What kind of event would cause the current Mitochondrial Eve to be replaced by a new one?

Question

Apparently all living humans are matrilineal descendants of a single woman who lived 200.000 years ago. She is called Mitochondrial Eve.

But at the time she lived there was a different matrilineal ancestor that all living humans shared. And between then and now I imagine many other women have been Mitochondrial Eves of the human population that lived at a particular time.

What I'm wondering is what kinds of events would cause a different, more recent matrilineal ancestor to become the Mitochondrial Eve of all living humans?

One scenario I can image is that the human population is on the brink of extinction and literally only a few people survive. Because there are so few people alive, I imagine that the chance that they share a more recent matrilineal ancestor is high.

Are there other scenarios possible? Does the chance of a new Mitochondrial Eve emerging decrease if the human population is large (billions) instead of small (hundreds or thousands)? Suppose that the human population will not decrease, is it likely future human populations will ever have a different Mitochondrial Eve than the current population? Does it depend on the amount of people that migrate?

The same questions could be posed for Y-Chromosomal Adam and the most recent common ancestor of all humans, I guess...

Answer 1

One possible scenario would be that a particular woman develops a mutation rendering her much more 'fit' (in the evolutionary sense) than her peers. For the sake of argument, lets take the very simplistic case of an X-linked recessive mutation causing the women who bear it to have say 10 children at each birth. If this mutation is then transferred to the offspring, it will very quickly spread across the entire population.

If this continues for a few generations, the crushing majority of the earth's population will be the descendants of that one woman who carried the original mutation, she would be the new mitochondrial Eve.

I want to stress that this is nothing but a very simplistic thought experiment. Such a mutation is i) very unlikely, probably impossible, ii) likely to render the carrying individual less fit rather than more as each birth would be exhausting, increasing the chances of dying during child birth.

---

Update:

I have specified that the mutation has to be X-linked and recessive. This means that it will only present a phenotype in women who are homozygous for it, women both of whose parents carried the mutation. This is because, as mgkrebbs very correctly pointed out in the comments below, otherwise her sons would mate with women who are not their sisters and their children would have the mutation but not her mDNA. If the phenotype is only expressed when the mutation is homozygous, only women whose mother carried the mutation (and Eve's mDNA) would show it and only then would a new mEve arise.

Answer 2

The new mitochondrial eve is alive right now.

When humans reproduce, mothers pass their mitochondria to their offspring without a contribution from the father. As a result, each human can trace back their mitochondrial lineage as an unbroken chain to a single individual woman in the past. Through random chance (genetic drift) or natural selection women in some mitochondrial lines will pass offspring to the next generation, while others will not. If a woman fails to pass offspring to the next generation then she represents the end (i.e., extinction) of a particular mitochondrial lineage. As time progresses more and more of the original mitochondrial lineages will go extinct until all humans represent one remaining linage.

Note that this is a continual process since lineages accumulate differences through mutation of the mitochondrial DNA. This mutation is the source of the different lineages that that will go forward. Based on the estimated rate of mitochondrial DNA mutation we can estimate how long it would take for extant mitochondria to have developed the genetic differences we now see.

If you take a snapshot of this process at one point in time (e.g., contemporary humans), you can use this estimated time to say how long it would take to accumulate the present differences - that is when the line began. Due to the continual extinction of extant mitochondrial lines, there is some point in the future where if we were to take the snapshot we would estimate that the mitochondrial eve was alive right now.

EDIT based on comment:

The rate that a new mitochondrial eve would be formed is based on the accumulation of mutations in non-coding regions of the mitochondrial DNA (i.e., the molecular clock) which is dependent on the population size only in so far as the probability that a given mitochondrial line will survive when all others go extinct is basically its initial frequency in the population. So in a large population with more initial diversity each line will have a lower probability of succeeding.

Also, if there is no selection for or against a given line then changes in frequency are due to chance (genetic drift). In a large population the effect of drift is smaller than it is on a small population. So if the population size is small, then the likelihood that a mitochondrial line will go extinct is greater and the rate of a single mitochondrial line succeeding to be the mitochondrial eve is greater.

Answer 3

What would need to happen for a single woman's mitochondrial genome to be in all living humans ?

All other women would suddenly or gradually need to be eliminated from the gene pool. No other woman could enter the gene pool of this future humanity. So the most likely scenario is one where we would have to be reduced to a single woman at some point.

Genetic isolation was brought up, but the words 'all living humans' doesn't really say 'isolation'. In isolation, there are other people still carrying on somewhere else. Here we are talking about only isolation in the sense that the breeders are the only ones left to reproduce.

Even so, any human woman will still be descended from MEve. That original Eve would still be the mitochondrial Eve, though the genomic record pointing to her would have disappeared.

A completely New Eve could happen if there were a woman unrelated to MEve were to come along, and her offspring were so much more successful than all other women that over time only this woman's daughters were chosen as mothers. She would be incredibly successful- like all her children were 6'6" and much smarter, attractive etc etc, then the rest of the gene pool could dwindle leaving only 'new Eve'. This could happen say if we got some Neanderthals going. They don't seem to be particularly likely to take over though.

This is similar to the scenario where one female lineage simply out-competes all other female lineages to assume the mantle of M-Eve. This seems less likely to me. This lineage would have to enjoy an advantage would have to out-weigh all over variants that exist at the time and would ever emerge over the following generations before our new M-Eve's final domination. Given the number of genes and possible variants, in the entire genome, that is statistically really improbable.

As far as a new Y Adam, this sort of thing nearly happened for men. Just to remind, the Y chromosome is transmitted exclusively from father to son, so a Y Adam would be the only ancestor of all men on the face of the planet.

A national geographic sponsored survey of genomes found that in central asia 8% of all men have the same Y chromosome. Over all humanity, 0.5% of all men have the same Y chromosome. The genetics point to the Mongol Warrior King, Genghis Khan whose campaigns across Asia to the doorstep of Europe included him bedding with one or more different women from the conquered peoples every night, one of whom is reputed to have killed him. Not incidentally killing many men too, eliminating much future competition for his progeny.

Still, all this is a mere fraction of the sort of effect you are talking about. 100% replacement by a single individuals descendants is hard to imagine in less than an extinction like scenario. Or a new race of superbeings coming from a single individual. Hard to imagine given the billions of people on the face of the planet now.

Its a little easier to imagine in the case of a new Adam only because a one man can have hundreds or even thousands of offspring in his lifetime. Even then, Male psychology seems to bring it up more than is rational. Its still extremely improbable (not going to happen).

Its also worth noting that few species have been able to come back from such a small gene pool and number...

Here's a tree of human Y chromosomes. You can see that fat grey blob which are descendants of GK.