Mitochondria and Eve

National Geographic have launched the Genographic Project in an effort to trace the journey of human beings across the globe.

Tracing DNA history is an area of science in which I have been interested for a while, although biology is certainly not my specialism. Here's a short explanation, which will hopefully give a sense of why cellular biology plays such an interesting and important part in tracing human history.

Inside every cell of your body are tiny structures called organelles (the nucleus is the most well known). One of these organelles is the mitochondrion. Mitochondria are often called 'cellular power plants' for their role in providing energy to the cell.

However, these little structures are important for another reason. Mitochondria are unusual because they have their own DNA, their own genetic code, and it is generally accepted that their history began as a separate organism. If this is the case, then these creatures have a symbiotic relationship with humans. But this is just part of the story, although an intriguing part!

Mitochondria reproduce like bacteria, by division ('binary fission'), essentially making them clones. They reproduce according to the energy needs of the cell and therefore their life cycle is not related to the life cycle of the cell. During human reproduction an egg nucleus and a sperm nucleus are joined and their genetic material combined to form a new genetic code. However, the mitochondria in the sperm cell are generally destroyed at fertilisation and so the embryo begins its life with mitochondria from the mother only.

So that's the background, here's where it gets interesting... As mitochondria are basically clones, very little genetic difference is seen between successive human generations, although mutation over time will introduce markers that enable a particular 'gene tree' to be identified. This makes them particularly useful in population genetic studies, such as the National Geographic project mentioned above, because they allow scientists to use these markers to trace the movements of populations across many generations and recognise links between widely dispersed peoples (e.g. between Koreans and Native Americans - see the Atlas of the Human Journey).

Comparing differences between mtDNA and working backwards to find the point at which it diverged also uncovers "the woman who is the most recent common matrilineal ancestor of all living humans", famously named Mitochondrial Eve. The age of 'Eve' can be approximated using the 'gene clock' technique, a way of tracing the point when two population groups have diverged by counting the number of genetic differences between their DNA sequences. She is thought to have lived in Africa about 150,000 years ago. While there will have been other women living at the same time, many of whom will have descendants today, only Mitochondrial Eve produced an unbroken line of daughters that persists to this day.

Controversial for some, perhaps, but certainly interesting - don't you agree?