Most human DNA is contained in 23 chromosomes inside the nucleus of the cell. The mitochondria were once separate cells (bacteria) that entered the cytoplasm of a host cell and they had their own circular DNA molecule. They lived in a symbiotic relationship with the host. They conferred a selective advantage to the host which allowed the host to process oxygen to produce energy.
Previous to the period the atmosphere of earth was reducing and the cells we evolved from used reduction of substrates like sugar (glycose) to produce energy via glycolysis much like yeast cells do today through fermentation. The early atmosphere of earth contained lots of methane and little oxygen – much as you find today on Jupiter or Saturn for example. So our ancestors – single celled organisms evolved in an oxygen poor environment. In fact oxygen was toxic to them. They lived by reducing molecules like glucose into two molecules of pyruvate.
Cyanobacteria evolved and started to capture the energy of sunlight and as a byproduct released oxygen via photosynthesis. Over time this changed the composition of the atmosphere such that it became very oxidative and toxic to many organisms. We talk about global climate change today but this event was on a whole different scale.
By incorporating these symbionts our cells gained the ability to process oxygen for metabolism so they could adapt to the new environment. The endosymbiotic bacteria took the byproduct of glycolysis i.e. pyruvate and converted it to CO2 as a byproduct via oxidative phosphorylation. That’s why humans can eat carbon food sources extract the energy and exhale the carbon waste product as CO2. Over many generations these symbiotic cells lost most of their original DNA and became organelles in the cytoplasm called mitochondria. But they still carry a ring of DNA that is just over 16,000 base pairs long. It is circular in structure just like most bacterial genomes – it is not packaged in a chromosome. There are many mitochondria in each cell’s cytoplasm and each mitochondria contains a single circular strand of DNA.
During reproduction the human sperm does not transmit mitochondria to the zygote. Therefore all the mitochondria and therefore all the mitochondrial DNA in the zygote is from the mother. Mitochondrial DNA unlike nuclear DNA never exchanges DNA. It remains the same from generation to generation unless there is a mutation.
About 24,000 years ago a mutation occurred in the mitochondrial genome of a single female. That mutation is diagnostic for the U5b mtDNA haplogroup. All living people (male or female) today within the U5b are descendants of that single female who lived 24,000 years ago. So yes all people with the U5b mtDNA haplogroup share the same maternal ancestor. But they may have different maternal lineages to that same female ancestor. Since that time there have be other single base-pair mutations in single females. All descendants of these females can unambiguously be distinguished from the descendants from all other females because they carry this mutation plus all previous mutation of U5b. Subsequently more mutations occurred over time. And you can see how this forms a phylogenetic tree with a terminal set of mutations identical to the tester in question. This is a fact and can be proven.
The person who tested posted some markers:
mtDNA Markers
16,224 C
16,270 T
16,311 C
73 G
150 T
263 G
279 C
315 CC
517 T
But they didn’t post which reference they were from. We have two major references for a “standard” mtDNA sequence. Without knowing which one these data are useless. So that’s another thing we need to include. What these tell us is at base-pair 517 the person who tested has a mutant-type T instead of the value of the reference. At 315 you can see they have a C instead of the wild type reference plus an insertion of an extra C. etc.
So our job is to take the genealogical tree based on paper records and overlay it onto the phylogenetic tree.
If the genealogical tree is certain than Francoise Martinet will have carried mitochondrial DNA with the same mutation carried the U5b haplogroup plus the terminal mutations of the person tested (possibly minus one). If the genealogical tree is certain than all other matrilineal descendants will also be in the same U5b haplogroup. However, they may be found to carry a new single point mutation. That can’t be ruled out. Mutations are natural. They just occur rarely in mitochondrial DNA. But we compare these mutations to the known tree to make sure this is a new mutation. That’s pretty simple.
We can’t assume the genealogical tree is certain. And we can’t assign it an arbitrary value of certainty from 0% to 100% at least not scientifically. The tree is a model and we find evidence to support or disprove the model. We use samples to scientifically prove the phylogenetic tree.
However, if a second tester has the exact same mutations as the first tester we can determine that they shared the exact same maternal ancestor. There would be no other samples necessary to reach this conclusion. That is to say they share the same maternal ancestor on the phylogenetic tree. And by testing more people we can refine the date for the emergence of the female who had the last mutation that formed the terminal branch in the phylogenetic tree.
Can we say with any certainty based on mtDNA genetics alone that the first two people who have matching results are both descendants of Francoise Martinet? No not really. For example her husband could have had an unrecorded first wife – the married date could have been recorded wrong or forged, etc. (In other words we could have had a green fish from Nevada). Or they may have both descended from a female who lived before Francoise Martinet back in France. And there is no additional number of tests that will ever prove it.
But we have a model and we look at the data we use in genealogy to construct this model and DNA is probably the strongest evidence we have to support it. We reason that there was a founding population with very few women and one of these women was Francoise Martinet. This was a bottleneck as Sharon pointed out. If two people who claim to be descendants have the same mtDNA mutations that’s darn strong evidence that their genealogies are correct! It is better evidence that most of us will ever have for an ancestor this ancient. So part of this is possibly how a biochemist like me uses the words “proof, support, predict” and how they are used in the social sciences and genealogy. I think most genealogists would call that “proof.” I prefer the words “strongly supports” this model. The bottleneck makes it almost undeniable but scientists don’t like to use the word “prove” unless we really mean it. Like I can prove 2+2 =4. I can prove energy is conserved when a photo elevates an electron to a higher energy orbital. That’s why we call evolution the “theory of evolution” – it is a model like genealogy and it is highly supported by the evidence. I use the word “disprove” much more often. If I can find a fact from DNA analysis that is not consistent with your genealogy I have disproven the model. DNA can be used to disprove genealogies easily. So I much prefer to disprove things than to try to prove things. But I do like evidence that supports a model too.
I agree with a lot of things Jan is saying – a negative match *is* data. That’s where the fun begins and we get to start testing our model. Usually a weak point will be found quickly.
So we use the DNA samples to prove the phylogenetic mtDNA tree. We overlay the genealogical tree onto the phylogenetic tree in order to support out model. We look for evidence that doesn’t match our model. Say we get a third person who doesn’t match? We start from the bottom of their genealogy and work our way up looking for an error. Alternately we use mtDNA, autosomal DNA and Y DNA testing to prove or disprove their genealogy a few generations at a time. Eventually we will find the error at least in many cases.
I’m not sure where this idea came about that autosomal DNA is only good for four generations. It can be critical in a case like this where we may get a third person who doesn’t match on a mtDNA test. Then we can use autosomal DNA to work our way up all three trees to look for evidence that doesn’t match.
Autosomal DNA can go back much further than four generations but not for all of our ancestors – only for some of our ancestors. But I have DNA that I can trace all the way back to the Mayflower. Each generation back the match gets smaller and smaller but it’s the same segment getting passed down. But I can’t trace autosomal segments back to other ancestors in my tree. Why? because I either don’t carry any DNA from those ancestors or the DNA I do carry from them either is no longer carried by any of my cousins or none of the cousins who do carry this same segment have taken a DNA test.
How far back does autosomal testing work? The answer is it depends on which part of your tree you are looking at. It will vary from branch to branch.
It is important when reading about autosomal matching to make the distinction between:
1) the probability of your personal DNA sample matching a specific remote cousin via your MRCA
2) the probability of your DNA matching *any* remote cousin via that same MRCA
3) the probability of any two cousins matching via that MRCA.
That’s really, really important because you don’t have to match a specific person to prove your MRCA. For example you might match their second cousin. Alternately that specific person may not match you but they may match your second cousin. And that’s all that is needed to prove the relationship assume you get enough matches to triangulate. You can prove you share a very recent common ancestor with a 2nd cousin and then use your 2nd cousin’s DNA to prove a relationship to a more distant ancestor using that closer ancestor as a gateway. So if you think about it you can “walk” your way from ancestor to ancestor! But only if you can share matches with multiple cousins! And that’s why we need DNA sharing on GENI. Because we can all use other GEDMATCH kits to walk our ancestors. This becomes a very powerful tool to track down mismatches in mtDNA or Y DNA. And in some cases it can be used to go very far back.
