Friday, July 07, 2017

Friday Science: Adam and the Genome 5

On to chapter 3 of a book by Dennis Venema and Scot McKnight called, Adam and the Genome: Reading Scripture after Genetic Science.

Previous posts
Personal Preface
Forward and Introduction
1. Evolution as a Scientific Theory
2. Genomes as Language, Genomes as Books

Chapter 3
1. Chapter 3 is titled, "Adam's Last Stand?" In it, Dennis Venema presents the genetic arguments for the claim that humanity as we know descended from a pool of some 10,000 individuals. That is to say, humanity does not descend from two individuals but from a population of several thousand.

McKnight will process this claim theologically in the second half of the book. Venema's purpose is to get the current understanding of genetics on the page. His assessment of the creationist response is worded very strongly: "There does not appear to be anyone in the antievolutionary camp at present with the necessary training to properly understand the evidence, much less offer a compelling case against it" (65).

2. The gist of the arguments relate to the fact that the number of variants present in a population seems to connect to the size of the population. Venema uses the example of the Tasmanian Devil as an example. The TD has a profound lack of genetic variability because at some point in the recent past it experienced a genetic bottleneck.

At some point in the last few hundred years, the population was so drastically reduced, that most TDs have exactly the same alleles (genetic options). This now includes a fatal cancer that TD immune systems do not recognize as harmful. In humans, an organ from another human is inevitably genetically different enough that it provokes a strong immune reaction. In Devils, the genetic material is so similar that a cancer does not.

3. So I counted five separate arguments from genetics in the chapter that all independently point to the same conclusion. "We descend from a population that has never dipped below about 10,000 individuals" (48).
  • First, if you take the human mutation rate. Then take the mathematical probability of new mutations spreading in a population or being lost. Then look at how many alleles (genetic variations) are currently present. This calculation suggests that at least 10,000 humans would be necessary to generate that many alleles at a particular point in the past.
  • linkage disequilibrium - When two genes are located close to each other on the same chromosome, the alleles present at both locations tend to be inherited together. It takes some time for "cross-overs" to happen in a population in relation to closely located genes. After crunching the data, it suggests a parent population of about 10,000 individuals.
  • A snapshot of genes not near each other at a point in the past can suggest a minimum population at that point in the past. If we choose around 200,000BC (when homo sapiens is alleged to have emerged), the pool is about 7,000 in Sub-Saharan Africa and about 3000 human ancestors elsewhere, which adds to 10,000.
  • Another approach using mutation frequency and many pairs of genes suggests a human bottleneck of about 1,200 between 40,000 and 20,000 years outside of sub-Saharan Africa, with a bottleneck in Africa of about 5,700 some 50,000 years ago. This adds up to about 6,900 in our total human ancestral pool.
  • Looking at more distant ancestors between humans, chimpanzees, and gorillas, we see that humans and gorillas can share some variants that did not survive in chimpanzees. This is called "incomplete lineage sorting" (ILS). The data suggests that about 50,000 common ancestors would be necessary to account for the genetic material of current humans, chimpanzees, and gorillas.
He concludes: "To date, every genetic analysis estimating ancestral population sizes has agreed that we descend from a population of thousands, not a single ancestral couple" (55).

Venema is basically arguing that if we assume that Adam and Eve are meant to be taken literally rather than literary expressions of the human condition, they were part of a larger population rather than the sole parents of humanity.

4. Much of the rest of the chapter is given to the current state of fossil research. Perhaps I have not yet conveyed Venema's correction of the idea that evolution happens in a straight line. Rather, he would say, you have a collection of variations in a population. Over time the average genetic characteristics of the population change in a certain direction, which is not a straight line.

Similarly, it is not likely that we have fossils that are in a direct line. Instead, we find distant cousins from other branches.

So the current composite theory is that homo sapiens arose in Africa about 200,000 years ago. About 50,000 years ago, significant numbers of humans left Africa (at least 1200). As they left, they encountered other "hominin" species like Neanderthals in Europe and Denisovans in Asia (who themselves may have earlier encountered homo erectus there).

Those descended directly from Africa have none of these other hominins in their genes. Those descended from Europeans have 1-4 percent Neanderthal in their genes. Those in Asia have some Neanderthal and 3-5 percent Denisovan. The Denisovan genome may incorporate some homo erectus genetics as well. All these hominins share a common ancestry that goes back to about 800,000 years ago in Africa.

5. One last thing of interest in the chapter is his explanation of "Mitochondrial Eve" and "Y Chromosome Adam." The Y gene is only passed down through males. If a man has only female offspring, then no genetics unique to the father's Y gene is passed down. Similarly, mitochondrial genes are only passed down through eggs. If a mother only has sons, her unique mitochondrial genes come to an end.

So all living beings are said to descend from a common "mitochondrial Eve" who lived in Africa somewhere between 100,000 and 150,000 years ago. Similarly, all men are said to descend from a common male who lived in Africa somewhere between 200,000 and 300,000 years ago. These two individuals are just two separate genetic lines for different aspects of human genetics. They are unrelated to each other (that is, they were not a couple), and they are different from the population from which our regular chromosomal DNA is said to emerge.

In short, "all humans can descend from one Mitochondrial Eve for our mitochondrial DNA, one Y-Chromosome Adam for our Y chromosomes, and 10,000 other ancestors for our regular chromosomal DNA" (64).

Very interested to see what McKnight does with all this!

1 comment:

Martin LaBar said...

I haven't read the book, but this is a good summary.