T4C: Population Genetics

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So if we can trace a continuous lineage from present day humans back to a time before Adam, how far back can we trace this lineage?

The answer to this question comes from a field of biology known as population genetics, and although it’s tempting to just blurt out an answer of 150,000 years, it would be useful to first briefly explain how population genetics works.

In the previous chapters we touched on genetics, which is the study of genes, genetic variation, and heredity in individual organisms. Population genetics focuses on studying the genetic make-up or variation within populations or species (as opposed to just individuals). We also saw how analysing the patterns of genetic characteristics in different populations has helped researchers to understand how (anatomically) modern humans intermingled with other ancient humans like Neanderthals and Denisovans. By analysing these patterns and the genetic variations within populations and species, these differences can be used to trace our evolutionary history, and to plot detailed evolutionary trees.

But another aspect of evolution that is often misunderstood, is that evolution is a population-wide phenomenon. Even though it is a slow process, evolution occurs through the accumulation of genetic differences that alter the “average” for a population’s characteristics. [1] Over time, new species arise when a population group diverges for whatever reasons - such as geographical or migratory factors - and the “averages” change enough that eventually two distinctly different populations occur, which in time become new species.

Thus, population genetics studies the genetic make-up of populations, looking at the genetic changes that have occurred within populations over time. It’s an important discipline, because populations are subject to change. The recent advances in genome sequencing and computer-based techniques for storing and comparing this information has allowed large-scale modelling to be done of how variations in population groups - and species - have evolved.

The insights most relevant to this discussion are those that concern human evolution, and in particular the evidence for other humans being on earth at the same time as Adam and Eve.

Doing the maths. . . 

It’s worth explaining a few more details about genetics to make the next point. Although there have been some differences of opinion on the exact number, the fact remains that a very large percentage of our genome is made up of non-coding DNA, i.e. genetic sequences that don’t encode proteins. There are various reasons for the existence of non-coding DNA, however the point is this: mutations that occur in these sequences will typically have no effect because these sequences don’t code for anything. So, because there is no effect - positive or negative - the mutation simply stays in place for posterity.

An example of a mutation that occurs relevantly frequently is called a single-nucleotide polymorphism or SNP (pronounced ‘snip’). These are instances where the nucleotide at a specific position in the gene changes; for example from an A to a C. This means that when looking at a specific position within a gene, one would typically expect to see a particular nucleotide (an A for example), but in some individuals there may be a C instead at that position.

These sort of mutations are very useful in population genetics, because they allow various SNPs within a population to be catalogued, and the characteristics for that population group to be defined. Measuring the amount of variation also helps to determine ancestry and heritage. Quite simply, the more variations there are between two genomes the less they are related, and the more similarities there are the more they are related.

By understanding the combinations or patterns in which these SNPs occur helps to determine population or family groups, and the number of pattern variations help to indicate the size of the populations from which those patterns came. These models have consistently returned very similar results, showing that the minimum population size from which all humans descend is about 10,000 individuals: approximately 8,000 ancestors are needed to explain SNP diversity in sub-Saharan Africa, and about 2,000 ancestors for everyone else. [2]

To put it simply (and to quote Dennis Venema), “SNP diversity in humans is far too large to result from one ancestral couple at any time in the last 200,000 years – we descend from a population. These values are also in good agreement with older, cruder methods of estimating population size from other types of genetic variation, giving us increased confidence that they are reasonable.” [3]

What about Mitochondrial Eve?

Some may have heard of a term called “Mitochondrial Eve” . . .  In January 1987, a paper was published in the journal Nature describing research that revealed all humans carried mitochondrial DNA in their cells that could be traced back to a single woman. [4] The term "Mitochondrial Eve" or even the name "Eve" was not used in the original paper; but it’s a catchy term that was popularised by the media, and it stuck.

Putting the research in plain English goes something like this:

Mitochondrial DNA is outside of the cell’s nucleus, and so during meiosis (cell division after fertilisation) it does not “mix” with the male genetic material. Therefore it allows us to trace every individual’s maternal lineage. Each one of us has our mother's mitochondrial DNA, she has her mother's, and so on and so on. . . 

As with other DNA, mutations in mitochondrial DNA do occur over a large population at a relatively constant rate, and so by using statistical models it was possible to trace the female links to a common ancestor through the generations. However, what has been most misunderstood about this research, is that it did not suggest Mitochondrial Eve was the first woman, or the first human, or the first member of a new species. It simply meant that hers was the mitochondrial DNA that is common to all population groups (also known as haplogroups).

It’s not an exact science applying a date to when this woman lived because a lot depends on the average age between generations (for example, between the ages of 15 to 40 a woman could have had numerous babies), however estimates vary from anywhere between 100,000 to 200,000 years ago. (Hence the throw away line of 150,000 years at the start of this chapter.)

The issue of “Mitochondrial Eve” is really more about statistics than anything else. A quick Google search will show just how badly misunderstood the concept is. If anyone would like to dig into this topic in more detail, this link at the TalkOrigins archive would be a good place to start.

The real takeout for us from this research is not this woman herself, but the generations involved. Even though the dates proposed by the various models are highly variable they still represent thousands of generations that have been traced. To put this into perspective, if we acknowledge that Adam and Eve lived 6,000 years ago, and we assume that on average every couple had their first child at twenty years, that only allows for 300 generations, or 300 mothers between Eve and someone living today. Our mitochondrial DNA is too diluted, and too varied to accommodate only 300 generations, and there is too much genetic diversity between population groups. Furthermore, as already noted, these genetic patterns also show that the population sizes across the earth never really fell lower than about 10,000 people.

A practical example. . . 

Organ transplants are a very real example that show the consequences of this genetic variation. At the time this chapter is being written there is an appeal on social media for organ donors from certain eastern European population groups to come forward as a person desperately needs a bone marrow transplant. When we need an organ transplant, first we need to find suitable donors, and then make use of anti-rejection drugs to ensure a viable transplant. This is because our bodies reject foreign tissue and attack it. However, researchers are beginning to understand how this works at a genetic level, and are using genome-sequencing to identify issues associated with transplants. [5]

Yet in complete contrast, some animals like cheetahs can accept transplants from any other cheetah with no issues at all. This is because they had a population bottleneck around 10,000 years ago and so genetic diversity is simply not an issue for them. [6]

A quick recap. . . 

Up to this point we have covered a fair bit of ground, so it’s worth taking a breather to recap some of the points we have looked at.

We started this section by noting three points from the early chapters of Genesis that offer compelling evidence of a wider human population co-existing with Adam and Eve. If this were so, what evidence is there to support this position?

To answer this question we began by looking at the archaeological evidence that shows there were numerous cultures around the world that date from the same time period as Adam and Eve. These cultures show that instead of some sort of mass extinction event prior to the creation of Adam and Eve, there was ongoing growth and development right throughout the Neolithic period.

This archaeological evidence is supported by the genetic evidence that shows how human populations expanded throughout the world, whilst at the same time also showing how we are directly descended to humans that lived hundreds, if not thousands of generations before Adam and Eve. This genetic evidence also shows that the human population of the world today is simply too genetically diverse to have originated from a single couple 300 generations ago.

So, does this evidence take God out of the picture? Hardly. On the contrary, it shows us something more remarkable about God’s creative methods than we have traditionally given him credit for.

It’s time to deal with the elephant in the room

Before we look more deeply at genomic evidence, there is an elephant in the room that we need to deal with. . . 

The idea of humans coming from monkeys is a huge challenge to some people, and rightly so! It’s a fallacious concept that shows how poorly this topic is understood. What was deduced from the fossil record (and later confirmed by genomics) is that apes, monkeys, and humans do have a common ancestor. Yet the scriptures say we are created in the image of God, so where would a common ancestor fit into that fact?

The point has already been made that there is no short answer to this question (hence this book!), but there are two thoughts worth considering:

Firstly, being created in the image of God does not necessarily preclude a progressive maturity to get to that image. Indeed, is that not the point of our spiritual journey? (1 Corinthians 15:49 and 2 Corinthians 3:18)

Secondly (and more importantly) the idea of God miraculously creating Adam is not that unrealistic in the grand scheme of things. The Bible is full of miracles - and by “miracles” we are talking about singular events that are especially notable because they happened in spite of God’s natural laws. For example, Jesus raised Lazarus, to show that resurrecting a dead person was possible (and to help his disciples believe it was possible when he was resurrected). But even after the example of Lazarus, Jesus’ resurrection was a singular, miraculous event. Singular, not in the sense that he was the first to be resurrected, but that he was the first to be resurrected never to die again.

Considering the examples of miracles in the Bible, God uses them to make a spiritual point. The creation of Adam is a miraculous event, and God — who exists beyond the natural realm — is perfectly capable of operating outside that realm if it serves to establish a suitable purpose or principle in his plan. However, just because God chose to act outside the boundaries of his natural laws in a given situation, doesn’t mean this singular act invalidates the evidence of other actions that are in accordance with his natural laws.

So, if God chose to create a human being as a special creation alongside the others that existed — rather than using that singular event to try and invalidate the evidence that he created other humans over a longer period and allowing the how debate to dominate our sensibilities, we should be asking ourselves why, and looking for the spiritual lessons.

We’ll delve deeper into the spiritual matters in Section Three, but for now let’s get back to the genomic evidence . . . 

  1. Dennis Venema, 'Adam and Eve and Human Population Genetics: Defining the Issues' (BioLogos, 2014). https://goo.gl/MYz2Wu
  2. Dennis Venema, Adam and Eve and Human Population Genetics: Signature in the SNPs (BioLogos, 2014). https://goo.gl/SxAcCB
  3. ibid: https://goo.gl/SxAcCB
  4. Rebecca Cann et al, Mitochondrial DNA and human evolution (Nature, January 1987) 325:31-36
  5. Jonathan Gitlin, Using DNA to Monitor Organ Transplant Rejection (National Human Genome Research Institute, April 2011) https://www.genome.gov/27544325/using-dna-sequencing-to-detect-early-organ-transplant-rejection/
  6. Christopher Emerling, Will Evolution Doom the Cheetah? (Understanding Evolution, February 2016) http://evolution.berkeley.edu/evolibrary/news/160201_cheetahs