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All those "rules" you cite are just conjecture. No one knows the fertility of Neandertal and sapiens offspring.

An adequate reason why there is no Neanderthal mitochondria in modern humans could be there was only a small amount to begin with and it was lost over the many generations since then. A small amount would have high odds against it to last very long.

Can we know for sure there isn't any neanderthal mitochondria in any human being with the sample taken in the studies?

At the same time, I can't really understand why if there is a 2% of neanderthal DNA in our bodies, there isn't any neanderthal mitochondria survivers in our bodies. To me, it just seem so remote thinking that there isn't any female straight line to survivors, if we can't explain it in terms of fertility...

I know that I am probably not seeing the full picture, I need much more information to understand it. I just can't find it right know.

20%??? More like 1-2%

Mitochondria pass down 'intact' from mother to child in the egg - there is no 'mixing' of DNA as there is with sperm-egg fertilisation, where the resulting nuclear DNA in the child is a mixture of paternal and maternal DNA.

For there to be no mitochondrial Neandarthal DNA in current humans, this means that there are no current offspring descended from a female Neandarthal ancestor. That is, there is no unbroken line of daughters.

This potentially implies (but doesn't guarantee) one or more of the following:

• Male Sapiens-Female Neanderthal reproduction did not produce female offspring, or produced sterile females.

• Male sapiens were unable to reproduce successfully with female Neandarthals

• There were Sapiens with Neandarthal mitochondria at one point, but none remain in our population (or have ever been discovered).

Not sure if this will help, but if I understand correctly, it goes like this:

Let's say a woman has only sons. The sons have her mitochondrial dna, and have children. The grandmother's mitochondrial DNA is left behind because, apparently, when the sperm hits the egg, it destroys the mtDNA it carried with it. And, because nature is devious, that doesn't always happen completely. But usually, then the grandchildren will have only their mothers' mitochondrial DNA. However, any of the granddaughters will have their paternal grandmother's X chromosome.

And for further fun, because our chromosomes line up and split apart without regard to whether they came from the mother or the father in the first place, a grandchild might easily have very little from one or another of their grandparents. Like the time the father had passed away, and his mother was sure one of the grandchildren wasn't his child. The DNA test didn't detect enough relation to the grandmother, so they tested against the siblings, which worked because they were full siblings, not half.

Thank you, that makes sense!!

So is mostly like, for whatever reason, it seems like female hybrids from Neardenthal mothers, probably won't be fertil or viable for any reason in a regular basis.

So the 2% DNA we have from Neardenthal comes from (1) Neardenthal father, Sapiens mother reprodution, or maybe, male hybrids from Neartdental mother, Sapiens father reprodution (less likely, since male male hydrids are more like to be sterile)

Am I right?

Thank you, your comment gave me some light. :)

> So is mostly like, for whatever reason, it seems like female hybrids from Neardenthal mothers, probably won’t be fertil in a regular basis.

That is not at all true. All modern humans have a single Mitochondrial Eve, and aside from her all other mitochondrial lineages from modern humans of her era are extinct. By your logic, that would mean that all other (fully human) females from that period were sterile, which is obviously not true.

Reading the wiki article linked above should help you understand what it could actually mean.

That’s not how mitochondrial DNA works. It is passed from mother to offspring 100% intact. Unlike nuclear DNA which mixes and you can end up with a “tiny bit”, mitochondrial DNA is all or nothing.

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Forgive my ignorance but is it not possible that there just wasn't a difference between Neanderthal mitochondrial DNA and H. Sapiens mitochondrial DNA? If it is passed along unchanged except for mutations is it possible there simply hadn't been any changes since our last common ancestor?

We have sequences of Neanderthal mitochondrial genomes, and they're significantly different enough that they can be used in reconstructing a phylogeny.

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It's difficult to say why. There are a number of possibilities:

Male sapien-female neanderthal (MS-FN) couplings did not happen. Perhaps FNs did not accept MSs for coupling

MS-FN offspring were sterile for whatever reason

MS-FN offspring were killed via infanticide

MS-FN offspring were not viable

MS-FN offspring caused some complications in birth.

MS-FN offspring were less hardy the pure N offspring and the N mothers couldn't adapt to keep them alive

We may never know why exactly.

Thank you, that helps. There are many possibilities.

The 20% figure is very relevant though. Per Wikipedia, an estimated 20% of distinctly Neanderthal genes are still extant.

The process here is not that different from mtDNA. When a couple produces a child, each gene from either parent has a roughly 50% chance to be passed to that child. (Ignoring mutations, where the gene might eventually become unrecognizable. Also its chance of being passed on further depends on whether the gene is adaptive or not, but let’s assume it’s neutral.) MtDNA is different, in that it is always passed from the mother, but the child has a roughly 50% chance to be female and thus to be capable of passing on those genes. Chromosomal genes can swap between chromosomes, but this mostly doesn’t affect the 50% probability of each gene being passed on. So the statistics for mtDNA (as a whole since it doesn’t recombine) and for other genes should be roughly similar, perhaps with different rate constants (eg due to men having a wider variation in how many children they have).

In any case, if only 1/5 of distinctly Neanderthal genes have survived this process, it’s not too surprising that their mtDNA didn’t make it (as far as we know).

Yes, thank you.

Okay so this reminded me of a question I had from watching too much Forensic Files:

If mitochondrial DNA is passed down identically from mother to child how is it that we don't all (all humans) have the same mitochondrial DNA from this Mitochondrial Eve?

>t all (all humans) have the same mitochondrial DNA from this Mitochondrial Eve?

You mean alive humans? If so, we have, the mitocondrial Eve is by definition the mother to dauther common line all the alive human beings have. But the mitocondrial DNA is not exactly the same for everyone since it mutates. Have in mind that mitocondrial Eve is not always the same individual, it can change any time a mother to dauther line ends.

If you mean all the human species, this would be a different concept, since the mitocondrial Eve concept comes from the ALIVE individuals. But you could theorically think about this new concept. You would have to go back so long to get back to the first ancenstor.

Have you met male sapiens? They will attempt to mate with anything animate and inanimate.

Other mechanisms are more likely than, "dey were ugly."

To add.. The egg has mitochondria thus mdna inside. The sperm has it only in its tail. 99.9% of the time just the head gets inside the egg during fertilzation thus no paternal mdna is passed on.

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Ah so it does mutate. So when Forensic Files says they got an "exact match" it's not necessarily true that one will perfectly match one's mother?

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It is also conceivable that the skulls of the hybrid children did not fit through the pelvis of the female Neanderthals.

That's right, it mutates even more than nuclear DNA.

The child would live and be adopted by the tribe and still pass on its genes, we would still have some floating around if that was the only reason.

I don't want to exclude the possibility that there may have been one or two caesarean sections 40k years ago, but the problem would have been passed on.

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The child would live?

Ah okay thanks that makes sense

Mitochondria are endosymbionts meaning at some point in time they made their home inside of us. On the timeline of neanderthals and mitochondria which one came first?

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So the only way mitochondria change through generations is via mutation?

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On average a mitochondrial sequence will have a single mutation every several hundred years. So exact matches are common if it's within ~20 generations or so.

If their skull did not fit through the mother's pelis, it would not live at all. This was some time before we invented surgery.

To get Neanderthal mitochondria today, you need an unbroken chain of mother-daughter ancestry going all the way back to the Neanderthal. At no point in that entire chain could there be a mother who had only sons, because then the mitochondrial line would be lost.

So we can't really tell much by the absence of mitochondrial dna....it's just too easy to lose by pure chance.

So matching mitochondrial DNA doesn't actually mean much in the context of forensics then, because you could also match with your 13th cousin 6 times removed and for all you know there are 17 of them in your village?

(I am zero surprised a TV show is wrong about science, by the way.)

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Definitely could be, but most data we have suggests Neanderthals had larger skulls. Shape and 'at birth' size could be a factor though.

Most likely Neanderthal women were broader and more able to handle larger skulls so I wouldn't count on this explanation if I was starting a thesis.

When you use DNA to match to a suspect, you usually use short tandem repeat (STR) length patterns in nuclear DNA, which change more frequently than nuclear DNA by several orders of magnitude.

Interestingly, there is a paper which suggests that at least some groups of Neanderthals had their y chromosome and mitochondrial DNA replaced via an earlier admixture with modern humans https://www.science.org/doi/epdf/10.1126/science.abb6460

This goes against the theory that ms-fn hybrids didn't happen, and suggests instead that they were selected against in modern humans populations

There are multiple episodes specifically about matching mitochondrial DNA with the suspect's mother, which was my specific question. :)

should definitely get a nice match with a mother, but i wouldn't put anyone on death row based on that alone :)

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This is the correct answer and no other conclusions are valid outside of this. I really recommend anyone interested in the subject to read "Who we are and how we got here" by David Reich*. The book explains how mtDNA works and talks about our Neanderthal genes, among other very interesting things, i.e. how they have "found" ghost ancestors in our DNA, people/species that have to have existed but there are no findings of them yet.

*P.S. I myself got the book recommendation from a fellow Redditor in this sub, thank you!

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Mitochondria 'merged' with other archaea to give rise to eukaryotes, probably around 2-3 billion years ago.

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Mitochondria became endosymbionts before there were even animals at all, let alone mammals or primates.

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Correct, and possibly not even then. The mitochondrial genome is highly conserved — functional defects would be catastrophic for an organism.

EDIT: That is, there’s enough variation for it to serve as a “molecular clock,” but there’s not a lot of tolerance for error. It would be more accurate for me to say most regions of the genome are conserved.

(Also, not a geneticist, so I’d be happy to be corrected by experts.)

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I disagree... If interspecies reproduction were close to 2%, which is possible given the molecular DNA we share, given the same ratio of survival (this is an assumption) the individuals with neardenthal mitochondrias should be close to this 2% nowadays. I think this is just an statistical thing... If you are a human being alive, there is for sure a straight chain of women above you. It's proven that all them are Sapiens. But at the same time, if there isn't any other explanation, it's highly unlikely there aren't any nearthental women in this chain for any alive human being today.

Mitochondria lines also die off just because of random chance. There was a woman who lived a couple hundred thousand years ago. Every woman alive is a direct female line descendent of hers. There were likely thousands of other women alive at the time, but every one of their female lines eventually died out, but hers survived. Why? No particular reason. Just random chance.

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And before anyone asks, yes humans and Neandertals diverged sometime after that.

This (only the head of the sperm enters the ovum) was once thought to be true but it is now known that the entire sperm cell, including the mitochondria and flagellum, enters the egg at fertilization.

Misconceptions about mitochondria ...

Nature - eliminating paternal mitochondrial dna

The egg contains over 100,000 mitochondria while the sperm contains 50 - 75. This could mean that the paternal mtDNA are either diluted out of the embryo or eliminated early in oocyte development.

The first paper suggests the paternal mtDNA could simply be diluted beyond detection:

>We simply do not yet know what happens to the paternal contribution of mtDNA in humans, but the simplest explanation is that it is diluted beyond recognition by researchers using relatively low resolution techniques of molecular biology.

The second one suggests elimination of the paternal mtDNA

>In mammals, the inheritance of mitochondrion and its DNA (mtDNA) is strictly maternal, despite the fact that a sperm can inject up to 100 functional mitochondria into the oocyte during fertilization. The mechanisms responsible for the elimination of the paternal mitochondria remain largely unknown.

2% DNA doesn't mean 2% interspecies reproduction....there's not a direct relationship between numbers like that. You can get to 2% DNA from a relative handful of crosses. As this paper shows only a few hundred crosses, basically one a generation across the whole range overlap, could account for the observed level of Neanderthal DNA in our genomes.

>But at the same time, if there isn't any other explanation, it's highly unlikely there aren't any nearthental women in this chain for any alive human being today.

Not really. It's very easy for mitochondrial DNA lineages to go extinct. For any particular lineage to stick around, it's like tossing a coin and coming up heads every single generation in a row. Even in the large population of humans, it's not surprising random chance would eliminate all neanderthal mitochondria.

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It may also be possible that neanderthal mitochondria are/were not compatible with homosapian physiology or cell chemistry

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There's ample evidence of Neanderthals with various severe, yet healed, injuries, including severed limbs. Suggesting they at least had enough medical knowledge to mend wounds/severed limbs and possibly even amputate.

It's not a large leap to think they would have had enough intuition to use a knife to cut out a baby if a mother died in child birth.

All this chatter about the viability of offspring of different pairings is all very interesting, but people are overlooking the social aspect to all of this. Mitochondrial DNA is passed down from mother to offspring so there needs to be an unbroken line of daughters for there to be modern Sapiens with Neanderthal DNA. But think back to the first hybrid offspring of that Neanderthal mother - it would have most likely stayed with the mother and her social group, so you are more likely to find hybrids with Neanderthal mitochondrial DNA in the Neanderthal population. Hybrids in the Sapiens population would be more likely to be the offspring of Sapiens mothers and Neanderthal fathers. For there to be Neanderthal mitochondrial DNA the Neanderthal mother would need to be socially integrated into the Sapiens group. Maybe someone more knowledgeable than me can say if there’s any archaeological evidence for this. But it seems likely to me that these were “one night stands” (allow me to be anachronistic haha) rather than “marriages”, otherwise Neanderthal mitochondrial DNA would be part of our modern genetic diversity.

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Amateur here. Is there evidence Neanderthal skulls were smaller at birth? In adulthood, their brain cases were 100ccm+ bigger than ours, on average.

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can you give me some link so i can read more on that woman?

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Great explanation. The last question mark is the relevant one.

The sexual attraction could also have been asymmetric, without requiring full on sterility for the neanderthal mother and sapiens father.

(sapiens hybrid skull size at birth as a third contender)

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Exactly what I was thinkin! We know ALOT of maternal mitochondrial lineages were lost. So with hybridization, they were either lost, or never existed in the first place.

This is smart. Sure, maybe there were a few successful caesareans, but what are the chances of continued success throughout the family lineage???

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Also possible, there is some neandertal mitochondrial DNA in some family that hasn't been tested yet. Some dude in South Carolina about a decade ago got a genetic test and learned that his Y chromosome diverged from everyone else ever tested like 250,000 years ago. Turns out this one patrilineal family survived with few members in Cameroon. It is perfectly possible some ancient undiscovered matrilineal line with Neandertal mitochondria is alive and well in some remote corner of the world.

She's called Mitochondrial Eve. Her male counterpart is Y-chromosome Adam.

The correct way to read this is, all living women have an unbroken daughter-to-mother line with Mitochondrial Eve, and all living men have an unbroken son-to-father line with Y-chromosome Adam.

That doesn't mean there were no other women or men in their time, nor that none of the others reproduced, nor that none of the other's bloodlines survived - just that all of Mitochondrial Eve's female contemporaries' bloodlines either died out or was solely carried by sons at some point, and a similar deal for Y-chromosome Adam.

They also never met. Actually, if I remember correctly, they lived several hundreds of thousands of years apart.

We also don't know anything about the individuals, and there's nothing special about them - if you went back to the time of Mitochondrial Eve (assuming you could nail it down to a specific generation, which I don't think we have with absolute certainty), and found her tribe somehow, you wouldn't be able to tell her apart from the other women. Actually, who she is changes all the time - If a woman who's carrying the last copy of a mitochondrial branch (imagine Eve has two daughters, and one of their female lines has only one surviving woman) dies daughterless (either doesn't have surviving children or only has sons), then the title of Mitochondrial Eve might move down a generation (if Eve had two daughters and one line dies out, then the daughter who's female lines survive is the new Mitochondrial Eve).

It's more of a statistics thing than anything - it's just if you trace back all the women you must eventually find a common mother. That's Mitochondrial Eve.

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Ehhh... Doubtful. C-sections weren't all that successful, even for the child, until fairly recently in the scope of human history.

Given that there'd be a very strong evolutionary pressure against needing a risky procedure to live, and you'd need it to be consistent, it's doubtful that that'd survive very long.

I liked this approach so much. We were simplifying it a lot.

Does that mean that every living human on this planet has the same mitochondria?

It is one family of mitochondria, but there are random mutations that happen regularly. Through tracking these mutations, you can tell how closely different people are related to each other, matrilineally speaking.

Wow thanks. I am already out of date, I learned that in genetics in college!

More likely as Sapain tribes moved north into Neanderthal territory they had less survival knowledge and ended up freezing or starving and the Neanderthals were more likely to take in the women because they were less threatening. Over time the hybrid humans kept mingling with new waves of Sapains coming from the south as the ice caps melted making their DNA increasingly less Neanderthal.

I think they meant a small amount on a population level, not on an individual organism level. And on a population level, a small amount of Neanderthal mdna carriers would indeed be easy to lose to history because they would have to have had an unbroken line of daughters leading to today, any carriers who only had sons would break the line, as others explained above. Thousands of years alone is plenty of time for that to occur, if the Neanderthal mdna female homo sapien population was small to begin with.

I think one of the misunderstandings going on here, other than misunderstanding how mtDNA is passed on to the next generation (always mother to daughter), is that we don't carry all of our our ancestors DNA, our genes are simply not many enough. Some ancestor DNA is lost from generation to generation, in time this will mean some DNA is gone forever. Scientists try to measure differences by comparing mutations, the differences in DNA samples, between different groups and then excluding a third group from 2 other groups. That's how they could assume Neanderthals share DNA with non-African people but not with Africans. (But they also discovered that Africans share ancestry with some archaic humans that maybe shared ancestry with Neanderthals, so maybe Neanderthals mixed with humans at different times).

To assume the fertility of different "hybrids" is just too much speculation. At several other (later) occasions, there was a clear disparity between the numbers of males and females from different group mixing with other groups, implying some kind of aggression ("war") or male-only migration.

Is think it would be the other way around. Neanderthals have bigger heads than we do.

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Successive generations and larger population dominance would eventually result in a singular case. The fact that mitochondrial eve isn't close in time probably indicates sufficient mixing and lineages.

If were going to extend biblical metaphor, it's better to also use the names of Noah and Namaah.

A y chromosomal male is better identified as Noah, whereas a mitochondrial Eve is Eve. After all Noah's sons brought their wives in the genetic bottleneck.

> all living women have an unbroken daughter-to-mother line with Mitochondrial Eve

Since all humans regardless of sex have mitochondrial DNA (unlike a Y chromosome), all living men also descend from the Mitochondrial Eve.

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That was a very good answer. You are right, I wasn't seeing the whole picture.

Eh, probably.

I think the Bible metaphor sucks because it gives the impression that no one else was around or passed on their genes, and that's very not true. It works on the surface level (in a "everyone's ancestors" sense), but it lends itself to being misunderstood and requiring a huge comment to clarify what it means.

I believe the original name was "lucky mother", which... Lost out for obvious reasons.

Do any of these variations [edit: mutations] change the form/fit/function of the mitochondria?

It seems to me that any marginal improvement there would yield a pretty significant boost to fitness.

(Thank you for fielding Qs. I’m only a moron, but this thread is brilliant.)

"white?" - there were no white people at this time

Doesn't even need to be remote, if your line hasn't been tested it could be you!

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This kind of explains why we don't have any neanderthal mitochondrial DNA though, doesn't it? If we are all descendents of this woman & she did not have neanderthal mitochondrial DNA then we wouldn't either...

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Just to clarify (because I’m a teacher):

mDNA = messenger DNA

mtDNA = mitochondrial DNA

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Given how “inbred” we as a species are (having all a common ancestor that lived 2000-3400 years ago and Neanderthals living together with us tens of thousands of years ago - why isn’t it reasonably safe to assume the third option?

An unbroken line of daughters feels immensely improbable (but statistics might be counterintuitive here)

Probably. Some mutations are good, some bad. If it provides selective advantage for ATP exchange then over a long period of time that mutation could become prevalent if that ATP exchange could become more common if it meant more breeding.

It’s worth noting that modern humans don’t necessarily rely on ATP exchange for “breeding” at this point. The movie idiocracy is an extreme example. But our survival selection method is pretty close to societally based at this point

Edit: for most of human history there are probably some tangible selective advantages to some mitochondrial mutations, just like anything else. I don’t have any specifics

Humans and Neandertals coexisted and interbred about 50,000 years ago. Mitochondrial Eve is thought to date back about 150,000 years ago. Of course, it is possible that there are as of yet undiscovered branches in the mitochondrial family, and Mitochondrial Eve dates back quite a bit further. Before the dude from South Carolina took his DNA test, Y chromosomal Adam was thought to date back about 150,000 years, but finding him pushes Adam back perhaps 250,000 years. And if some Neandertal mitochondrial lineages are found in humans, that could push Mitochondrial Eve back to more than 500,000 years or so, to before humans and Neandertals split.

remote with the regard to the current discovered set, not the commenter.

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What about mitochondrial Eve's mother?

Isnt that odd? Or maybe just seems strange at face value. Nature rarely does anything in ones, why would all other lines vanish rather than have a mix of many lines

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There's no word for her.

But Mitochondrial Eve is the most recent such woman - If her mom had multiple daughters it's entirely possible she used to be Mitochondrial Eve, but no longer is because one of her daughter's female lines was broken.

It is just a natural effect of lots of time combined with a population that doesn't grow quickly (as human population didn't for the majority of its existence). Take all the women living at a specific time in history and track each of their lines. Over time, just by random chance, one line will grow in members, which means another line will shrink. Every so often this random growing and shrinking will mean a line will shrink to nothing. However, once it is gone, it is gone forever, and so will never grow again. One by one they are snuffed out, until only one remains. And then Mitochondrial Eve moves forward in time. Since populations started growing considerably, lines have been dying out less.

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Man, I wish discoveries like this were the only possible consequence of widespread genetic testing.

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I personally liked the theory that Neanderthal females found sapien males to be small and weak, and weren't particularly interested in mating with them.

Mitochondrial Eve likely hasn't changed in quite a while. With rapid population increases since agriculture became a thing, lines are much less likely to die off.

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I believe mitochondrial Eve’s mother would be a singular mitochondria outside of a human cell.

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I mean, knowing if you have markers for certain medical conditions so you can make informed decisions for yourself are pretty good. Just as long as we don’t go gattaca with it

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Ehhhh... Maybe.

Fertility rates among some demographics is dropping pretty quickly, which could easily end a line.

And lines end by chance too - even with an increasing population, an endangered line just needs a few accidents, or a few lesbians, or a few child free daughters, or even just a random generation of sons to come to an end.

Now, most lines that end won't be ancient enough to move mitochondrial eve - since it can only move down if exactly one of her daughters has a surviving female line.

But, the point is that "Mitochondrial Eve" isn't a permanent title, or one specific super important person - it's flexible. Who she is changes with time.

A lot of people overstate the importance of Mitochondrial Eve - not that she isn't important to our development, obviously everyone on earth literally has a part of her in us - but she isn't unique. If you went back in time and shot her, another woman would take her place, and you'd come back to an earth that's probably very different, but probably also is still populated by many humans - And there are probably plenty of humans back then you could have shot and changed history even more.

Indeed, any given woman alive today who has/will have multiple daughters could become mitochondrial eve at some point in the very distant future - It's not impossible, though it's kind of unlikely at this point.

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It isn't just the growth, it is the sheer population. Suppose you had an ancient generation that was down to 2 women with matrilinear descendants. One line accounts for 99.9999% of the current population, and the other the rest. If your world population is 1,000,000, then the likelihood you will have a new Eve pretty soon is high, as there is currently only 1 woman left of the minority line, and the chance of any given woman not having daughters, or her daughters not having daughters is pretty high. But if the world population is 8,000,000,000 then the chance of a new Eve is low any time soon as you would need 8000 such occurrences.

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A few things; 1, there's seems to be two inter breeding "events" 140k years ago, and 40k years ago, and the mitochondria DNA of Neanderthal prior to the 140k time period appears very similar to Denisovans, while after looked more like Sapiens, suggesting a replacement of Neanderthal mothers with Sapien ones, likely do to our more rounded shaped skull and adapted birth canal (also we have a few fossilized examples of Neanderthal mothers with near full term Sapien "hybrids." 2, there's a growing (slowly) position that Sapiens, Neanderthal, Denisovans, etc should actually be considered subspecies of Homo Erectus, as we appear to be able to fully, and successfully breed with one another, meaning a speciation event never happened (although I would argue by now that has likely occurred.)

Also there's the misconceptions of the genetic bottle neck, that supposedly cause only 50 to 5000 pairs (depending of the source) of humans to be alive at one point, and that's completely false. If a father has only daughters (or only his daughters have offsprings) then his Y legacy ends, and in modern genetic testing he never existed. Same with a mother, if she only has sons (or only her sons successfully procreate) then her mitochondria DNA ends, and we will have no evidence of her genetic existence.

Just remember, we, as a collective, have at least 6 other hominids DNA within our genome (2 of which we only know from genetic testing).

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quoting @za419: Actually, who she is changes all the time - If a woman who's carrying the last copy of a mitochondrial branch (imagine Eve has two daughters, and one of their female lines has only one surviving woman) dies daughterless (either doesn't have surviving children or only has sons), then the title of Mitochondrial Eve might move down a generation (if Eve had two daughters and one line dies out, then the daughter who's female lines survive is the new Mitochondrial Eve).

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I understand some lines will snuff out and some will progress but why would there be exactly one line that survives? Wont you expect a mixture of lines to survive?

The answer lies in time. In a given period of time, just from random fluctuations there is a chance the number of surviving lines will decrease, based on how many lines are left and the population. So, given enough time, assuming the population doesn't grow, the chance the number of lines will decrease eventually approaches 100% just like theoretically you can flip a coin as many times as you want and always get head, the chance you will eventually get tails approaches 100%