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Wednesday, August 31, 2011

Quest for the Primal Couple: ICR's response to Dennis Venema

Genetics is a topic I have avoided here for one simple reason: I am not a geneticist.* But I could not help but to comment on some of the recent reactions to an article by Dennis Venema and Darrel Falk, entitled Does Genetics Point to a Single Primal Couple?. The Biologos authors argued therein that data from modern genetics do not corroborate the traditional biblical postulate of a single, primal couple (i.e. Adam and Eve) who gave rise to the world's population. National Public Radio (NPR) followed up on the controversy, citing Dr. Venema from an interview. Responses from various evangelical camps have been predictably adamant, and while some tried to focus on a scientific response, others—the focus of my post today—skewed the scientific data and questioned Dr. Venema's motives.

By way of preface, I am aware that many of you side against Biologos on this topic, and would prefer that I stick to geological issues. So I want to be clear that my aim is not to persuade you otherwise, necessarily, but simply to promote critical discussion (to the exclusion of some fruitless arguments). Since a firm position on the historical Adam is so treasured by many, it is vital to maintain fairness all around.

Does genetics point to a single primal couple?

Although Drs. Venema and Falk reject the historicity of Adam and Eve** on biblical and scientific grounds, the purpose of their article was not to wrestle exhaustively with that evidence. From what I gathered, the article could be summarized quite simply:

The existence of a primal couple can be discussed apart from the question of common descent (i.e. human evolution). Christians are not the only ones interested in the original population size of the human race, and geneticists have devised several independent methods by which to estimate that number. Three of those methods (discussed here) point to a population bottleneck in human history, but of several thousands individuals—not two. The existing genetic data, therefore, do not corroborate the traditional biblical picture. Any model that presupposes that picture must account for this evidence.

It would be misleading, therefore, to claim 1) anything from this article about the authors' motives or presuppositions with respect to human evolution; 2) that the authors rest arbitrarily on 'evolutionary assumptions' and ruled out a priori the existence of Adam/Eve; and 3) most importantly, that a refutation of this article lends positive support to the historical Adam/Eve.

ICR skirts the issue, adds to the clamor

At the risk of exposing my bias, I want to reiterate that I highly appreciate the clarity and cordiality with which Dennis Venema (and others at Biologos) have presented their respective positions. As a non-specialist in biology, I have been frustrated by 'experts' on all sides so desperate for concurrence that they overlook pivotal, looming questions from their audience. A recent article by Brian Thomas, entitled Christian Professor Claims Genetics Disproves Historical Adam, exemplified that frustration.

Mr. Thomas begins with a threefold objection to Dr. Venema's claim that genetic evidence falsifies the hypothesis that humans could have derived from a single couple:

"First, it relies on the presumption of "evolutionary history," not scientific data. Second, the idea that an initial group of 10,000 humans evolved from primates is mathematically impossible. Third, a descent from Adam and Eve actually does explain the patterns in modern human genetics."

Seemingly, Mr. Thomas cannot distinguish between scientific data and scientific theories built to explain those data. Nonetheless, it is rather disingenuous to dismiss an argument just because the data on which it rests require some interpretation. To be consistent, we would then have to reject every scientific argument. Lastly, Dr. Venema's case is not without assumptions, but "evolutionary history" is not one of them, as we shall see.

"Shouldn't such a person at least attempt to examine the genetic possibility of an Adamic ancestry before completely ruling it out?"

Caricatures are never helpful. Not merely because they are false, but also because they provoke hostile emotions without warrant. If Mr. Thomas had so much as clicked Dennis Venema's name at the top of the Biologos article (which I'm not sure he read), he would have found the 5-part series entitled From Intelligent Design to BioLogos. It is very misleading to claim that Dr. Venema has never considered "genetic possibility of an Adamic ancestry".

Method I – Can total genetic diversity arise from a single pair in ~6,000 years?

The first method cited by Venema and Falk considered the origin of genetic diversity in the modern human population. They explain:

"First we ask how many different alleles there are for a number of genes within the current population. Correcting for the rate at which we know new forms of genes appear (mutation), we can calculate the minimum number of people needed to generate the current amount of diversity."

To which Dr. Venema added in the NPR interview:

"You would have to postulate that there's been this absolutely astronomical mutation rate that has produced all these new variants in an incredibly short period of time."

But Mr. Thomas responds by appealing to a false premise:

"Evolutionists assume that all genetic differences between individuals resulted from mutations...But if Adam was created with DNA variations, then one would not have to postulate astronomical mutation rates."

They assume that all genetic differences result from mutation? I don't think this is accurate. Regardless, I wonder what kind of DNA variations Mr. Thomas envisions within Adam's genome. As the Biologos article explained:

"At maximum, four gene-forms (two from each parent) would be passed on by Adam and Eve."

An individual genome cannot carry more than two forms of any gene, meaning that the maximum 'created' diversity would entail complete heterozygosity in each parent. Since the Biologos article stated this fact at the outset, Mr. Thomas's argument is again without foundation.

A more quantitative response came from Dr. Robert Carter at Creation Ministries International (CMI), who argued that directly measured rates of mutation are much higher than those measured indirectly by comparing the human and chimpanzee genomes. Mr. Thomas seems to have based much of his critique on the CMI article, with investigating further. Dr. Carter writes, for example, that:

"The mutation rates used in the calculations generally depend on assumptions of common ancestry...Measurable mutation rates are generally several orders of magnitude faster than those used in evolutionary studies. Using a measured rate would shrink the size of the bottleneck population." (emphasis added)

Using a higher mutation rate to estimate the original human population would indeed reduce the size of that population—on this point, everyone agrees. But Dr. Carter does not cite any studies where mutation rates in the human genome vary by several orders of magnitude, depending on the technique. So I decided to investigate. My brief search yielded a highly collaborative paper by Conrad et al. (2011) in Nature Geoscience, which reported "the first direct comparative analysis of male and female germline mutation rates from the complete genome sequences of two parent-offspring trios":

"The sex-averaged germline mutation rate estimates we derived agree very closely with three other recent studies focusing on sex-averaged mutation rates in the most recent generation. Averaging across these four studies gave a more precise sex-averaged mutation rate of 1.18 × 10−8 [per base pair]...which is less than half of the frequently cited sex-averaged mutation rate derived from the human-chimpanzee sequence divergence of 2.5 × 10−8. These apparently discordant estimates can be largely reconciled if the age of the human-chimpanzee divergence is pushed back to 7 million years, as suggested by some interpretations of recent fossil finds..."

Mutation rates can be measured directly by comparing parent and offspring genomes (as in this study) or by dividing the number of genetic variations between species by the estimated years since their last common ancestor. Based on the references cited within the article, previous estimates of human germline (i.e. passed on through reproduction) mutation rates varied by a factor of three—not orders of magnitude. Moreover, the uncertainty was due not to conflicting results so much as pricy technology and limited data, as well as variable estimates for the last common ancestor between humans and chimpanzees (4–7 million years).

Drs. Venema and Falk did not rely on "evolutionary assumptions" therefore, since measured mutation rates are comparable to, if not lower than, those estimated from the postulated timeline of human evolution. As far as I can tell, Dr. Venema's original point still stands: our current understanding of genetics suggests that the original human population was much larger than two, and lived much longer ago than ICR would have us believe.

Fuzzy population genetics?

The second objection made by Mr. Thomas seems to be rooted in a complete misunderstanding of population genetics and evolution. He argues that it would be "mathematically impossible" for a population of 10,000 humans to have "evolved from primates". Since humans are themselves primates, I believe the problem is also rhetorical, and question whether Mr. Thomas has an accurate view of human evolution.

For one, the several thousand individuals being discussed constitute a bottleneck in human history—not an original population that somehow received "human-like mutations", as Mr. Thomas puts it:

"About 700 million information-packed DNA differences exist...between humans and chimpanzees. Each of these changes would need to become "fixed" into the whole population of primates in order to transform them into humans."

Are each of these DNA differences truly "information-packed"? Many of them are neutral point mutations, found in pseudogenes or genetic redundancies. Before I address the issue of "transforming" humans, consider the following hypothetical:

"But even if a single human-like mutation fortuitously occurred in both members of a reproducing pair, it would have virtually no chance of spreading to all 10,000 "emerging" humans. Instead, through interbreeding with non-mutants, the mutation would diffuse and disappear after only a few generations."

The functional genetic signatures unique to humans are supposed to have arisen through a rather active force called natural selection—not the "fortuitous" insertion to and diffusion throughout the entire hominid population. Furthermore, it is strange to refer to 'mutants' and 'non-mutants'. Every human offspring contains several dozen mutations not found in their parents. We are all mutants! As long as any population survives with time, there will be thousands to millions of mutations that define the genetic diversity of that population. The extent and nature of that diversity give clues as to how large and diverse that population was in history.

Lastly, genetic drift is sufficient to explain the sporadic introduction of new alleles (arising through mutation) into a large population of early humans. Conversely, the chance disappearance of alleles through genetic drift is only exacerbated by inbreeding that would necessarily occur following a 2-person or 8-person bottleneck (Adam and Noah, respectively).

Adding 'biblical' parameters

The final objection offered by Mr. Thomas can almost be termed a tautology. He assures his readers that assuming biblical parameters, modern genetic data really are consistent with the young-Earth picture. But adding no relevant information, his argument appears rather to be an appeal to his starting position. He begins by citing that "all people are 99 percent genetically similar"—a fact never disputed by either party but seemingly intended to make genetic variation in humans appear small. Consider that when citing the differences between human and chimpanzees, he used an absolute number instead: 700 million. He continues:

"For example, any Vietnamese is 99 percent genetically identical to any Ethiopian. Of the one percent DNA sequence difference between the two, the large majority is shared by all within their ethnic group, whether Vietnamese or Ethiopian. A small minority of that one percent is unique to each individual person."

As I recall, the genetic variation within human subpopulations can exceed that between individuals of different ethnic groups (making categories of 'race' a cloudy topic in biology). It may range from ~98% to nearly 100%—but what does this add to the discussion? The fact remains that total genetic diversity in humans, even if limited to variations that necessarily arose from mutations, cannot be accounted for by a single couple living less than 10,000 years ago.

Mr. Thomas concludes by promoting the working model of Dr. Robert Carter at CMI, which, although novel and possibly deserving of our consideration, is not without serious challenges and is hardly conclusive. Perhaps I have missed something important, but the assessment offered by ICR regarding the historical Adam controversy seems to be premature at best, and misleading at worst. I pray that discussion might continue with more substance and integrity for the sake of all non-experts at the mercy of respectable Christians in the biological sciences.

*I am not an expert in genetics, but I know a few. So I am deferring partly to their expertise and input, which I highly appreciate!
**As a specially created couple from which all humans today could trace their lineage.

1 comment:

  1. Thanks for this. You took some heady discussion and made it a little easier to digest. Appreciate it.