Adam, Eve and human population genetics, Part 13: addressing critics - Poythress, population genomics, and locating the historical Adam (continued) | The BioLogos Forum


(system) #1

Note: In this series, we explore the genetic evidence that indicates humans became a separate species as a substantial population, rather than descending uniquely from an ancestral pair.

In the last few posts in this series, we’ve explored Vern Poythress’s arguments in his short book Did Adam Exist? – specifically, his arguments against human – chimpanzee common ancestry. While we now turn to his arguments against the conclusion of human population genetics that our species does not descend uniquely from an ancestral pair, it’s worth noting, in passing, that Poythress’s arguments do not even attempt to refute the most obvious and straightforward evidence for common ancestry. For example, we see mutations in genes that form nested hierarchies in the human, chimpanzee, gorilla, and orangutan genomes. By far and away the simplest reason for this pattern is common ancestry, and as it turns out it’s the exact same pattern of common ancestry predicted by DNA sequence identity between these primates. We also see the remains of a gene in the human genome devoted to large-scale egg yolk production – something placental mammals such as humans simply do not need. If Poythress is to build a convincing case against human common ancestry with primates, these and other similar lines of evidence are the issues to tackle. As it stands, he has not even attempted to address them.

Having dealt with common ancestry to his satisfaction, Poythress then shifts to discussing human population genetics. As I see it, the three main arguments Poythress puts forward in this area are as follows:

  1. Population genomics methods report only long-term average population sizes. These methods could not detect a bottleneck to two individuals, even if one existed, since they report only long-term averages.
  2. Population genomics methods report sizes for more recent human populations, and do not address more distant history. As such, Adam and Eve may have lived further back in time than these analyses measure. This view is consistent with Scripture since the biblical genealogies may have gaps.
  3. The findings of population genomics are based on uniformitarian assumptions that may not in fact be true if allowance is made for miracles.

While the third argument requires us to delve into the philosophy of science and how science might interact with theology, the first two claims are strictly scientific. As we will see, these scientific claims – like Poythress’s claims relevant to common ancestry – also fail to stand up under scrutiny.

Population bottlenecks and genetic variability over time

Poythress’s first claim, that population size estimates are long-term averages only and thus could not detect an original pair, appears a few times in the book:

Another paper uses genetic diversity among humans today to estimate average population size over the remote past, and offers nine different estimates in the region of 10,000. But these numbers depend on models that assume a constant population size through many generations. The figures are in fact giving us rough averages over long periods of time, so they say nothing about the possibility of two original individuals.

the analysis always results in figures that represent a rough average over many generations in the human population. Consequently, the principal figures, like 3,100 for non-African populations and 7,500 for the African population, represent average populations over many generations. They say nothing one way or the other about whether the size decreased rapidly to two individuals in the more distant past.

This argument fails to understand how genetics works within a population. A population bottleneck to two individuals (the most extreme bottleneck that a mammalian population can experience) would not merely affect one or two generations, nor even a few hundred generations, but rather mark the genome of a species for hundreds of thousands of years. As such, it would easily be detected by modern population genomics methods. Let’s examine how such an event would shape genetic variability in a species.

The first effect an extreme bottleneck would have would be to greatly reduce genetic variability – the number of alleles that the species in question would possess. Recall that an allele is an alternative version of a DNA sequence. In mammals, each member of the species can have a maximum of only two DNA variants for any given DNA sequence – one allele inherited from mom, and one from dad. For a population to maintain high genetic diversity, you need a large number of individuals that have alternative DNA sequences to reproduce. If a population is reduced to only two individuals (or, as Poythress argues, was specially created starting with only two individuals) then the maximum possible DNA variation for any location in the genome is only four different alleles (two each in the male and female, with all four alleles being different from the others). Also, all alleles in genome of this species would coalesce to approximately the same time point, since all genetic diversity above this baseline (that would later accumulate through mutations) would be effectively the same age. However, as we have seen, humans are highly diverse genetically, and our alleles do not uniformly coalesce to a specific point in our history, but rather to a wide range of times, consistent with our lineage maintaining a large population over time.

A second effect of a reduction to (or start from) two individuals would be to place the original alleles into four chromosomal linkage patterns. These linkage patterns would slowly be broken up by recombination over successive generations. As with allele coalescence, we would see a pattern in present-day genomes that could be tracked back to four ancestral linkage patterns indicative of two original ancestors. These patterns in the human genome, however, indicate thousands of ancestors, as we have seen.

(Now it is of course possible - to jump ahead to Poythress’s third argument to state that the patterns we see in the present-day human genome are the results of God’s miraculous intervention superimposed on what is in fact descent from an ancestral pair. I will address this possibility in a future post, but for now I’ll merely state that I do not find this argument plausible.)

So, if indeed humans descended from two individuals we would expect our present-day genetic diversity to be low, and the pattern of diversity to reflect an origin from a maximum of four original chromosomal arrangements. We do not, however, observe anything like this for present-day human genetic diversity.

Perhaps one way to appreciate that population genetics methods can (and do) detect extreme population bottlenecks would be to examine other mammalian species that do have the features we would expect for such an event. Cheetahs are one well-known example of a species with greatly reduced genetic diversity. Population genetics models for cheetahs support the hypothesis that they experienced a strong population bottleneck, which has recently been estimated to have occurred prior to 32,000 – 67,000 years ago, nearly eliminating their genetic variability. This signature remains with them to this day, since there has not been adequate time for new variation to arise through mutation. A second example would be the more recent bottleneck in Hawaiian monk seals that occurred in the 1800s, which is easily (and dramatically) seen in their extreme lack of present-day genetic variability. So, contrary to Poythress’s claim, the methods of population genetics are fully capable of detecting a reduction to (or start from) two individuals, since such an event would shape the genetic variability of that species for a very long time, well within the detection limit of current methods. The fact that our genome does not show these features thus remains a problem for his argument.

In the next post in this series, we’ll discuss Poythress’s second claim – that perhaps Adam and Eve lived long ago enough to avoid detection by current methods.

Further reading on the scientific and theological issues related to Adam and Eve:

Note: this list is mostly drawn from my BioLogos colleague Ted Davis's excellent series on Evolution and Original Sin, found here.


This is a companion discussion topic for the original entry at https://biologos.org/blog/adam-eve-and-human-population-genetics-part-13-addressing-critics-poythress

(Dcscccc) #3

here is one problem. a simple calculation give us no more then 15,000 years for the first human. the current population doubling time is about 50 years. lets say that in the past it was even 1000% slower because of wars, diseases and so on (one doubling in about 500 years/ 500 years to get from 10 people to 20). so we need only 30 doubling to get to the current size. or 15,000 years.


(James Stump) #5

@dcscccc You can’t just invoke uniformitarianism when it suits your conclusion. There is strong evidence that the rate of population growth has not been steady throughout human history. And even today it differs wildly between people groups. Prior to the advent of agriculture, the hunter-gatherer societies had very, very low population growth. Your “simple calculation” is going to have to be a lot more complex to account for these facts.


(Dcscccc) #6

jstump- we realy need to believe that in the past its take 500 years to get from 10 people to 20? if extrapolation isnt a good method, why we should believe in radiometric dating?(that also base on extrapolation). we also have evidence that radiometric dating can be wrong in a factor of 10^9 from the real age. so its not realy a scientific method either.


(James Stump) #7

No, you cannot extrapolate from population growth rates today, when we have factory farming, dietary supplements, and pre-natal care, to 10,000 BC when agriculture was just developing. World population sizes were very stable for a very long time prior to that.


#8

James, put yourself in different eras (in your mind). If population was stable at the time, would you argue that it could never increase (as Malthus suggested). If plagues were rampant, would you argue that humans were doomed? If population increases or doubles in thirty years, would you argue that it has always done so? dcscccc has reasonably extrapolated a growth rate 1000 times slower than the present 50 yrs doubling time. Scripture also indicates many people having six to ten male children, thousands of years ago.

Of course, the rate of growth has not been steady, and dscssss acknowledged this in his statement. His statement does not have to be more complex. It only needs to be right. Would you suggest a million times slower growth rate, and on what basis?

When there is lots of food, relatively little competition for deer and rabbits, much land left to be explored and converted to food production, then you don’t need intensive farming, herbicides, fertilizer, irrigation to provide sufficient food. With less dense human population, spread of disease is slower and has lower impact. As land becomes more limiting, then crop yields per acre must double or triple to sustain population. But the interesting thing is that with more limitations showing up all the time, population is likely growing faster than it ever has, even though the more “advanced and developed” societies have a lower fertility rate and lower population growth.

Uniformitarian principles with generous assumptions… when it suits whose conclusions? What if it was your conclusion? I admit I don’t understand your objection.


(James Stump) #9

Do a quick google search on “human population over time” and select images. Compare the slope of the graph today with the slope of the graph from 10,000 BC to about 1AD. You’ll see the objection of extrapolating from today’s rates.

Perhaps you dispute the data on which these graphs are built. That’s fine. But you should at least start with the data and explanations for that data, instead of making wild guesses. These are pretty established models (no, not perfect and immune to revisions).


#10

I’m not sure if I dispute the data from 10,000 BC. I’m not sure there is any real data to dispute.

I agree that if you take certain data from after 1000 AD (hopefully it is real data), that an exponential curve is indicated. But it is also true that recently, that curve is not so smooth, since the last doubling took as long (or longer) as the doubling before it. While population growth may fit a particular curve for a while, whether linear, or exponential, it is not certain that this curve is really constant over time… in fact, it likely is not. We probably do not have a guarantee that there were serious plagues (that decimated more than 5%-10% of the population) in the distant past, or that there were not this type of plague.

The flood did decimate the population, obviously, from millions down to almost nothing. It is easily conceivable that the population could increase from that time to what it is today. This is more data than the estimations for 10,000 BC.

An accurate estimate of the number of humans who have ever lived is difficult to produce for numerous reasons. …More importantly, robust population data only exist for the last two or three centuries.

Wikipedia summary, which I agree with. Except that certain census in late BC and early AD were good indicators, even though often they didn’t number the women and children. But they were not global census, did not include Americas nor Africa nor numerous other areas. So, no data to dispute.


(GJDS) #11

Hi jstump

A site that provides an informative discussion is: http://www.prb.org/Publications/Articles/2002/HowManyPeopleHaveEverLivedonEarth.aspx

From this site, the following comment is instructive and shows us this subject is speculative:

This semi-scientific approach yields an estimate of about 108 billion births since the dawn of the human race. Clearly, the period 8000 B.C. to 1 A.D. is key to the magnitude of our number, but, unfortunately, little is known about that era. Some readers may disagree with some aspects—or perhaps nearly all aspects—of the table, but at least it offers one approach to this elusive issue.”


(system) #12

This topic was automatically closed 7 days after the last reply. New replies are no longer allowed.