Friday, 3 April 2015

Ancestral Adam & Eve Probably Never Even Met

The Y-chromosome point mutation rate in humans : Nature Genetics : Nature Publishing Group

Scientists have revised their estimate of when the last common ancestor of men lived, bringing it much closer to the estimate of when the last common ancestor of women lived. The basis for this estimate is the range of diversity in the Y-chromosome and the estimated rate of mutation.

The calculation for women is based on the same principle but based on mitochondrial DNA (mDNA). We inherit our mDNA from our mothers but males inherit their Y-chromosome from their father. Women don't have a Y-chromosome.

By 'last common ancestor', we mean the unknown person to which we must all be related. The estimate now is that this male ancestor lived between 174,000 and 321,000 years ago, revised downwards from about 350,000 years ago. The last female common ancestor lived about 200,000 years ago.

The new estimate by a team from dCode Genetics in Iceland and from Iceland University, Reykjavik, was arrived at by examining the Y-chromosomes of 753 Icelandic men, who fell into 274 paternal lines.

The apparent difference between the estimates for male and female last common ancestors was difficult to explain because, if the 'molecular clock' was correct, they should have been about the same anyway because, on average, and regardless of how many partners a man or a woman might have, they will have an equal number of male and female offspring, although this will vary at the individual level, of course, so this new research may have resolved that discrepancy.

The logical idea of a last common male and female ancestor is highly significant in terms of evolutionary theory because, if we extend the principle back a little, it also shows that we are all descended from all or almost all our species genetic ancestors, so, any mutation which conveyed an advantage, or any neutral mutation which randomly increased by genetic drift, has an increased probability of being in our genome, so hugely improbable mutations can be concentrated up the improbability gradient by natural selection or genetic drift until the probability of us inheriting it become virtually certain.

The idea of a last common ancestor is essentially mathematical, based on the fact that, as we go back in our ancestry the number of our individual ancestors doubles with each generation. Ignoring for the moment any marriages between cousins, second cousins, etc, and assuming four generations per century, the number of ancestors you will have who lived 1000 years ago is 2^40, which is 1,099,511,627,776 - far more people than have ever lived, and that's only the last 1000 years. Imagine how many ancestors you would have from 200,000 years ago based on that simplistic calculation.

So how come that figure is vastly more people than ever lived? Because as you go further back, the chances of your great, great... grandparents being 1st, 2nd, 3rd, 4th cousins increases until it becomes virtually certain that at least one of your ancestral parents were related, no matter how distantly, in other words they had a common ancestor and you and all their descendants have that same common ancestor.

You can also look at this the other way round.

On average, how many descendants will this common ancestor have had? That depends of course on infant mortality rates, birth rate, etc, but let's assume for simplicity that the population remained stable (even though we know it has tended to increase and that the rate of increase has changed over time). This would mean that this ancestral couple will, on average, have produced 2^40 descendants in just 1000 years. Again 1,099,511,627,776 - and again, far more people than have ever lived.

If we are again considering an ancestral couple from 200,000 years ago, the figure is again astronomical. The reason for this of course is that, although the genes of this ancestral couple have been spreading out into the genepool and would have been present in all those theoretical descendants, as time went on the probability of children being produced by two related people, no matter how distantly, increased until it became virtually certain.

At some point if you start far enough back in our history, any two people would be related, no matter how distantly, so all children would have a single common ancestor. So it should be easy to see how we must all be related to a single last common ancestral male and a last common ancestral female from some point in our history. There two people need never have met, nor even have been contemporaneous but we will all carry the direct descendants of their genes.

In fact, it's harder to see how this couldn't be so. And given natural selection ensuring the genes which made our ancestors better at surviving and producing descendants, it easy to see how advantageous genes become more and more concentrated in each generation as each generation is passed through this sieve of selection and the genes from our increasing number of ancestors are filtered and end up in us.

So, that's how we know we share a common ancestor and what the implications of this are in terms of changing allele frequency over time - in other words, our evolution.

But there is another interesting aspect to this: note that these last common ancestors lived about 250,000 years ago. This is close to the time when some estimates say modern humans evolved from Homo heidelbergensis in East Africa but, given that modern humans evolved gradually from their archaic ancestors, there is no reason at all why our last common male and female ancestors need even have been H. sapiens.

Our last common ancestors could even have been an earlier hominid. After all, almost all our genes have spent far longer in another species than they have spent in us. There are very few genes which can be said to be uniquely H. sapiens genes. Most differences between us and most of our ancestors are not new genes but variations on, and different combination of, very old ones.

But then we also have a last common ancestor with every chimpanzee, every mammal, every vertebrate, every beetle, every blade of grass, every tree and every bacterium. It's just a matter of degrees of separation.

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