Out of Eden: The Peopling of the World by Stephen Oppenheimer Page B
Authors: Stephen Oppenheimer
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recentmtDNA 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 ( Figure 0.3 ). 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.
It is important to realize that because of the random nature of individual mutations, the dating is only approximate. There are various mathematical ways of dating population migrations, which were tried with varying degrees of success during the 1990s, but one method established in 1996, which dates each branch of the gene tree by averaging the number of new mutations in daughter types of that branch, 36 has stood the test of time and is the main one I use in this book.
Figure 0.3
Real maternal gene tree of 52 randomly selected individual people from around the world. Note the age of Mitochondrial Eve. Branch dating by author based on complete sequence data; the chimp–human coalescent date arises from analysis, i.e. not assumed from fossil evidence – see note 22 in Chapter 1 .
Y chromosome: the Adam gene
Analogous to the maternally transmitted mtDNA residing outside our cell nuclei, there is a set of genes packaged within the nucleus that is only passed down the male line. This is the Y chromosome, the defining chromosome for maleness. With the exception of a small segment, the unpaired Y chromosome plays no part in the promiscuous exchange of DNA indulged in by other chromosomes. This means that, like mtDNA, the non-recombining part of the Y chromosome remains uncorrupted with each generation, and can be traced back in an unbroken line to our original male ancestor.
Y chromosomes have been used for reconstructing trees for less time than mtDNA has, and there are more problems in estimating time depth. When these are solved, the NRY method may have a much greater power of time and geographical resolution than mtDNA, for both the recent and the distant past. This is simply because the NRY is much larger than mtDNA and consequently has potential for more variation.
Yet Y chromosomes have already helped to chart a genetic trail parallel to the mtDNA trail. At the major geographical branch points they support the story told by mtDNA: they point to a shared ancestor in Africa for all modern humans, and a more recent ancestor in Asia for all non-Africans. In addition, because men’s behaviour differs in certain key ways from women’s, the story told by the Adam genes adds interesting detail. One difference is that men have more variation in the number of their offspring than women: a few men father considerably more children than the rest. Women, in contrast, tend to be more even and ‘equal’ in the number of children they have. The main effect of this is
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 ( Figure 0.3 ). 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.
It is important to realize that because of the random nature of individual mutations, the dating is only approximate. There are various mathematical ways of dating population migrations, which were tried with varying degrees of success during the 1990s, but one method established in 1996, which dates each branch of the gene tree by averaging the number of new mutations in daughter types of that branch, 36 has stood the test of time and is the main one I use in this book.
Figure 0.3
Real maternal gene tree of 52 randomly selected individual people from around the world. Note the age of Mitochondrial Eve. Branch dating by author based on complete sequence data; the chimp–human coalescent date arises from analysis, i.e. not assumed from fossil evidence – see note 22 in Chapter 1 .
Y chromosome: the Adam gene
Analogous to the maternally transmitted mtDNA residing outside our cell nuclei, there is a set of genes packaged within the nucleus that is only passed down the male line. This is the Y chromosome, the defining chromosome for maleness. With the exception of a small segment, the unpaired Y chromosome plays no part in the promiscuous exchange of DNA indulged in by other chromosomes. This means that, like mtDNA, the non-recombining part of the Y chromosome remains uncorrupted with each generation, and can be traced back in an unbroken line to our original male ancestor.
Y chromosomes have been used for reconstructing trees for less time than mtDNA has, and there are more problems in estimating time depth. When these are solved, the NRY method may have a much greater power of time and geographical resolution than mtDNA, for both the recent and the distant past. This is simply because the NRY is much larger than mtDNA and consequently has potential for more variation.
Yet Y chromosomes have already helped to chart a genetic trail parallel to the mtDNA trail. At the major geographical branch points they support the story told by mtDNA: they point to a shared ancestor in Africa for all modern humans, and a more recent ancestor in Asia for all non-Africans. In addition, because men’s behaviour differs in certain key ways from women’s, the story told by the Adam genes adds interesting detail. One difference is that men have more variation in the number of their offspring than women: a few men father considerably more children than the rest. Women, in contrast, tend to be more even and ‘equal’ in the number of children they have. The main effect of this is
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