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Author Topic:   Human Origins: Let's Talk Mitochondrial Eve
ramoss
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Posts: 3123
Joined: 08-11-2004


Message 16 of 29 (284869)
02-08-2006 8:36 AM
Reply to: Message 1 by Pensees
12-24-2005 12:28 AM


the 'mitochondria eve' is merely the date approximation of the woman whose mitochondria genes have replaced all the other variations from her time period. Other genes will have a different time frame in which a specific strain became dominate. The 'Y' chromosome dates back to the man who is the great great great (unknown number of greats) of all the men of hte world.

Let's look at it this way. YOu have 5 woman. Each woman has 3 kids. One woman has nothing but boys. Her mitochondria dna no longer is in the gene pool, yet her genes are. One woman has 1 girl, and two boys, and her girl has nothing but boys. .. her mitochondria DNA is elminate from the gene pool. Repeat for a long enough time.. and sooner or later, just one of those 5 woman will have their mitochondria DNA prominent over the rest of them.


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Faith
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Posts: 33872
From: Nevada, USA
Joined: 10-06-2001
Member Rating: 1.2


Message 17 of 29 (326137)
06-25-2006 1:57 PM
Reply to: Message 1 by Pensees
12-24-2005 12:28 AM


Tracking back to "Mitochondrial Eve"
Since Mitochondrial Eve came up on a thread yesterday in reference to a human bottleneck, I've been trying to understand this bit of science, and I am getting nowhere.

First let me say that I don't have any problem with the idea that the mitochondria is inherited through the mother all the way back, so that it can only go back to a single female for each and every one of us. Father's mothers don't count and all that. That much I get.

What I don't get is not all that clear or easy to put into words. I understand that samples of mitochondria were taken from individuals all over the planet, and that this is what led to this particular woman in Africa 150 to 200 thousand years ago. I've read some discussions about this supposed first woman and all the speculations surrounding that, but that is the CONCLUSION of this study that was done, and what I don't get is the specifics of that study, the evidentiary and mental route taken to arrive there. I'm trying to picture what was actually DONE in this study, and it's driving me crazy. I want to be able to TRACK all the thinking that went into arriving at the conclusion.

Sorry if the question is vague, but I'd really appreciate anyone's effort to sort it out and answer it.

{Edit:
Like, what exactly are they seeing when they look at the mitochrondria of each person in the study?

Like, some question is raised about the value of a mere sample, no matter how diversely collected, considering that it's still only a very small portion of the living human population.

Like, how much guesswork is involved and at what points in the study.

Edited by Faith, : No reason given.

Edited by Faith, : No reason given.

Edited by Faith, : No reason given.


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Faith
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Posts: 33872
From: Nevada, USA
Joined: 10-06-2001
Member Rating: 1.2


Message 18 of 29 (326138)
06-25-2006 2:07 PM
Reply to: Message 16 by ramoss
02-08-2006 8:36 AM


a bunch of questions
the 'mitochondria eve' is merely the date approximation of the woman whose mitochondria genes have replaced all the other variations from her time period.

Replaced? Only ones that survived, doesn't that mean? And what's important about this anyway? I just don't get it. It seems whimsically arbitrary to track this phenomenon.

Other genes will have a different time frame in which a specific strain became dominate. The 'Y' chromosome dates back to the man who is the great great great (unknown number of greats) of all the men of hte world.

Yeah, but I see no conceivable significance to this. It's just an odd artifact of genetic structure.

Let's look at it this way. YOu have 5 woman. Each woman has 3 kids. One woman has nothing but boys. Her mitochondria dna no longer is in the gene pool, yet her genes are. One woman has 1 girl, and two boys, and her girl has nothing but boys. .. her mitochondria DNA is elminate from the gene pool. Repeat for a long enough time.. and sooner or later, just one of those 5 woman will have their mitochondria DNA prominent over the rest of them.

I get it, I get it, but so what? It's not as if billions of women hadn't existed and passed on their genes. It's just that one particular woman's mitochondria happened to survive a complex chain of genetic transmission -- and all that means is that there just happened to be an unbroken chain of mother-daughter transmission, an accidental and not at all significant fact that I can see. I see absolutely no significance to this fact. I can't see how this indicates a bottleneck. I just don't get the whole scenario.

I also do not see how anyone could possibly figure out the exact SOURCE of the particular woman. Location, timing etc. So they found this one kind of mitochondria in all these people all over the world. Well, it does apparently indicate, according to the basics about mitochondria, that it does all go back to just one particular woman back sometime or other, but how on earth could anybody guess where? And again, why does it matter????

Edited by Faith, : No reason given.

Edited by Faith, : No reason given.

Edited by Faith, : No reason given.


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NosyNed
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Posts: 8868
From: Canada
Joined: 04-04-2003
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Message 19 of 29 (326140)
06-25-2006 2:15 PM
Reply to: Message 17 by Faith
06-25-2006 1:57 PM


First woman
I'll let others go into details but this is wrong:

I've read some discussions about this supposed first woman and all the speculations surrounding that, but that is the CONCLUSION of this study that was done

(bolding mine)

There is no suggestion that this is a first woman. It is a last woman. The last woman for whom there are current female descendants.

Let me see if I can explain how this can happen.

Let's go to a small village some 100's of years ago. There are 100 males in it, each with a different surname. Smith, Jones, Alson, etc. etc. The totally firm rule is that sons get their dad's last name and that women change theirs on marriage. The village stays small because they have very small families (or they move away or whatever)

They have children; some have both sons and daughters, some have sons and some have only daughters.

If in one generation all the Jones's happen to have only daughters there will never be any Jones's after that. You can, I think, calculate how long it will take until you have only one surname left.

However, all of the original males may well have descendants in the population through the female lines.

The mitochondrial DNA is like the surname but for women and it is really a firm rule. :)

If the original population is small enough you will, I think, end up with only one of those with surviving direct female descendants.

That does NOT say that the other women don't have (tense is funny here) surveying descendants. But somewhere in the line there was a generation of only males.

Edited by NosyNed, : to connect this to the females and correct spelling of surname


This message is a reply to:
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Faith
Member
Posts: 33872
From: Nevada, USA
Joined: 10-06-2001
Member Rating: 1.2


Message 20 of 29 (326141)
06-25-2006 2:19 PM
Reply to: Message 19 by NosyNed
06-25-2006 2:15 PM


Re: First woman
Sorry if I phrased it wrong. I think I get all that. I just don't get why it MATTERS that there just happened to be this one unbroken chain of mother-daughter transmission. It's not as if all the other women whose mitochondria DIDN'T get passed on, because the chain of daughters was broken, didn't pass on their genes, or didn't exist. Millions, billions of others. Where's the "bottleneck?" It seems like an arbitrary oddness of genetics that ends up meaning nothing.

Edited by Faith, : No reason given.


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NosyNed
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Posts: 8868
From: Canada
Joined: 04-04-2003
Member Rating: 7.2


Message 21 of 29 (326142)
06-25-2006 2:23 PM
Reply to: Message 18 by Faith
06-25-2006 2:07 PM


A bottleneck
I see absolutely no significance to this fact. I can't see how this indicates a bottleneck. I just don't get the whole scenario.

I don't know the right answer for sure but I think this has to do with population genetics as I hinted at in my earlier post. I believe it is possible to do the calculations and determine that there has to be a small enough population at some point for only one DNA line to reach to today. I'm sure someone will clear that up.

It is by a comparison of the differences in the DNA that the likely location of the "eve" is determined. It is part of learning about our past.


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Faith
Member
Posts: 33872
From: Nevada, USA
Joined: 10-06-2001
Member Rating: 1.2


Message 22 of 29 (326144)
06-25-2006 2:28 PM


I'd really like someone to walk me through the process of thinking that led back to this woman, including all the evidence gathered along the way. I get the basic idea about the inheritance of mitochondria from mother to daughter.

It would help to understand how it is known that there is only one type of mitochondria per woman, perhaps; how it was surmised which population group it originated in etc.


  
Faith
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Posts: 33872
From: Nevada, USA
Joined: 10-06-2001
Member Rating: 1.2


Message 23 of 29 (326145)
06-25-2006 2:30 PM
Reply to: Message 21 by NosyNed
06-25-2006 2:23 PM


Re: A bottleneck
I think this has to do with population genetics as I hinted at in my earlier post. I believe it is possible to do the calculations and determine that there has to be a small enough population at some point for only one DNA line to reach to today.

OK that's helpful. Something to do with probabilities then.

It is by a comparison of the differences in the DNA that the likely location of the "eve" is determined. It is part of learning about our past.

Differences in the DNA? Don't get it.


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Faith
Member
Posts: 33872
From: Nevada, USA
Joined: 10-06-2001
Member Rating: 1.2


Message 24 of 29 (326151)
06-25-2006 2:47 PM


a couple more questions
Probably answered somewhere I overlooked but anyway:

Is it the idea that ALL women living today have this particular mitochondrial DNA?

Were all the samples taken of women? Men DO have this mitochondrial DNA, right, they just don't pass it on, right?


Replies to this message:
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nwr
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Posts: 5587
From: Geneva, Illinois
Joined: 08-08-2005


Message 25 of 29 (326162)
06-25-2006 4:00 PM
Reply to: Message 17 by Faith
06-25-2006 1:57 PM


Re: Tracking back to "Mitochondrial Eve"
I'm trying to picture what was actually DONE in this study, and it's driving me crazy. I want to be able to TRACK all the thinking that went into arriving at the conclusion.

Firstly, let me be clear that I am not a biologist. So what I say below is probably wrong is some details.

Where are our biologists today? I have seen several answers. But they seem to be answering a different question.

Here is my understanding.

If we take two siblings, and compare their mitochondrial DNA, then there is a good chance it is the same. After all, both got if from the same mother.

If we take two cousins twice removed (or whatever the term is), and compare the mDNA, then we would expect slight differences. Both were inherited from the same female ancestor. But there could have been some mutation in between, perhaps a change in one or two base pairs.

What the scientists are doing, is taking mDNA from two people, and comparing. Assuming a common ancestor, the number of differences is a measure of the number of mutations in the mDNA since the time of the common ancestor.

Now it happens that, at least in some parts of the mDNA, the mutations occur at a relatively stable rate. Thus the number of base pair differences between two individuals is measure of the amount of time since their common ancestor.

Apparently, when you do these comparisons, you never get anything further back than around 200,000 years. It will be often shorter, due to a more recent common ancestor. If the longest is 200,000 years, and if you only get this long by including african mDNA, then that suggests a common ancestor in Africa at around 200,000 years ago.


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Modulous
Member (Idle past 444 days)
Posts: 7789
From: Manchester, UK
Joined: 05-01-2005


Message 26 of 29 (326164)
06-25-2006 4:13 PM
Reply to: Message 24 by Faith
06-25-2006 2:47 PM


Re: a couple more questions
Is it the idea that ALL women living today have this particular mitochondrial DNA?

No - but all people alive have inhereted it from their mother. Like other DNA, it is not the same for all people. As nwr briefly explained above, the relative differences can be used to determine approximately how distantly related two people are.

Men DO have this mitochondrial DNA, right, they just don't pass it on, right?

Yup, the sperm isn't big enough to carry it - but the egg is.


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MangyTiger
Member (Idle past 4693 days)
Posts: 989
From: Leicester, UK
Joined: 07-30-2004


Message 27 of 29 (326165)
06-25-2006 4:14 PM
Reply to: Message 24 by Faith
06-25-2006 2:47 PM


Re: a couple more questions
Men DO have this mitochondrial DNA, right, they just don't pass it on, right?

As I understand it that's right. Sperm only contributes to the nuclear DNA of a fertilised egg cell - the mitochondrial DNA is already in the mitochondria of the egg cell.

I just Googled and it seems in mammals at least that sperm does contain mitochondria (and so mitochondrial DNA) but they are destoyed by the egg after fertilisation.


Oops! Wrong Planet

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Nuggin
Member (Idle past 832 days)
Posts: 2965
From: Los Angeles, CA USA
Joined: 08-09-2005


Message 28 of 29 (326170)
06-25-2006 6:08 PM
Reply to: Message 17 by Faith
06-25-2006 1:57 PM


Re: Tracking back to "Mitochondrial Eve"
Hey Faith,

Let me see if I can explain the study...

The section dna know as the Mdna can be broken down into a string of letters based on the proteins in the string. For simplicity, rather than: gattaggacaacatgat.... I'm going to use a simple binary number sequence that's 10 digits long:

Starting with: 0000000000

Now, we know that MDna is not subject to change through the normal sexual mixing pot that gives us most of the rest of the DNA. Most of the time if your mom has 0000000000, then you will also have 0000000000

However, rarely changes creep up. Somewhere back in time some 000000000000 mother gave birth to a daughter which had 0000100000

(keep in mind, we're actually talking about a much longer, much more complicated code).

If we tested people living today, we could say that everyone who had a "1" in the 5th position was descended from that daughter, and that those that didn't have the "1" were descended from a much earlier woman.

Now more time passes and the 0000000000 people produce another child, this time 0010000000. And the 0000100000 people produce and additional child 0000100001.

A cross section of the population shows that we have 4 groups. And we certain groups broke off from one another because they are more similiar.

0000000000 lead to both 0010000000 and 0000100000
0000100000 lead to 0000100001
but we know that 0010000000 is not a forefather of 0000100001 because they dont have similarities.

That's the basic concept behind this, and from it we could look at something like: 0100101001 and figure out which chain it belongs in.

Now in the real study, the codes have 4 variable and are much longer. Further, it not be just a single digit that changed, but a whole sequence.

But hopefully that gives you an idea about how they were able to determine which groups split off from who.

As for a time table, because mDNA changes so slowly, we can calculate the average time it would take a string to develop a mutation. If we say, here's a string with 17 mutations from the original, we can say it took roughly 17x (however long the average rate of mutation is) and come up with an idea about how long ago that string split off.

Hope that clears stuff up a bit


This message is a reply to:
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crashfrog
Inactive Member


Message 29 of 29 (326174)
06-25-2006 7:09 PM
Reply to: Message 17 by Faith
06-25-2006 1:57 PM


PCR-RFLP: Polymerase Chain Reaction/Restriction Fragement Length Polymorphism
Like, what exactly are they seeing when they look at the mitochrondria of each person in the study?

They're seeing this:

What is this? That's a little complicated. The short answer is - it's a bunch of segments of DNA, marked with a flourescent dye and sorted by segment length from longest at the top to smallest at the bottom.

Basically, we know that most mutations increase the length of of highly repetitive sequences already found within DNA. Using a technique called PCR-RFLP, we can amplify a sample of DNA - make a bunch of copies of it - and then cut the DNA with enzymes around the repetitive sequences.

These sequences are loaded into holes at one end of an agarose gel and then exposed to an electric field for a certain period of time. Since DNA is electrically charged, the sequence fragments travel from the holes through the gel. Since the gel is viscous, smaller sequences travel faster - longer sequences get "tangled up" in the gel. The DNA is also dyed with a florescent dye so we can see where it winds up.

The result is a series of bands representing the relative length of different people's samples of mtDNA. This represents the number, and position, of mutations in a human's maternal genetic ancestry.

Like, some question is raised about the value of a mere sample, no matter how diversely collected, considering that it's still only a very small portion of the living human population.

The odds that a random sample of human beings from all around the world all have the same maternal ancestor are extremely, extremely low, - unless all human beings have the same maternal ancestor. It's just a matter of basic statistics.


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