Does Genetics Point to a Single Primal Couple?

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April 5, 2010 Tags: Human Origins

Today's entry was written by Dennis Venema and Darrel Falk. You can read more about what we believe here.

Does Genetics Point to a Single Primal Couple?

A Single Primal Couple?

Most Christians who have grappled with the science of genomics (the branch of biology that compares the DNA sequences of different organisms to one another) have done so with the question of common ancestry in mind: do humans share an ancestor with other forms of life, such as chimpanzees?

Here the evidence is very compelling, and reasonably accessible to non-specialists. For example, the human genome has numerous defective genes embedded it, and the vast majority of these defective genes are also present in the chimpanzee genome in the same relative positions with identical mutations. This sort of evidence is easily understood due to its qualitative nature.

A second question, and one that is less frequently explored even by Christians who accept common ancestry, is the issue of human/hominid population sizes during our evolutionary history. Specifically, is the human race descended from one ancestral pair in the recent past? Are we, as C.S. Lewis puts it in his Chronicles of Narnia, the “sons of Adam and daughters of Eve”? Is there genomic evidence to suggest that the human race is genetically derived from a primal pair? Here the evidence is more difficult for non-specialists to appreciate, because it is quantitative in nature.

Genomics can be used as an estimate of population sizes in the past by measuring genetic variation in the present. Genes come in different forms, or alleles: for example, the human ABO blood types are determined by three alleles of one gene. Some genes in human populations exist in hundreds of forms.

The catch, however, is that any individual person can only carry at most two different varieties of any one gene: one from mom, the other from dad. It therefore follows that a large population can pass on a large number of gene forms (alleles), but a population that passes through a population “bottleneck”—where only a small number individuals survive—will fail to pass on most of its genetic variation to future generations.

Attempting to square the Genesis account and common ancestry by positing a literal Adam and Eve who were the progenitors of the entire human race is, biologically speaking, looking for the most extreme population bottleneck a sexually reproducing species can experience: a reduction to one breeding pair.

Is there evidence that such a bottleneck has ever occurred? Dr. Peter Enns has been exploring whether this is even the right question to be asking from a biblical perspective (here, here, here, and here). Here we explore three independent ways of answering the question, this time from a biological point of view.

Method I:

The genetic consequences of a bottleneck required by a literal reading of Genesis 2-3 would be severe: at maximum, four gene-forms (two from each parent) would be passed on by Adam and Eve. Interbreeding in the (necessarily very small) population after the bottleneck would result in the further loss of some alleles due to chance alone. In short, the genetic impact of such an event would leave a stamp on the genome of that species that would persist for tens of thousands of generations as mutations slowly generated genetic diversity.

We can use this information, then, to estimate the minimum number of people that could have existed at any point in time. 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. Numerous studies analyzing many different genes all point to a bottleneck. However, these studies are all clear: during the bottleneck, there were several thousand individuals, not two.

Method II:

In earlier posts, we have discussed the fact that DNA segments known as Alu repeats, can insert themselves at various locations the genome. It turns out that the Alu sequence comes in various forms, like different makes of cars—Fords, Toyota, etc. There are several thousand families of Alu.

Consider just one family, which we will call Ya5. Members of this family have been inserted into human chromosomes at 57 mapped locations. If all humans descended from a single pair of individuals, all humans would have each of the 57 elements in pretty much the same locations, since individual members of the family almost never move. However, the human population consists of groups of people who share some insertion points but not others. The multiple shared categories make it clear that although a human population bottleneck occurred, it was definitely never as small as two. In fact, this line of evidence also indicates that there were at least several thousand people when the population was at its smallest.

This method is much different than Method I since it does not depend upon mutation rate, but the answer is similar.

Method III:

A third independent estimate makes use of a concerted research effort called the HapMap project. Humans have 3 billion bits of information in their genomes. (The official term for one bit is a “nucleotide.”) The bits between any two individuals differ at many sites, which is, of course, why we don’t all look the same.

In the HapMap project, one million of these differences have been analyzed by examining something called linkage disequilibrium The technical details are beyond the scope of our discussion, but to give you a feeling for how it works, imagine that you have a gene for blue eyes and a gene for a bent finger, both of which you inherited from your dad. Assume these genes reside in the same “neighborhood” on chromosome 2. Because these genes are close to one another, chances are that if your brother got the blue eye allele from your dad, he would have received the bent finger allele as well. After all they are neighboring genes, both on chromosome 2. Why? Blocks of genes in the same neighborhood on a chromosome are usually inherited together. Alleles that are very close together on chromosomes tend to stay together for many generations before they are “mixed and matched” through a process called recombination.

Now pretend that someone analyzes both your DNA and that of your brother in a double blind experiment. The investigator would, upon examining the results, be able to say, “I’ll bet these two people are related to each other.” And he would be right.

Now picture being able to do this, not for two differences, but for a million differences all at once and not just for two people, but for many people from all over the world. Using this approach, it is possible to tell how many people gave rise to all the prevalent combinations of differences. In short, we can tell if everyone came from just two people at any time in the last 200,000 years. So did we?

No.

This third independent method tells us that everyone alive today is related, but not to a single pair of people. We are related to a population that consisted of several thousand people with their several thousand combinations of these million genetic differences.

Here’s the real point of this. When you have one way of doing a calculation and you get a certain answer, perhaps you are justified in being a little skeptical. Perhaps you made a mathematical mistake, or maybe you made a faulty assumption. However, when you do your calculation using two totally different approaches, using methods with completely different assumptions, and each method gives you the same answer, you become convinced it is correct. Three, of course is just icing on the cake.

So that’s the situation we are in with regard to the human population size in ancient history. There was a bottleneck. There were likely fewer people alive during that time than the number of fans attending a typical NHL hockey game. (We don’t know if they were all together in one village, of course, but the total was small.) However, it was not two people. Our species diverged as a population. The data are absolutely clear about that.

References:

  1. Relethford, JH. 1998. Genetics of modern human origins and diversity. Annu. Rev. Anthropol. 27: 1-23.
  2. Tenesa A, Navarro P, Hayes BJ, Duffy DL, Clarke GM, Goddard ME, Visscher PM. 2007. Recent human effective population size estimated from linkage disequilibrium. Genome Res. 17:520–526. (available free here)
  3. Sherry ST, Harpending HC, Batzer MA, Stoneking M. 1997. Alu evolution in human populations: using the coalescent to estimate effective population size. Genetics 147:1977-1982. (available free here)
  4. http://hapmap.ncbi.nlm.nih.gov/index.html.en

 

 


Dennis Venema is Fellow of Biology for The BioLogos Foundation and associate professor of biology at Trinity Western University in Langley, British Columbia. His research is focused on the genetics of pattern formation and signalling.
Darrel Falk is former president of The BioLogos Foundation. He transitioned into Christian higher education 25 years ago and has given numerous talks about the relationship between science and faith at many universities and seminaries. He is the author of Coming to Peace with Science.

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Bilbo - #9075

April 8th 2010

Or if you don’t like Adam marrying his sister, we can always go with the rib story.


Bilbo - #9080

April 8th 2010

So have I offered a plausible scenario that satisfies both our scientific knowledge and our traditional understanding of Adam and Eve?


pds - #9090

April 8th 2010

Bilbo,

I appreciate your questions and proposal.  Here is my problem with it.  All of Scripture is imbued with the concept of the uniqueness of humanity.  To kill and eat a human is murder and cannibalism.  To kill and eat a cow is perfectly moral hunting and gathering.  In your scenario, if Adam killed and ate his great grandfather, was that murder or hunting?  How about if Adam used his grandparents for scientific experimentation?  If human rights were relative at the beginning, why shouldn’t they be relative now?  We have already seen an episode in history when many of the brightest people on the planet started thinking that way, because they thought science supported it.

Humans are special, and I think that means God did something very special when he created them.


Bilbo - #9105

April 8th 2010

Hi pds,

You wrote: In your scenario, if Adam killed and ate his great grandfather, was that murder or hunting?  How about if Adam used his grandparents for scientific experimentation? 

Good question.  Assuming that Adam’s grandparents had not been endowed with souls or whatever it is that supposedly makes us different from animals, then I think we would have to say no, it was not murder, and I guess scientific experimentation (assuming God approves of that, and He may not) would also be permitted.  But I imagine that Adam’s grandparents would have looked so much like Adam that he would have been disinclined to eat them or experiment on them, unless he had fallen very low, indeed.

If human rights were relative at the beginning, why shouldn’t they be relative now?

Adam would be the first of a genetically distinct species, that we would call truly human.  All ancestors would be pre-human.  So human rights would not have been relative at the beginning.

Cont…


Bilbo - #9106

April 8th 2010

cont…

We have already seen an episode in history when many of the brightest people on the planet started thinking that way, because they thought science supported it.>/i>

Yes, and we should fight against that.  I agree that the fight would be easier if we could show that common descent is false, but I don’t think we can.  If you are who I suspect you are, then I won’t waste my time trying to convince you of that.

Humans are special, and I think that means God did something very special when he created them.

I and, no doubt, Biologos agree.


Bilbo - #9107

April 8th 2010

But why is nobody else responding to my question?  I have tried to provide an intellectually honest scenario that conforms to the genetic evidence and to the view that Adam and Eve were the first of a new species.  Have I succeeded or not?


pds - #9124

April 8th 2010

Bilbo,

You said,

“I agree that the fight would be easier if we could show that common descent is false, but I don’t think we can.”

I am open to “common descent” of some kind defined properly.  But if God intervened like I see Lewis proposing, I am not sure that “common descent” has much meaning.  What would it mean if it is not driven by natural selection?

I doubt that we can show that common descent is false.  But that is not the question I ask.  I ask:  is common descent by random mutation and natural selection the best explanation of the evidence?  Is it a conclusion that is demanded by the evidence given all the possible historical contingencies?  I don’t see the evidence demanding a strong inference of common descent by RMNS.


pds - #9125

April 8th 2010

(cont.)

Lewis says:

Then, in the fullness of time, God caused to descend upon this organism, both on its psychology and physiology, a new kind of consciousness which could say “I” and “me”, which could look upon itself as an object, which knew God, which could make judgements of truth, beauty, and goodness, and which was so far above time that it could perceive time flowing past. This new consciousness ruled and illuminated the whole organism, flooding every part of it with light, and was not, like ours, limited to a selection of the movements going on in one part of the organism, namely the brain.

Does this sound like common descent to you?  Please explain.


pds - #9126

April 8th 2010

Also, is it plausible that God did what Lewis proposes (“both on its psychology and physiology”) and it did not affect the genetic make up of the creature?


unaplogetic catholic - #9141

April 8th 2010

” I have tried to provide an intellectually honest scenario that conforms to the genetic evidence and to the view that Adam and Eve were the first of a new species.  Have I succeeded or not?”

No.


You need to review the idea of what a “species” is.  It is normally not possible to determine that a particualr individual is a differnt species than its parent.  Such an event would not occur in the animal kingdom, unless you consider hybrids new species.


Savage - #9168

April 8th 2010

“Humans are special, and I think that means God did something very special when he created them.”
“I and, no doubt, Biologos agree.”

Where is the proof that God created humans and that we are special? Remember, you cannot use the Bible stories as proof. And agreeing with the premise cuts no scientific cheese. 

“I have tried to provide an intellectually honest scenario that conforms to the genetic evidence and to the view that Adam and Eve were the first of a new species.  Have I succeeded or not?”

Most definitely not! School yourself in the very basics of biology and you will see why your premise is false.


BenYachov(Jim Scott 4th) - #9186

April 8th 2010

>Where is the proof that God created humans and that we are special?

I reply: Beg the question much?  If God, whose nature is understood in the Classic sense, exists, then logically He is the creator of man either by means of a direct efficient cause brought about supernaturally or by means of a providentially guided natural process.  Good grief the above is as bad as someone asking “What proof do you have life evolved from pre-existing matter”.


Savage - #9201

April 9th 2010

“Good grief the above is as bad as someone asking “What proof do you have life evolved from pre-existing matter””.

No, it is not as bad. You are referring to abiogenesis, i.e. the theory of how life could have arisen from inanimate matter. The sequence of chemical events that led to the first building blocks of life is not known. There are many thing scientists do not know about Nature at present, but continued research constantly fill in the gaps. The religiose call these gaps god-did-it, and expect to be taken seriously.

Richard Lenski, an evolutionary biologist, has bred a new species in the laboratory proving, among other things, that species evolve and were not all created by God as creationist literature indicate. The geneticists and palaeontologists have shown us the evolution of all living (and extinct) species evolved from a common ancestor, but these scientific findings are rejected by ID proponents and YEC. And you want to reconcile science and religion? No chance.


Bilbo - #9205

April 9th 2010

Hi UC and Savage,

Please correct me if I’m wrong, but I assume that it usually takes several mutations to make a new species.  So if we designate the original species as S, and say it takes 100 mutations to produce a new species, T, which is reproductively distinct from S, then all the intermediates might be capable of interbreeding with either S or T.  Nevertheless, the 100th mutation would result in a new species.  If so, then my thesis still stands, and there would have been an original species designated homo sapiens as the result of one mutation.


Savage - #9209

April 9th 2010

Bilbo

Let us define “species” as a group of interbreeding organisms. A new species (say B) has evolved from species A, at a time when A and B can’t reproduce sexually anymore. Now how does this happen? It happens when A is split into two separate populations (for whatever reason) that now live and evolve apart without any contact; call them A1 and A2. After many generations and genetic mutations happening independently in population A1 and population A2, there will come a time when if A1 and A2 get into contact again, they will not be able to reproduce sexually, and two separate species have evolved. (It can happen that SOME individuals in A1 can reproduce offspring with SOME individuals in A2; this is as a result of the relatively large genetic variation that exists within a species. But if A1 and A2 have been separated long enough, this will not be possible.)

It has not been observed in nature, or the laboratory, that a new species developed from its parents.


pds - #9221

April 9th 2010

Bilbo,

I am curious to hear your answer to my questions about Lewis and common descent.

Do you mean your proposal to be a variation on the one Lewis proposes, or something different?

Also, based on your answer to my question, Adam could have killed and eaten his son in law, and it would not have been murder, because his son in law was not human.  That does not seem to fit the overall teaching of Scripture about the Imago Dei.


BenYachov(Jim Scott 4th) - #9230

April 9th 2010

>No, it is not as bad. You are referring to abiogenesis,

I reply: No you have completely missed the point & abiogenesis is irrelevant since if you read my post #8810 I state that “It is possible to believe in regards to abiogenesis that God might have brought about the existence of biological life by acting threw secondary natural causes.” 
  You asked “Where is the proof that God created humans” & that is clearly question begging since if the classic Theistic God exists then by definition He created humans.  Just as if pre-existent matter exists by definition life comes from it.  You real question is “How do you know there is a God” your other question implies that some “god” could exist that didn’t create humans which is silly.  It seems IMHO you have the same Mechanistic post Enlightenment philosophical errors that dog the ID people.  I’m a Thomist your brand of contra-theistic polemics won’t work on me anymore then I launching into a polemic against Lemarkian Evolution will move a Darwinian.


BenYachov(Jim Scott 4th) - #9234

April 9th 2010

>Richard Lenski, an evolutionary biologist, has bred a new species in the laboratory proving, among other things, that species evolve….

I reply: Yeh he did that with a bacterium, single cell organism.  I’m not an ID advocate(indeed only Thomists can successfully polemic ID) nor am I an evolutionary opponent but that is a lame example.  So a single cell organism changed it’s genetic structure but maintained it’s basic form.  Big deal come back when you turn a mouse into a Bat.

>And you want to reconcile science and religion? No chance.

I reply: Replace “religion” with philosophical naturalism & the above statement would be accurate since all Thomists know there is no conflict.


Savage - #9247

April 9th 2010

“It is possible to believe in regards to abiogenesis that God might have brought about the existence of biological life by acting threw secondary natural causes.” 

It is possible to believe anything, but science functions on experiment to supply proof. All religions function on a belief system that requires no proof, thus their incompatibility with science. 

St. Thomas Aquinas was a Catholic priest and philosopher and made no contribution to science. To assert that “replac[ing]  “religion” with philosophical naturalism & the above statement would be accurate since all Thomists know there is no conflict”, is just a philosophical thought and supplies no argument.

The mouse and bat are both mammals and evolved from the common ancestor, the therapsids, but it took longer to evolve than what is possible in a laboratory experiment.


BenYachov(Jim Scott 4th) - #9248

April 9th 2010

The following would be a better scientific example of Evolution then the lame Bacteria example given by Savage IMHO.
http://www.talkorigins.org/faqs/faq-speciation.html

QUOTE"the biological species concept(BSC) has undergone a number of changes over the years.”

I should note Darwinians notoriously have different historical definitions of the term “species” unlike let’s say Thomism which has had the same definition for the last 800 years.


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