|by Stephen Oppenheimer
To say that we get exactly half of our DNA from our father and half from our mother is not quite true. One tiny piece of our DNA is inherited only down the female line. It is called mitochondrial DNA because it is held as a unique circular strand in small tubular packets known as mitochondria that function rather like batteries within the cell cytoplasm. Some molecular biologists say that, aeons ago, the mitochondrion was a free-living organism with its own DNA, and possessed the secret of generating lots of energy. It invaded single celled nucleated organisms and has stayed on ever since, dividing, like yeast, by binary fission. Males, although they receive and use their mother’s mitochondrial DNA, cannot pass it on to their children. The sperm has its own mitochondria to power the long journey from the vagina to the ovum but, on entry into the ovum, the male mitochondria wither and die. It is as if the man had to leave his guns at the door.
So each of us inherits our mtDNA from our own mother, who inherited her mtDNA intact from her mother, and so on back through the generations hence mtDNA’s popular name, ‘the Eve gene’. Ultimately, every person alive today has inherited their mitochondrial DNA from one single great-great-great-. . .-grandmother, nearly 200,000 years ago. This mtDNA provides us with a rare point of stability among the shifting sands of DNA inheritance. However, if all the Eve chromosomes in the world today were an exact copy of that original Eve mtDNA, then clearly they would all be identical. This would be miraculous, but it would mean that mtDNA is incapable of telling us much about our prehistory. Just knowing that all women can be traced back to one common ancestral Eve is exciting, but does not get us very far in tracing the different lives of her daughters. We need something with a bit of variety.
This is where DNA point mutations come in. When mtDNA is inherited from our mother, occasionally there is a change or mutation in one or more of the ‘letters’ of the mtDNA code about one mutation every thousand generations. The new letter, called a point mutation, will then be transmitted through all subsequent daughters. Although a new mutation is a rare event within a single family line, the overall probability of mutations is clearly increased by the number of mothers having daughters. So, within one generation, a million mothers could have more than a thousand daughters with a new mutation, each different from the rest. This is why, unless we share a recent maternal ancestor over the past 10,000 years or so, we each have a slightly different code from everyone else around us.
The diagram above shows the drawing of
Reconstucting a gene tree using mutations
Although it is simple to draw on the back of an envelope a recent mtDNA tree with only a couple of mutations to play with, the problem becomes much more complex when dealing with the whole human race, with thousands of combinations of mutations. So computers are used for the reconstruction. By looking at the DNA code in a sample of people alive today, and piecing together the changes in the code that have arisen down the generations, biologists can trace the line of descent back in time to a distant shared ancestor. Because we inherit mtDNA only from our mother, this line of descent is a picture of the female genealogy of the human species. Not only can we retrace the tree, but by taking into account where the sampled people came from, we can see where certain mutations occurred for example, whether in Europe, or Asia, or Africa. What’s more, because the changes happen at a statistically consistent (though random) rate, we can approximate the time when they happened. This has made it possible, during the late 1990s and in the new century, for us to do something that anthropologists of the past could only have dreamt of: we can now trace the migrations of modern humans around our planet. It turns out that the oldest changes in our mtDNA took place in Africa 150,000 - 190,000 years ago. Then new mutations start to appear in Asia, about 60,000 80,000 years ago. This tells us that modern humans evolved in Africa, and that some of us migrated out of Africa into Asia after 80,000 years ago.
The diagram above shows the tracing
A final point on the methods of genetic tracking of migrations: it is important to distinguish this new approach to tracing the history of molecules on a DNA tree, known as phylogeography (literally ‘tree-geography’), from the mathematical study of the history of whole human populations, which has been used for decades and is known as classical population genetics. The two disciplines are based on the same Mendelian biological principles, but have quite different aims and assumptions, and the difference is the source of much misunderstanding and controversy. The simplest way of explaining it is that phylogeography studies the prehistory of individual DNA molecules, while population genetics studies the prehistory of populations. Put another way, each human population contains multiple versions of any particular DNA molecule, each with its own history and different origin. Although these two approaches to human prehistory cannot represent exactly the same thing, their shared aim is to trace human migrations. Tracing the individual molecules we carry is just much easier than trying to follow whole groups.