Mitochondrial DNA

In women there is a DNA sequence that is passed on just through them. This is the sequence called mitochondrial DNA. The fact that is passed only by mothers to their sons and daughters has to do with the mechanics of fertilization. The sperm is extremely small compared to the egg. The sperm has its DNA concentrated in a very small area, the head. That’s indeed the only part of the sperm that gets into the egg. Included in the parts that don’t get into the egg is an organelle called mitochondria, which has a very small amount of DNA.

A molecular clock looks at differences between DNA sequences or proteins between two organisms, and then calculates back to their last common ancestor. If we have 20 differences between them, and their last common ancestor lived 20 million years ago (according to fossil evidence), one difference per million years is piling up. These differences are caused by non-selected mutations that are simply piling up over time. Then, considering a constant rate of mutations, if we have two organisms that have only 5 differences between them, we can infer that their common ancestor lived 5 million years ago.

If we want to make this argument for human genetics we have to use DNA sequences that are not subject to natural selection. If they are selected they would not be piling up over time. The sequences we can use in humans for looking at evolutionary clocks are sequences on the Y chromosome that is passed through males, and the mitochondrial DNA that is passed only through females. Both of these DNA sequences are not subject to selection.

First, let’s look at the Y chromosome sequence. On the Y chromosome are regions that are non-selected and mutate at a constant rate. The Y chromosome has very few functional genes, the most notable of which is that one that determines maleness. If we look at these DNA sequences, my brothers and I probably have identical sequences. Our last common ancestor is our father. We didn’t give our DNA much time to change; we just got it from our father.

My first cousins and I have a common grandfather, so maybe there might be a change. My third cousins might have slight differences, because we have a common ancestor longer time ago. And so on, to the most diverse humans.

A survey of Y chromosome sequences in different men from all over the world was conducted, and the group with most diversity, who obviously had a common ancestor the longest time ago, was from Africa. That says that humans’ last common ancestor for this particular sequence lived in Africa. How long ago? We can calculate the rate at which it changes. The Y chromosome “Adam” lived, according to Spencer Wells, 60000 years ago.

This is not necessarily the first man, but the one who’s Y chromosome was passed on uninterrupted to the current Africans.

Mitochondrial Eve

The small sequence of DNA in the mitochondria is not subject to natural selection and is passed only from mother to offspring. With mitochondrial DNA we can do the same analysis as with the Y chromosome in males. We can look at sequences of DNA, compare Africans to Africans, Asians to Asians, and so on. Using this method, we find that the greater diversity in mitochondrial DNA is found in Africans. The mitochondrial “Eve”, who’s DNA has been passed uninterrupted all the way to us lived about 150000 years ago. Wait a minute; she didn’t live at the same time as Y chromosome Adam. That’s just because of who passed on what. If a woman doesn’t reproduce, her sequence doesn’t get passed on.

We don’t have the first man or the first woman, but this is an interesting calculation. Maybe it is a pointer to where we originated.