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

They've read the markers in mitochondrial DNA and mapped the exodus of the human race from one woman in Africa 150,000 years ago. That's what I've learned today (following an hourlong investigation into Germs, thanks Icy North and Kea). But how did they map those markers? Are they in the skeletal remains that have been found, or is this all theoretical?

Thanks
SW

Have you a link to what you read, it would be useful to answer your question?

mitochondrial Eve was not human.

First off, mitochondrial DNA is *only* passed through the female line. Mitochondira are like miniature power packs in your cells, which come from the egg (and not the sperm). Therefore we can be sure that they're passed solely from the mother.

Using this approach, it seems likely that *all* humans' mitochondrial (Mt) DNA does come from one woman (or more likely a small family unit) approx 150kya. AIUI, this is done by looking at different ethnic groups and checking the mutations among genes. Using statistical analysis, it's possible to work a 'family tree' for the *genes* involved. The family tree is probabilistic, so it's a statement of likelihood rather than of fact. Obviously, 99.999% likelihood isn't that far away from fact, but I don't know what the actual probabilities are.

Other genes have different family trees, so the Mt family tree isn't the only one. For example, blood groups are genetically determined, and the A,B,O system is identical in chimps. So those genes' family tree would have be totally different from the Mt one.

In Richard Dawkins' 'The Ancestor's Tale', he discusses this at some length. Some of our genes' family trees date from as long as 1.8 million years ago. Presumably our ancestors moved from Africa to Asia (or somewhere), but kept in genetic contact through migration over the millenia. It's really not as simple as it first looks.

Might as well say what I know on mitchondrial DNA though off the top of my head.

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Firstly it might be an idea to know what the mitochondria are.

They are little 'organelles' within each of our cells where primary metabolism takes place. That is where fuel is burnt and energy is generated and stored by the synthesis of lots of molecules of a chemical called ATP.

Mitochondria are *thought* to have originated as a bacterium living inside our cells and over time they got subsumed and made into an integral part of the cell. One of the reasons this is thought to be the case is that they have a small stretch of their own DNA which is entirely independant of the DNA in the cells nucleus.

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Because mitochondrial DNA is not in the nucleus it does not get divided at cell division and is not mixed with DNA from the father at conception. Hence DNA from mitchondria comes solely from the mother and remains 'unchanged'.

I put 'unchanged' in quotation marks because in fact it does get gradually changed through random mutation over long stretches of time and because it is random and not sexually mutated.

Because the changes in it are random, over extremely long periods of time these changes average out to happen at a predictable rate and with enough samples of this type of DNA from specimens of known age (e.g carbon dated) we can plot a graph of known mutations against time.

With this graph as calibration, the age of an unkown sample of mitochondrial DNA can be estimated.

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However, each mother has slightly different mitochondrial DNA so this complicates things but if one takes enough samples one can interpolate this graph back through time to show that certain people have common ancestors.

To my knowledge it's been estimated that the whole human race descends from five different woment from abotu 600,000 years ago.

It is important to note that this is reasonably controversial in genetecist circles though. Don't take what you read as gospel, it's not.



I'm not sure what you mean about tracking migrations though. I'd like to see that, that would be interesting.

I was misremembering what I saw on TV about this. More like 60,000 than 600,000 years.

http://www.bbc.co.uk/science/horizon/1999/supervolcanoes_script.shtml

I've been reading a book on this subject - Out of Eden - which indicates that not only was there an African Eve, but also there was an "Eve" known as L3, who was responsible for populating the rest of the world about 80,000 years ago. Non-Africans are no more than a branch on a large genetic tree that is mainly African based. White Europeans are two relatively small twigs on this genetic branch, having only started the population of Europe about 50,000 years ago.

The migrations I mentioned, Orcus? Read the book, I'm not really qualified to comment, but...

The genetic family tree showed splits which then joined up again. The most plausible 'real world' explanation for this would be populations splitting up then joining again. At 1.8mya, the first members of Homo are found outside Africa, in Georgia (H. Djaminensis?), so it seems migration, then migration back to Africa (or from Africa again) is the most likely explanation for this.

This wasn't the only split, it seems that another split and rejoining happened more recently as well, about 1mya. But I'm not sure when, or what those particular implications were.

Obviously, any cock-ups in this post are mine and not Mr Dawkins.

I was talking about the migrations the original poster made, ours were simulposts.

Yup - original info here: http://www.raceandhistory.com/cgi-bin/forum/webbbs_config.pl/noframes/read/665

The bit that made me curious was "we know that we all carry the genetic marker of one woman who lived over 150,000 years ago in Africa, but ...how did human beings first leave Africa and where did they go to then?" How can they tell? I think Orcus has explained since, but I find it difficult to understand their leaps of faith. "From Yemen they went to the Gulf of Arabia...all the way round the coast all the way down to Australia and then all the way up the coast to China."

Oh, Ictoan, I'm not an expert, but I *think* she was supposed to be human. Certainly not martian...

I seem* to recall that they determine the time at which those peoples origionally leaving the birthplace of humans got to other particular locations, by the level of simularity/disimulirity between the mitochondrial DNA of those populations as compaired to other populations closer to, or furhter from the point of origion. As the MtDNA has a set rate of mutation (as it is not affected by evolutionary pressures in the same way as chromosomal DNA), this is used by maths I can't find to work out the age at which the MtDNA in those populations got into that particular area

The term 'Eve' implies that we are all descended from MtEve and have no other woman as a common ancestor - there was only one Eve. Surely that can't be true? Or at least, the odds are incredibly highly against it being true. It would mean that every branch descended from every other human female at the time has died out. Even seeing Eve as a small family group of women, it seems unlikely that they are the only common ancestors from that time.


>>mitochondrial Eve was not human<<

My dictionary calls humans any of the Homo species. Are they not all human?

Mt Eve is the common ancestor of all our mitochondrial DNA, it doesn't mean that all women are descended from her and nobody else. As we have millions of genes, most of them have different ancestors. It's the same deal with the Y chromosome, Y-Adam may be the sole ancestor of that chromosome, but he isn't the sole male ancestor of all men.

It's a bit like surnames, which are only inherited from the male line. You have eight great-grandparents, but your surname only derives from one of them. Overall, though, you will have roughly 1/8 of their genes, just not in the case of Mt DNA or the Y-chromosome.

Thanks pedro. I guess the name "Eve" is a bit misleading.

I think this I have confused things somewhat.
Mitochondrial eve was the first being to have mitochondria - i.e. the being who is the ancestor of all humans and had the original mutation where mitchondria was in the cells, which we all now have.

That being was not human.

What is being talked about here, it seems, is some human ancestor within the human species lifetime who is being touted as the mother of us all.
That is not mitochondrial Eve because she was not the first with mitochondria in the evolution of humans from whatever.

Although see Pedro's post aboput think of this as being an actual individual creature.

Appols for confusing the issue. I agree Kea, the term 'eve' is rather misleading and it seems I was mislead as to what was being discussed here.

So "mitochondrial Eve" was in fact a protozoan species? Have I got that right?

RF

Aha, that's why I was confused! I was imagining some woman packing her cases, going forth and multiplying. I see I should have been thinking on a much smaller scale...

RF - Not sure what the general consensus is, but my understanding is that it was later than that, more likely some small furry pre-mammalian speciecs, possibly from around the time of pangea. I.e. the first creature (which bred and whose offspring continued to breed to the current day) that had mDNA.

However, just to confuse matters even further, it does seem that some people and sources do indeed use the term for the matralineal most recent common ancestor, who indeed be of the human species. Wiki does, for example.

Don't all Eukaryotic cells have mitochondrial DNA then? Which include plants and fungi. Although plant cells contain other plastids too of course.

I think the mitochondrial common ancestor should not be confused with the first organism to contain mitochondria - which is what seems to be under discussion also.