Out of Eden: The Peopling of the World by Stephen Oppenheimer Page A
Authors: Stephen Oppenheimer
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that do not recombine. Non-recombining DNA is easier to trace back since the information is uncorrupted during transmission from one generation to the next. The two portions are knownas mitochondrial DNA (mtDNA) and the non-recombining part of the Y chromosome (NRY).
Mitochondrial DNA: the Eve gene
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 intracing the different geographic 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. 35 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 within the past 10,000 years or so, we each have a slightly different code from everyone else around us.
Using mutations to build a tree
Over a period of nearly 200,000 years, a number of tiny random mutations have thus steadily accumulated on different human mtDNA molecules being passed down to daughters of Eve all around the world. For each of us this represents between seven and fifteen mutational changes on our own personal Eve record. Mutations are thus a cumulative dossier of our own maternal prehistory. The main task of DNA is to copy itself to each new generation. We can use these mutations to reconstruct a genetic tree of mtDNA, because each new mtDNA mutation in a prospective mother’s ovum will be transferred in perpetuity to all her descendants down the female line. Each new female line is thus defined by the old mutations as well as the new ones. As a result, by knowing all the different combinations of mutations in living females around the world, we can logically reconstruct a family tree right back to our first mother.
Although it is simple to draw on the back of an envelope a
Mitochondrial DNA: the Eve gene
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 intracing the different geographic 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. 35 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 within the past 10,000 years or so, we each have a slightly different code from everyone else around us.
Using mutations to build a tree
Over a period of nearly 200,000 years, a number of tiny random mutations have thus steadily accumulated on different human mtDNA molecules being passed down to daughters of Eve all around the world. For each of us this represents between seven and fifteen mutational changes on our own personal Eve record. Mutations are thus a cumulative dossier of our own maternal prehistory. The main task of DNA is to copy itself to each new generation. We can use these mutations to reconstruct a genetic tree of mtDNA, because each new mtDNA mutation in a prospective mother’s ovum will be transferred in perpetuity to all her descendants down the female line. Each new female line is thus defined by the old mutations as well as the new ones. As a result, by knowing all the different combinations of mutations in living females around the world, we can logically reconstruct a family tree right back to our first mother.
Although it is simple to draw on the back of an envelope a
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