h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Entry: Mitochondrial Eve - An Explanation - A703199
Author: Frogbit - U175610

A short, simple piece on a hugely misunderstood theory.

Hiya,

Good stuff, very understandable to a science moron like myself.

One little typo - >>an startling result<<

and a repetition of a sentence just after; dang, I've not got the text in front of me now, but the bit about advanatageous traits of Eve's offspring, I think.

Otherwise, nothing more constructive than my best wishes for this one.

seeya
spiff

Cheers Spiff - got the typo, got the repetition (that'll teach me to copy and paste rather than cut and paste).
Frogbit.

I liked it, not really a constructive comment, but oh well.

SallyM

Welcome nonetheless - thanks Sally.

Nice article - a couple of small points, then a couple of more random thoughts.

I'm not sure saying the mitochodria produce electricity is the best way of explaining things - saying they consume sugars and store some of the energy produced in an chemical form that the rest of the cell can easily use to drive chemical reactions may be more accurate.

The paragraph starting "But why is it considered separate from the cell..." might be better just stating that mitochondria contain their own DNA (seperate from the nucleus), and reproduce by themselves, *almost as if* they were separate creatures.


Many people do have the strong impression from the mitochondrial Eve theory that even if there wasn't a point in time where only one woman bred, there must have been a time when there were only a small handful of people, and they may wrongly assume that all the nuclear genes must have gone through a tiny bottleneck at the same time, which doesn't necessarily follow.
Given any woman (not necessarily an Eve) who has a couple of daughters, who themselves have one or more daughters, and so on for a few generations in an expanding population, it would be easy to end up with many women with the same mitochondria, but with all their nuclear genes being in much the same proportions as in the rest of the population, as a result of outbreeding with males. In fact, after only a few generations, some of the descendents might carry no nuclear genes inherited from the initial woman. If some later disaster wiped out all the women who weren't female-line descendents of that initial woman, there would be a single type of mitochondria, but potentially lots of variety in nuclear DNA.

Also, how sure are we that if some mitochondria DNA layouts do confer a selective advantage, that advantage doesn't operate over prolonged time periods, and help 'prune off' mutations in mDNA, giving a projected Eve date more recent than might actually be the case?
Are there specific regions where it's known that random variations don't make much difference to fitness and which can therefore be used for timing, or is an average timing for mammalian mDNA mutation worked out relative to known (or assumed) dates of species diverging?

"I'm not sure saying the mitochodria produce electricity is the best way of explaining things - saying they consume sugars and store some of the energy produced in an chemical form that the rest of the cell can easily use to drive chemical reactions may be more accurate."
I'll work on it.

"The paragraph starting "But why is it considered separate from the cell..." might be better just stating that mitochondria contain their own DNA (seperate from the nucleus), and reproduce by themselves, *almost as if* they were separate creatures."
I wanted to keep this simple, and step by step. It's the least complicated way of describing their important differences.


"it would be easy to end up with many women with the same mitochondria, but with all their nuclear genes being in much the same proportions as in the rest of the population, as a result of outbreeding with males"

This is the case. The samples were of mitochondrial DNA. Outbreeding? - Breeding with males from without the community? You'll have to help me out there.

"If some later disaster wiped out all the women who weren't female-line descendents of that initial woman, there would be a single type of mitochondria, but potentially lots of variety in nuclear DNA."

Again, no problem - as long as the mitochondrial DNA was intact. You'd have to have lots of variety in the nuclear DNA otherwise we wouldn't have ended up with Masai, Inuit and Viking characterisitcs from the same source.


"Also, how sure are we that if some mitochondria DNA layouts do confer a selective advantage, that advantage doesn't operate over prolonged time periods, and help 'prune off' mutations in mDNA, giving a projected Eve date more recent than might actually be the case?"
It's not the mitochondrial DNA that confered the selective advantage. The nuclear DNA would have conferred the advantage - the mitochondrial DNA merely records when this happened. The dtae has been confirmed at 171,000 years precisely. I daresay it'll be different before long. There are an abundance of Creationist sites who use the same reasoning to get a date of 6500 years. Oddly enough

"Are there specific regions where it's known that random variations don't make much difference to fitness and which can therefore be used for timing, or is an average timing for mammalian mDNA mutation worked out relative to known (or assumed) dates of species diverging?"

It's an average. mtDNA mutation rate is calculated to be between 2 and 4 percent per million years (so that's just a leeway of 100%).
The more I delve into this subject, the less it seems worth delving into. This is why I put the final paragraph 'Is any of this true' in. Although I have an interest in the theory, my aim was to attempt a clear explanation for those who were confused about it. I wanted to introduce the hourglass idea - the when isn't important. I wanted to emphasise the fact that there have been very close scrapes in the past. History is contingent - science is provisional.
Cheers Potholer.
Frogbit.

Ps - thanks for the comments, and apologies for the formatting of that last posting.

The mention of 'outbreeding' was meant to indicate that DNA unrelated to Eve (or any of her descendents) could seep in to her female line to the point where none of her nuclear DNA was left.

What I was really wondering about was how much of a nuclear DNA bottleneck there might have been.
Clearly, there could well have been a severe one, which many people might assume follows from the Eve theory, but there could have been something more complicated.

Given that even after only a few generations, most of the nuclear DNA in Eve's female-line descendents could be pretty much similar to that in the rest of the local population, if a very tight bottleneck did happen, it seems possible that it could have been as much a matter of luck as selective advantage that no other women survived whose female line carried on to the present day.

Expanding on that, as you said, the theory doesn't actually say there weren't any other women who survived the bottleneck, just that going back up the pure maternal line, everyone can trace back to the same woman, but if we could follow up the incredibly more numerous mixed male/female lines, (which isn't possible with mDNA), we could probably all trace back to some (or many) other women living at the same time.

Similarly, considering most of the genes in which we all differ from the common chimp/human ancestor, given perfect information I guess we could all trace our ancestry back to some random individual who happened to have that particular mutation.

You say the date is not important. But I thought that the date was the main objection to the theory of mitochondrial Eve. At 200,000 years, it's too recent, considering that the Aborigines populated Australia 50 - 100,000 years ago. This is not my opinion, it's one I've heard. I don't know anything about this myself.

Another excellent entry Frogbit on an very interesting subject.

A query - I'd just like to know a bit more about how people know this. I'm assuming there are ways of determining the lineage? Is it from material found in bones or what? Prior to 200,000 years ago or thereabouts, were there other types of mitochondria around that dies out because of the population crash. I know that evidence for human origin goes back much further that this time.

Thanks Miss Fish, hi Gnomon!

"I'm assuming there are ways of determining the lineage? Is it from material found in bones or what? Prior to 200,000 years ago or thereabouts, were there other types of mitochondria around that dies out because of the population crash. I know that evidence for human origin goes back much further that this time."

There are no fossilised mitochondria - even if any were discovered it would almost certainly be impossible to extract any data from it. The research done was all based on mtDNA taken from living populations. Given the mutation rate of 2 and 4% per million years, it has been calculated that our most recent common ancestor lived around 200,000 years ago (171,000 years ago according to a more recent figure). These calculation rates assume that the rate of change is steady. Given that the leeway for for the mutation rate is 100%, and that it is unknown whether the rate is constant, it strikes me as somewhat rash to give a precise date.
The main contention is between molecular biologists and palaeoanthropologist. The palaeoanthropologists have the fossilised evidence of hom's exodus (probably exodi) out of Africa, and the molecular biologists have sequencing machines and extrapolations from modern humans.
The arguements between them for the date rage back and forth in an undignified manner, with both sides attempting to prove that their science is more accurate than the other's.
This is a real turn-off for me. Although the date seems to hold tremendous importance for those attempting to find a precise time for the exodus, it strikes me as more important to aknowledge that the event occured, and that there are striking implications for the event. The date wrangle seems to me as nothing more than squabbling over who's science is better, with a lot of the results being published hurriedly in order to bloody the other camp's nose. The important thing is to establish, without doubt, that the first idea is correct, before attempting to prove the second point using incomplete data. The experiment used only a few thousand people. Some of the energy and finances being used to prove when could better be spent proving *whether*.
(IMHO)

ps Miss Fish, as you rightly say the evidence of human origin goes back much further - I believe 6 million years is the date currently being bandied about after the discovery of a certain fossil. However, using cladistics, we cannot say that he/she was an ancestor, but only a relative. The fossil is only proof that *a* homonid was wandering about at that time - it may not have had anything to do with us.
But...but if it is a relative, then given that the estimate for our splitting off from our common chimp ancestor is 8 million years ago, we're getting a bit closer. Who knows, one day we may even have a population that widely accepts our position in the animal kingdom.

Thanks Frogbit. Very interesting.

I'd still like to know how the calculation is done. How do they know? I'm not trying to be difficult. I'm really interested. Is it too difficult to explain (or have you explained it already and I don't understand)?

If you have a rough date for when two species diverged, (or two populations of a species became permanently isolated), you can look at the modern distribution of mutations in (and between) the two groups, and get an estimate of how much divergence happens over time.

However, I guess you have to make some kind of assumption that at the time of the postulated split, if there was much variation present in the mtDNA, there was a common pool of variation which the two groups shared roughly equally, and that subsequent changes in the mtDNA are mainly a matter of random mutations, rather than being driven largely by evolution.

Assuming the mutation rate is similar for many species, some averaging of results should improve the accuracy of the estimate of the mutation rate, but the average limit of accuracy in dating when divergence of any two groups happened seems likely to put an upper limit on how subsequent calculations can be.

On a much shorter timescale, I guess you could take samples from down the female line of animals of known ancestry, but I'm not sure how well that data could be extrapolated to longer timescales.
Also, possibly the mutation rate might vary with the nature of the animal - larger, slower, longer-lived creatures might have a different rate of mutation (relative to time, rather than generations) than smaller fast-living ones.

Browsing around, there does seem to be quite a bit of uncertainty about the actual rate of mtDNA mutation.

OK, ish! Thanks Potholer,

However I still have a query. Mitochondria mutates at a *very* slow rate, if I understand it arightly. So if it is the same for all peoples on earth at present, couldn't it be that mitochondria is mitochondria is mitochondria, ie there never were variations? Or are there minute differences now, but if we trace them back, they become identical?

Bear with me on this one. I am trying to understand.

There are differences between humans living now. There are also differences between the group of variants of mtDNA posessed by one species, and the group of variants posessed by another species.

Generally, the more closesly related two species are, the more similar their collections of mtDNA are to each other, though I've read on a couple of websites that mtDNA-based family trees don't always coincide exactly with those based on nuclear DNA.

OK, I'm getting a little clearer, however I'm not there yet.

How did they show we are all related to this woman. Fundamental similarities? How do they trace back the mitochondria back?

The project found that all human mtDNA had a surprising degree of relatedness, indicating a common source. To estimate a time when this common source lived, an approximate mutation rate of mtDNA was established (a rate of 2 and 4% per million years). From there they arrived at an estimate of how much mutation had occured since the ancestor lived (171,000 years ago given the above factors).
You may have noticed that a terrifically precise number has been calculated from two estimates. Initially this was a smallish experiment, the results of which were interested only to palaeoanthropogists. However, a newspaper ran the story under the 'Eve found' banner, and the story caught. Since then the idea has been misrepresented, castugated and clung to by many parties, both scientific and religious (although you don't have to be non-religious to be a scientist). This is the reason why the entry finishes with a 'Is any of this true?' paragraph. I wanted to highlight the point that the original science was done on approximations, and still is. A precise date for the bottleneck is likely to be impossible. Until more tests are done, it cannot be said that there was definately one Mitochondrial Eve. If the full tests are done, they may well show that there were several.
Or one. The pity is that the haggling concerns the date, as opposed to the social implications of this discovery, and conjecture over the reason for the bottleneck.

Hi, Frogbit!

One very small point first: you say 'There is no reason to suppose that she had more than one female child.' But if she had only one female child, *that* child is Eve as far as we're concerned!

More generally, I have a serious problem with the paragraph where you list various reasons why she was so successful. This seems to imply that Eve's genotype must have been in some way particularly succesful. I think you're underplaying the whole random effect due to the fact that men's mitochondrial DNA disappears from the line of descent.

Consider this scenario:
Eve is on the dull side of normal but happens to produce two daughters. Ada is highly intelligent, unusually strong and healthy etc. etc. but only manages to produce one child (a boy) before she's struck by lightning. The descendents of Ada's son and Eve's daughters wipe the floor with everyone else, due to the genes they inherit from Ada, but all have Eve's mitochondria. In this case, the traits that made her progeny successful didn't come from Eve.

You could set up a simple theoretical model to show this kind of effect:
-Initial population of 10 men and 10 women, each with a different mitochondrial line.
-Women and men pair off so that all produce offspring.
-Each woman has two children.
-If a woman's children are both boys, her mitochondrial line disappears.
Run a model like that for 10 generations or more, crossbreeding each time, and your final population should still be around 20, all carrying a mixture of the original generation's nuclear genes, but the number of mitochondrial lines should have dropped from 20 to probably 4 or less, purely due to chance.

Azara

Hi Azara
"One very small point first: you say 'There is no reason to suppose that she had more than one female child.' But if she had only one female child, *that* child is Eve as far as we're concerned!"

"Eve is on the dull side of normal but happens to produce two daughters. Ada is highly intelligent, unusually strong and healthy etc. etc. but only manages to produce one child (a boy) before she's struck by lightning. The descendents of Ada's son and Eve's daughters wipe the floor with everyone else, due to the genes they inherit from Ada, but all have Eve's mitochondria. In this case, the traits that made her progeny successful didn't come from Eve."

OK - let me nick this and I'll settle for dual authorship.