Most of our DNA is useless for distant ancestry analysis because it changes too much from generation to generation. These changes ensure that humans have a lot of genetic diversity. But it can be a real pain when you want to study your distant past.
The Y chromosome and mtDNA are thought to be different though. Scientists have shown that these weird outliers of our DNA don't change much between generations. And the work the Genographic Project has done confirms this.
Before getting to the specifics of their results though, we need to understand more about what makes mtDNA and the Y chromosome so special. And to understand this, we need to dig a bit deeper into why the rest of our DNA does change so much.
The Y chromosome is the chromosome that determines the sex of a person. A person with an X and a Y chromosome is usually male. And a person with two X chromosomes is usually female.
Mitochondrial DNA is a cool relic from our evolutionary past (click here
to learn more). Unlike the rest of our DNA, mtDNA is found outside of the nucleus in little compartments in our cells called mitochondria.
Both mtDNA and the Y chromosome are lonersthey don't have a partner they are paired with. This is different than the rest of our DNA. And this is the reason why mtDNA and the Y chromosome don't change much between generations.
The rest of our DNA recombines. That's a fancy genetic term for shuffling between partners.
See, at birth you get half your DNA from your mother and half from your father. In fact, with the exception of the Y chromosome and mtDNA, you get one of each chromosome from each parent.
But you don't get the exact same chromosomes from your mother. Her own two copies of chromosome 1 mix and match with each other first. Then you end up with a mix of both of her chromosome 1's. You also get a mix of her chromosomes 2-22 and her X chromosome as well.
Because the mtDNA and the Y chromosome don't have a partner, they don't swap DNA with anyone. They pass virtually unchanged from generation to generation.
Why recombination causes problems
Recombination really messes up any attempts to look into our distant past with DNA. To understand why recombination can make things difficult, we need to go into a bit more detail about what these tests are looking at.
Humans are genetically almost identical to one another. In order to tell different groups apart, we have to look at small sections of the DNA that tend to have a lot of differences. We call these regions "genetic markers".
Genetic markers are really just mutations or random DNA changes that happened in our past. There is no one mutation that makes one ethnic group stand apart from the rest. Instead, we have to look at many of these markers and try to find a pattern that fits each ethnic group.
Let's think of these markers as stamps on a passport, and the DNA as the traveler. Imagine there is a group of travelers that all begin traveling together. Their passports will start out looking exactly the same. Then, let's say they start to split up and travel on their own or in small groups to different countries. With time, each traveler will acquire new stamps of where they have been.
At the end you can look at all the passports. All the travelers should have the same stamps, up until they split from the group. You can then estimate how long it has been since each person was last in the group.
This is basically how we believe ethnic groups came about. All humans originally started out in one place, but then started splitting up. With time, each group can be considered a different ethnicity with a different set of hereditary markers.
This is all fine and good until the DNA starts recombining. This quickly messes up everything and hides our ancestor's tracks.
Let's imagine our DNA as world travelers again. Imagine that before each traveler splits from the group, they exchange half of their passport pages with another person.
And then, they split them again. And again. It would then be really hard, after a few months, to tell when each person split!
This is why we can't trace our ancestry back very far using most of our DNA. Now a number of studies have shown that mtDNA doesn't recombine between generations. The folks at the Genographic Project decided to look for evidence of recombination in their 78,500 samples. And they saw no evidence that it happens.
The way they did this was to compare their samples to some known reference samples. If any DNA got shuffled, then some of the mtDNA the scientists looked at should look like a combination of two of the reference sequences. And none of them do.
Actually, 538 of them looked different than the reference DNA. But when they looked closer, they could all be explained by a mutation event. What I mean is that a mutation happened that changed one base into the base of another reference sequence. This isn't surprising given the high mutation rate of mtDNA (click here
for more details).
So they confirmed that mtDNA is very useful for studying our past. These data are important because some people think that the evidence that mtDNA does not recombine is weak.
Well, it looks like they aren't right. Which means that our ancestors did probably start out in Africa and spread across the globe over the last 80,000 or so years.