Adam — Once More, With Feeling

Adam — Once More, With Feeling November 4, 2019

By Dennis Venema. Dennis and I are co-authors of Adam and the Genome, and in this post he responds to recent news about an original couple.

I’m not sure what gets your inbox pinging, but one of the things that lights up mine is folks claiming that humans can descend from just two people. Just this week saw another example of this, and it seems to be getting some press over at the Gospel Coalition:

(E)vangelicals have been told for many years now that they are being anti-intellectual and anti-scientific if they don’t recognize that, because science shows that humans evolved from a large population over a period of millions of years, therefore we must reject the idea that one human race could have descended from just two human beings. Why do you reject that claim regarding the science?

This latest attempt to explain the genetic data in light of just two progenitors comes from the Discovery Institute – the main organization that promotes Intelligent Design – though, as we shall see, the attempt here is more straight-up creationism than ID. But I’m getting ahead of myself.
As I argued in Adam and the Genome, which Scot and I coauthored back in 2017, it is highly unlikely that that humans descend uniquely from just two people. As I put it in the book (after discussing the evidence that the human ancestral population size was never below a few thousand):

As our methodology becomes more sophisticated and more data are examined, we will likely revise our estimates in the future. That said, we can be confident that finding evidence that we were created separately from other animals or that we descend only from two people just isn’t going to happen. Some ideas in science are so well-supported that it is highly unlikely that 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 (Adam and the Genome, 55).

Not surprisingly, this particular quote became a flash point for those who desire for theological reasons, one presumes, to have our species descend from a single starting couple. One response came from a geneticist in the UK – Dr. Richard Buggs. His critique was focused on one of the techniques used to estimate ancestral population sizes from present-day genetic diversity that I discuss in the book, and that routinely gives estimates in the thousands. These techniques, Buggs argued, present only a long-term average of population sizes. Because of this, perhaps, hiding in these averages, is a particular event that can give us a single couple despite the average in the thousands. What Buggs had in mind was what population geneticists call a “bottleneck” – a reduction in population size. Now, the bottleneck that Buggs had in mind was a very unconventional one: a reduction from a standing population (a few thousand, say) to just two in a single generation. In other words, instantaneously.

Normally, a population bottleneck that severe would greatly reduce the genetic diversity of the population and show up in the sort of studies I describe in Adam and the Genome – mostly because such an event would force a small population size for a long time as the population slowly rebuilt itself. But Buggs proposed that immediately after the bottleneck, the population would grow, at an exponential rate. In this way, most of the genetic diversity in the founding pair would be preserved. Put another way, the effect of the bottleneck would be minimized as much as possible.

Buggs, myself, and some other biologists kicked the idea around for some time. I reached out to someone who did population genetics modeling of the sort I described in the book. Based on some new simulations and some other published studies that we drew on, our group came to an agreement – that if an event like this had 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. But population genetics isn’t always intuitive, and we were torture-testing only one modeling approach. Now, hear me well – there is no positive evidence at all that such an event occurred. Moreover, there is no mechanism that I, nor Buggs, nor anyone else that I am aware of, has conceived of that could accomplish such an amazing feat. At 500,000 years ago, hominins are widespread over Africa and Asia. In order for this to work, one would have to propose that in one generation all of them were obliterated, save two. Moreover, immediately thereafter, conditions have to be just right for exponential population growth. I can’t think of a plausible mechanism for that, and no one else has proposed one.

Another thing to keep in mind is that at 500,000 years ago, there are no Homo sapiens on the planet. At that time Homo erectus is widespread, so, if we’re talking about a bottleneck at this time, then Adam is not Homo sapiens, but rather Homo erectus or a close relative. It should also go without saying that the Genesis narratives (with their agriculture, domesticated animals, advanced metallurgy, cities, and so on) look nothing like 500,000 years ago, when stone tools were the order of the day.

So, this discussion provided support for my conclusion in Adam and the Genome – humans (defined in the book as Homo sapiens, our own species[1]) are too diverse to have descended from one couple. If you want that, you have to go back before our species emerges, and you have to postulate that some very (very!) unlikely things happened – an instantaneous bottleneck to two across two continents that nonetheless leaves no trace in the geological record, immediately followed by exponential population growth.

From where I sit, that looks like special pleading. Would we be comfortable with apologists from other religions making similar moves to prop up a certain piece of their theology? Or would we call them out on it? I have my suspicions.

Gauger and Hossjer: a single-couple human origin?

With that context in hand, we can now explore this recent work by Gauger and Hossjer to try to find an Adam and Eve in our history that we uniquely descend from.

They propose two approaches. One, like the Buggs hypothesis, requires an instantaneous bottleneck followed by exponential growth. A second option is openly creationist – that Adam and Eve are created, de novo, as a starting couple, and they founded the human race. In both cases, they place the couple back at 500,000 years ago in order for enough time for genetic diversity, under the measures they use, to approximate present-day diversity levels. And interestingly, the measures they use are ones that do not use information derived from our shared ancestry with other primates.

The first option, that of a bottleneck, runs into the same concerns we noted for Buggs’s hypothesis. In order to have sole genetic progenitors, all other hominins that could interbreed with Adam and Eve’s offspring must be eliminated in some way, or Adam and Eve must be reproductively isolated. Gauger and Hossjer put it this way:

One can imagine this scenario in a world still populated by other hominids, but the pair was isolated in a valley or gorge; one can imagine a scenario more extreme where all hominids except those two were wiped out.

Nope – geographic isolation isn’t going to cut it – after all, eventually this pair will populate the whole globe, which requires not being isolated. So, we’re back to trying to find an event that wipes out every other hominin across Africa and Asia – and as before, a mechanism is not forthcoming.

The second option, that of fiat special creation of a first pair, avoids the problem of a bottleneck mechanism by positing a miracle. While that does solve one problem, it causes another. The human genome is replete with evidence that we share common ancestors with other species, such as chimpanzees and gorillas. I spend a significant amount of time in Adam and the Genome laying out that evidence, and even that discussion is merely a drop in the bucket. If Adam and Eve are created de novo, why does our genome look like we evolved? Gauger and Hossjer do not say – though it is clear from their published work that they, like many (but not all) ID proponents, do not accept common ancestry.

Because the evidence for humans and chimpanzees sharing a common ancestral population is so strong (and honestly, only folks with prior theological commitments argue against that, given the strength of the evidence) we can use the chimpanzee genome to help us understand mutations in the human genome. (As an aside, since many of the readers of Jesus Creed will be familiar with textual criticism, the principles here are exactly the same, just swapping DNA sequences for variant manuscripts). If we see a “variant text” in present-day humans – a place in the genome where we see differences between present-day people – we can use the chimpanzee genome as a reference to determine what the “original text” for that region most likely was (or the gorilla genome, or the orangutan genome, and so on. Comparing multiple “manuscripts” is often helpful).

When we do this analysis, we see patterns – patterns that arise because certain types of DNA changes (mutations) are more common than others. With DNA copying, our “scribes” – the enzymes that copy DNA chromosomes – have particular quirks that make certain types of mutations more common than others. They rarely make errors – human scribes would be thrilled to match their performance – but when they do, they have a distinctive pattern.

In Gauger and Hossjer’s de novo model, they propose that present-day human variation comes from two sources: the original created couple, with their created (and maxed-out) genetic diversity, and subsequent mutations that arise after that starting point. The original created variants go on to be a large proportion of present-day variation that is common – i.e. present in a lot of people. Rare genetic variants are explained as arising from new mutations after the original creation event. Now, as geneticist Steve Schaffner aptly points out, it’s reasonable to expect that the rare variants should have the distinctive pattern suggesting that they are copying errors that arose over time though the work of our “DNA scribes”, but there is no reason to expect that same distinctive pattern for the originally created DNA variants. But, of course, the same pattern applies to all variants – both rare and common. The variation that Gauger and Hossjer claim was created directly by God looks just like the variation produced slowly over time by DNA copying errors when one uses the chimpanzee genome as a reference.

So, the short answer (if you can call this short) is that this sort of thing is not convincing to scientists who understand the genetic evidence, and who understand what sort of events in our past are plausible. It also shows just how far one has to go to try to make sole genetic progenitorship work – you need to avoid evidence for common ancestry, pull back to species that predate our own by several hundred thousand years, and propose extinction events that span two continents but nonetheless are invisible (genetically or geologically). And note well – there is no evidence in favor of any of this – none at all.

To sum up, I can’t think of a better way to put this than I did in Adam and the Genome:

… we can be confident that finding evidence that we were created separately from other animals or that we descend only from two people just isn’t going to happen. Some ideas in science are so well-supported that it is highly unlikely that 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.

If new evidence (and notice how important “evidence” is in that quote above) comes along that changes the landscape, I (and every other scientist out there) would of course take that into consideration. But that’s not what we’re dealing with here, alas. It looks to me like special pleading in service of a predetermined theological system.

[1] In the book I used the standard, non-specialist convention of referring to humans as Homo sapiens. In science, you will sometimes see “humans” used to refer to all species within Homo, including Homo erectus, Neanderthals, and so on. But for a popular audience, equating Homo sapiens with human is more common, so I went with that approach. When Gauger claims that single-couple human origin is possible, she is using “human” in the broader, all-of-Homo sense – but many people will not understand that and hear “single-couple origin for Homo sapiens”.

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