improve the “mitochondrial Eve” dates by leaving out chimpanzee data. They did however use chimpanzee data as a standard to help sort out some of the human Y-chromosome sequences.2 Francalacci’s group calibrated their work in accordance with assumptions about the divergence of humans and chimpanzees from a common ancestor.4 Thus the biblically untenable and scientifically unverifiable belief that humans and chimps evolved from a common ancestor is foundational to molecular clock dating.

Molecular clock dating is not an absolute, independent measurement of time. The “clock” must be calibrated. The mutation rate must be known and assumed to be constant. The authors of these studies explain how they calibrate their clocks by determining how long it takes for a certain number of point mutations to accumulate. Francalacci writes, “The inferred phylogenetic rate and dating estimates presented here remain tentative, because the calibration date was deduced from archaeological data, which may be incomplete and typically covers a relatively large temporal interval. In the future, a more precise calibration point might be obtained by sequencing ancient DNAs from prehistoric Sardinian remains dated by radiocarbon methods. Further limitations derive from the scarcity of related samples for rare lineages.”4

Bustamente’s group expresses considerably more confidence in their calibration point—the time the first immigrants crossed the Bering Strait from Asia to America. They use that information in conjunction with the genomes of two men with Mayan ancestry to calculate a mutation rate. They write, “we estimated their respective mutation rates by calibrating phylogeographic patterns from the initial peopling of the Americas, a recent human event with high-confidence archaeological dating. Archaeological evidence indicates that humans first colonized the Americas ~15 kya via a rapid coastal migration that reached Monte Verde II in southern Chile by 14.6 kya. . . . Using entry to the Americas as a calibration point, we estimate a mutation rate.”2

Circular confidence

But does the date America was peopled deserve such great confidence? If we look at the source of this information, we find that the scientists who came up with that number did so by making a lot of guesses among a host of discrepancies. They relied on radiocarbon dates, evolutionary assumptions about the timing of the Ice Age, molecular clock dates, and guesses based on the primitive tools present. They concluded, “The peopling of the Americas debate is far from resolved. . . . Genetic studies will also be key as more is learned about modern and ancient haplogroup subclades in combination with full mtDNA genome sequencing and identification of patterns of nuclear DNA variation.”5

Errors are thus introduced by the unverifiable assumptions foundational to radiocarbon date determinations and the Ice Age. The dates assigned to ancient tools are based on dates encumbered by other assumptions. The fact that the latest numbers for male and female common ancestry agree (more or less) should not inspire confidence since they are based on the same unverifiable assumptions. Analogously, a broken clock is right twice a day but still should not be trusted. Moreover, the circular reasoning inherent in this blind confidence should be obvious: the newest molecular clock dates are confidently calibrated using information tentatively based on molecular clock dates.

Biblical confidence without compromise

All human beings do share two common ancestors—the real Adam and Eve. All human diversity developed from the human genome God fashioned when He created the historical Adam and Eve. And the people descended from Noah’s family, including those people who dispersed from the Tower of Babel, are the source of all the genetic diversity we have among humans today. Thus it is no surprise the human genomes have so much in common.

In fact, an understanding of biblical history reveals how genetic discoveries like these—apart from the “clocks” imposed on them—are actually consistent with Scripture. Molecular geneticist Dr. Georgia Purdom of Answers in Genesis explains:

We know that the biblical Adam and Eve lived approximately 6,000 years ago because of biblical chronology, and science confirms a young human race. The fact that there is a low level of variation found in mitochondrial DNA and Y chromosome DNA from many different people groups all over the world is because only a short time has passed between humans today and Adam and Eve. If there was more time, like the hundreds of thousands of years that the evolutionists suggest (in addition to the millions of years since humans evolved from an ape-like ancestor), we should see a lot more variation in the DNA and that is simply not what we observe.

God’s Word records our Creator’s eyewitness account of human history. Bible-believing Christians must be careful to avoid attaching any other biblical significance to the molecular clock dates so confidently reported just because of the popularity of tagging them with the names of our first parents.

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Footnotes

  1. R. Cann, “Y Weigh In Again on Modern Humans,” Science 341 (2 August 2013): 465–467, doi:10.1126/science.1242899. Back (1) Back (2)
  2. G. Poznik et al., “Sequencing Y Chromosomes Resolves Discrepancy in Time to Common Ancestor of Males Versus Females,” Science 341 (2 August 2013): 562–565, doi: 10.1126/science.1237619. Back (1) Back (2) Back (3) Back (4) Back (5) Back (6)
  3. www.sciencedaily.com/releases/2013/08/130801142148.htm Back
  4. P. Francalacci et al., “Low-Pass DNA Sequencing of 1200 Sardinians Reconstructs European Y-Chromosome Phylogeny,” Science 341 (2 August 2013): 565–569, doi:10.1126/science.1237947. Back (1) Back (2)
  5. T. Goebel et al., “The Late Pleistocene Dispersal of Modern Humans in the Americas,” Science 319 (14 March 2008): 1497–1502, doi:10.1126/science.1153569. Back