You, me and mitochondrial ‘Eve’
Your DNA comes from three sources
In each of your cells you keep a record of your grandmother’s genome. It was passed on to you by your mother. Your grandmother also had a copy of her grandmother’s genome, as did her grandmother, and her grandmother, and so on, stretching all the way back through time along the maternal line.
How come? Surely your genome is a mixture containing DNA from both of your parents? Well, it is. But there’s another type of DNA you might not have heard of.
Each newly fertilised egg cell is host to a nucleus that contains a mixture of DNA from the egg itself and the sperm that fertilised it. But another compartment in the cell contains multiple copies of a tiny loop of DNA encoding just 37 genes – compared to the 20,000 in the central genome.
This ‘mitochondrial DNA’ is housed in the energy-producing powerhouses of the cell called mitochondria. It is inherited directly from the mother’s egg, without mixing or shuffling, and is needed to produce proteins involved in the energy-generating process. As the egg cell divides and becomes an embryo, its mitochondrial DNA sequence is transferred into each and every cell in the developing body as a living record of the mother.
Knowing that we all keep this record has given rise to the concept of ‘Mitochondrial Eve’ – the idea that everyone alive today is descended from one woman and that our mitochondrial DNA is ultimately inherited from her. (Though this doesn’t mean she was the first woman.)
If this were the case, then each person’s mitochondrial DNA should be the same. It isn’t though, because DNA isn’t perfectly copied, so over millions of years ‘Eve’s’ DNA has been changed by mutation. Mutations occur in both nuclear DNA and mitochondrial DNA, but they happen about 100 times more frequently in mitochondrial DNA.
In this sense mitochondrial DNA is a really useful tool for tracking human evolution – it changes over time, but it doesn’t undergo a complete reshuffle with every new generation. It’s also abundant in cells and stabilised by its ring structure, so that in old bones, for instance, mitochondrial DNA remains intact longer than nuclear DNA.
Based on sequences from all over the world and the mutation rate of mitochondrial DNA, scientists have calculated that all of the mitochondrial DNA sequences in living humans could have been inherited from one woman. Studies suggest this woman lived in Africa around 200,000 years ago. Mitochondrial DNA also provides us with more recent details about human origins. According to mitochondrial DNA analyses, seven hunter-gatherer women arrived in Europe during the last Ice Age and gave birth to the ancestors of modern Europeans.Lead image:
Maurits Verbiest/Flickr CC BY