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Welcome to Defenders, the teaching class of Dr. William Lane Craig. Today the Doctrine of Man, Part 17. For more information and resources from Dr. Craig, go to reasonablefaith.org welcome to Defender's Class.
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Thank you for joining us. It occurred to me that during this time of the coronavirus pandemic, some of you joining us today might actually be suffering from the coronavirus yourselves. And you're taking advantage of this podcast to be able to be part of our Defenders class. I want to give a special welcome to any of you who are dealing with such a trial. Now, in our last session, I argued that Adam and Eve should be thought of as the most recent common ancestor of both Neanderthals and Homo sapiens. This traditional view of Adam and Eve as the fountainhead of all humanity has been vigorously challenged by some on the basis of the science of population genetics. This morning. I'd like to look at this challenge with you. Now, in order to understand this challenge, it's important to understand that according to the theory of evolution, and, and perhaps contrary to popular impression, evolution does not proceed along an isolated individual line. It's not as though there is some sequence of individual reptiles that evolved, for example, into the first bird. Rather, the idea is that whole populations evolve over time. So the ongoing front of evolution is not like the point of a spear. Rather it is like a broad front as a whole population of organisms evolves together over time. In this sense, a whole population of reptiles would be evolving bird like characteristics. Similarly, in the case of human evolution, there is a whole population of hominins, that is to say man like primates, which is gradually evolving characteristics of modern human beings. So it's typically thought there weren't originally two human persons, an original couple that were the ancestors of everybody else. Rather, modern human beings evolved through a whole population of hominins, moving gradually toward some more recognizable modern human forms. But why couldn't there have been, within this wider population of non human hominins a first couple who made the transition to humanity and whose descendants then became the human race? Some critics have alleged that this scenario is impossible, since it contradicts the data of population genetics. Now, in order to understand this challenge, let's briefly review what we learned in high school biology. Human beings have in each nucleus of each cell, 23 pairs of chromosomes containing the DNA that determines our genes. A segment of DNA is called a locus, which is Latin for place, the sequence of DNA letters, and at any locus is called an allele. Since our chromosomes come in pairs, we therefore have one allele at A locus on one chromosome and another allele at a similar locus on the other chromosome. These alleles determine features like eye color, height, skin color, and so on. Now, the claim is that when we look at the genetic profile of the human population today, it is impossible that it could have stemmed from an original couple alone. There had to be numerous ancestors from the very beginning in order for the human race to to have arrived at its present condition. So what exactly is the problem supposed to be here? Computational biologist Joshua Swamidass distinguishes at least four different problems that have been put forward in the popular literature as incompatible with the existence of of an original human couple. The multiplicity of alleles There are just too many different alleles in the present population to have all come from an original human couple within the last 18 million years, which is long before human beings ever appeared on this planet. Two effective population size estimates, various independent methods of estimating past population size all concur that the human population in the past was never fewer than around 10,000 breeding people. Three trans species variation in order for all the alleles which we have in common with chimpanzees to be passed on to us from a common ancestor, there needed to have been more than one couple who transmitted these genetic lines from that ancestor to us. In other words, a single couple could not have passed on to us all the genetic material which we share with chimps. And finally, number four, the divergence of alleles. To grasp this point, it's vital to differentiate genetic divergence from genetic diversity. I shall take genetic diversity or variation to refer to the multiplicity of alleles in a population. Genetic divergence, on the other hand, has to do with the mutational distances between alleles in a population. We can visualize divergence by representing alleles as dots plotted on a plane. The more mutations separating two alleles, the greater the distance between them on the plot. One way to measure divergence would be to measure the distances of the farthest alleles from the most central allele. Draw a circle around all the dots, and the radius of that circle provides a measure of their genetic divergence. Now, notice that the multiplicity of alleles, what I'm calling genetic diversity, is irrelevant. What matters is the spread of the alleles genetic divergence. The wider the spread, the more ancient, the most recent common ancestor of those alleles. We want to compute genetic divergence across the whole human genome. We can then ask how, how long and how rapidly mutations must have been occurring for the distances separating alleles in the present population to arise from a pair of sole genetic progenitors. Given the known mutation rate, we can use genetic divergence to calculate the time back to the most recent common ancestor. And the argument is that it would take millions and millions of years for the observed divergence of alleles to have arisen from a common ancestor. We cannot reach a single pair of sole genetic progenitors of the human race, but within the time during which hominins have existed on Earth, as one writer put it, this would make Adam literally a monkey's uncle. Therefore, there could not have been an original human pair from whom we all descend. On the basis of evidence such as the above, Denis Venema expresses supreme confidence that humanity did not descend from a single human couple. He writes, some ideas in science are so well supported that it is highly unlikely new evidence will substantially modify them. And these are among them. The sun is at the center of our solar system. Humans evolved, and we evolved as a population. Here he expresses what has been called heliocentric certainty against an original human pair. Swamidass has subjected the above four arguments to searching criticism. Let's look at responses to each. First, the multiplicity of alleles and ancient population sizes. Swamidass dismisses arguments based upon 1 and 2 as, and I quote, just wrong headed. These arguments are total misdirections that have nothing to do with the key question. They are category errors from the mere number or variety of alleles in the human population today. Nothing at all follows about population sizes in the deep past. What matters is the divergence of alleles in the population. The argument from genetic diversity as I've defined the term, is a red herring. As for arguments based on estimated effective population size, these are misleading because such estimates are averages over a window of time and so are consistent with peaks and valleys within the interval. Venema consistently errs in taking these estimates to concern minimum population size rather than average population size. In 2017, geneticist Richard Buggs pointed out that the hypothesis of a bottleneck of two people had in fact never been tested scientifically. Indeed, as Swamidass observes, we know that at some point in the past, the number of human beings goes to zero, and therefore to fewer than 10,000 individuals. In thinking otherwise, Venema is guilty of a critical equivocation between ancestors and humans. Even if the ancestral population of hominins leading to humans remains constantly above some minimal size of several thousand, it doesn't follow that there were not at some time exactly two humans who emerged within that population. It is entirely possible that at some time in the past, the total number of humans was exactly too. Even though the total population of hominins at the time was much greater, the descendants of these early humans might or might not have interbred with their non human contemporaries. If they did, then that founding couple would not be our sole genetic progenitors, since outsiders would have had genetic input into the human race. On the other hand, if such interbreeding never occurred, the founding couple would be the sole genetic progenitors of the human race, since there would be no outside input. In either case, though, there would have been an original founding pair. What then, about the third argument? The argument from trans species variation? While initially plausible, the argument dissolves upon examination. Since every human being has two sets of similar chromosomes, not counting the X and Y chromosomes determining our sex, a founding human pair can together carry at most four alleles at any locus into the descendant population. So if it could be shown that there are more than four allele lineages exhibiting trans species variation, then we should have a strong argument against an originating human pair. But apart from an outdated study by Francisco Ayala, there is no evidence for this. Although this issue is still under debate, Swamidass reports that other studies have failed to uncover evidence of trans species variation between humans and non human ancestors and involving more than four allele lineages. Even if such variation should be discovered, it could indicate no more than the fact that Adam and Eve were not our sole genetic progenitors, but that there was interbreeding with outsiders who introduced more alleles into the human population. Moreover, there is a plausible alternative explanation of trans species variation among hominins, namely convergent evolution. In convergent evolution, similar alleles evolve independently in different species. Convergent evolution seems to be common among hominin species, producing similarities from, for example, bipedalism among independent species that may be misleading in determining lines of descent. Swamidass notes that the particular gene studied by Ayala is, and I quote, one of the genes with clear evidence of convergent evolution, end quote. This finding robs the argument from trans species variation of of its force. Finally, then, the fourth argument from genetic divergence. The decisive question then concerns the divergence of alleles in the present human population. Swamidass points out that a founding couple could have been heterozygous, each carrying two different alleles and at any locus of their chromosome pairs, for a total of four alleles between the two of them for any locus to be passed on to their descendants. In that case, the relevant time is not the time to the most recent common ancestor, but the time to the most recent 4 alleles. Population genetics has been concerned only with with the time to the most recent common ancestor, so that no studies of the time to the most recent four alleles had been published prior to Swamidassa's work, requiring him to do his own original modeling in order to obtain a date. Now I'll skip the fascinating details and cut to the chase. Swamidass determines a date of of 495,000 plus or minus 100,000 years ago. Four the most recent four alleles. So there could have been a founding couple about 500,000 years ago who were the sole genetic progenitors of humankind. On the basis of this work, Richard Buggs agreed to that Swamidass for the first time had tested the hypothesis of an original human pair. More recently, these findings have been confirmed by Ola Herscher and Ann Ganger, who explore what they call a single couple origin model of the human race. They find that the data are consistent with at least two single couple origin models. One, a model featuring a homozygous first couple dating to about 2 million years ago, and two, a model featuring a heterozygous first couple who lived about 500,000 years ago. Thus, they conclude, given common assumptions shared by evolutionary geneticists, a single couple origin is possible despite claims to the contrary. So while a recent bottleneck is ruled out by the genetic divergence exhibited by today's population, a bottleneck of two before 500,000 years ago is possible, in which case the founding pair would be the common ancestors of Homo sapiens, Neanderthals, and Denisovans. The dust has yet to settle on the scientific details, says Swamidass, but it looks at this point likely that a bottleneck anytime before 700,000 years ago is undetectable in the genetic data. Such a date is well within the range of our proposed identification of Adam as Heidelberg man. After extended discussion with bugs, Swamidass and others, Venema came to acknowledge in 2019 the failure of his arguments against a single couple origin. I quote, based on some new simulations and other published studies that we drew on, our group came to an agreement that if an event like this has happened, we would be able to detect it if it happened more recently than 500,000 years ago. That was surprising to me. To be sure. I thought beforehand that an event like that would show up even further back in time. Venema nonetheless insists that despite the possibility of a founding pair before 500,000 years ago, the existence of such a couple is highly improbable. In order for this to work, one would have to propose that in one generation of all of them were obliterated save two. This bold claim is obviously false, since a founding pair could have existed as part of a wider population with whom the founding pair's descendants may or may not have interbred. It's important to understand that the existence of a historical Adam and Eve need not imply their sole genetic progenitorship, especially over tens of thousands of years, even if their descendants were, for a time, reproductively isolated. Such isolation could result not from population reduction, but from social distancing due to a myriad of factors, including geographic isolation, tribalism, language barriers, xenophobia, differences in cognitive capacity, racism, just plain revulsion, and so on, as well as any population reduction we might imagine. Perhaps these barriers were sometimes breached, but then we do not have any idea whether there were offspring of such unions that had genetic input into the human line. Of course, once Adam and Eve's descendants replaced Heidelberg Mann, we know that there was interbreeding among the extended human family, but we can only conjecture as to what happened before that point. In conclusion, then, Adam and Eve may be plausibly identified as as members of the species Homo heidelbergensis and as the founding pair at the root of all human species. Challenges to this hypothesis from population genetics fail, principally because we cannot rule out, on the basis of the genetic divergence exhibited by the contemporary human population, that our most recent common ancestors, situated more than 500,000 years ago, are the sole genetic progenitors of the entire human race, whether past or present. Next time I'm going to conclude by reflecting theologically upon what we have discovered.
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The copyright for the preceding material is held by Dr. William Lane Craig. For more, go to reasonablefaith.org.