There are just two kinds of DNA that pass down virtually unchanged. One is the DNA in the mitochondria in our cells. Mitochondria are energy-generating bodies in our cells. When a sperm fuses with an egg, it leaves its mitochondria behind. As a result, the DNA in our (both for males and females) mitochondria is an exact copy of our mother’s mitochondrial DNA (mtDNA). The second is the DNA that sits on the Y-chromosome, the chromosome that determines the male sex in humans. Unlike the DNA on other chromosomes, much of the Y-chromosome is passed on unchanged from father to son. This non-recombining (NR) part is often referred to as NRY-chromosome in scientific literature.
If the mtDNA and the NRY were to remain entirely unchanged, they would be of little help, as it would be difficult to distinguish a man’s NRY from his distant male ancestor’s NRY or a person’s mtDNA from her or his distant female ancestor. But recombination is not the only way the DNA changes; in each generation, there is the small probability of a mutation, which changes the DNA at one point. Once a mutation takes place, it is passed down to the next generation. This mutation then becomes a marker separating the generation when it occurred and its descendants from the generations that preceded it.
Consider a population A where one individual undergoes a point mutation (let us call it ‘b’) while others don’t. Let us term his descendants B, so now the population consists of A and B where B differs from A by the marker b. Some generations later, one A individual undergoes a point mutation ‘c’ that is passed on, while a B individual develops another point mutation ‘d’ that is passed on. As we can see, this splits the population into four groups A, B (identified by the marker b), C (identified by the marker c), and D (identified by the marker b followed by d). A haplogroup then is nothing but the set of markers that distinguishes one group from another, in this case b, c and b>d will be the haplogroups identifying B, C, and D, respectively. In actual practice, the naming of these point mutations is more complicated but the essential picture is the same.
In addition to distinguishing groups, since we have reasonable estimates for the rate of mutation, we can tell roughly how long ago A and B split, how long ago B and D split. Not only does the science, then, give us a way of tracing the ancestor who defines our haplogroup, it also gives us an estimate for how long ago did s/he live.
In much of the article we have followed the terminology that is standard in the field. We have used results from the National Geographic Project (NGP) and used their explanations for what the results indicate. The field, though, is changing so fast that conclusions the project tentatively started with some years ago have themselves undergone change. We have made every effort to reflect that change. In the end, the data being collected by projects such as the NGP will be one of the vital tools in finding conclusive answers to the questions that are still being debated. If you are interested in the story of your own ancestry and how you can submit your own DNA sample for testing (all it needs is a simple scrape of the cheeks) you can read more at https://genographic.nationalgeographic.com/genographic/lan/en/index.html. Each new test will only add to the evidence that will help clarify the story of our origins.