Free The Journey of Man: A Genetic Odyssey by Spencer Wells

Steinem
    In the last chapter we met ‘Eve’ – the female ancestor of everyone alive today, who lived in Africa around 150,000 years ago. Based on the populations that seem to have retained the clearest genetic signals from our distant grandmother, we’ve begun our search for the location of the Garden of Eden. But before we go any further, we need to clarify Eve’s uniqueness. She represents the root of the mitochondrial family tree, and as such she unites everyone around the world in a shared maternal history. However, it isn’t necessarily the case that every part of our DNA should tell the same story. Because of sexual recombination, our genome is composed of a large number of blocks that have each evolved pretty much independently. Perhaps one region of DNA traces back to an origin in Indonesia, while another began its journey in Mexico. So is Eve’s lineage unique in tracing a recent journey out of Africa?
    The answer is that the test of our genome shows essentially the same pattern as the mtDNA, although it tends to have a lower degree of resolution. Studies of polymorphisms in the beta-globin gene (which encodes the oxygen-carrying component of blood), the CD 4 gene (which encodes a protein that helps to regulate the immune system) and a region of DNA on chromosome 21 all show that African populations are much more diverse than those living outside of Africa, and provide dates that are substantially less than 2 million years for the age of our common African ancestor. But the problem with using markers like these – from the 22 pairs of chromosomes that comprisethe majority of our genome – is that the information tends to be shuffled over time. The further apart the polymorphisms are, the more likely it is that they have been shuffled. And because shuffling obscures the historical signal, this means that most of our genome isn’t terribly useful for tracing migrations.
    There is one piece of DNA, though, that has recently proven to be an invaluable tool for inferring details about human history – providing us with far greater resolution than we ever thought possible about the paths followed by our ancestors during their wanderings. It is the male equivalent of mtDNA, in that it is only passed from father to son. For this reason, it defines a uniquely male lineage – a counterpart to the female line illuminated by studying mtDNA. It is the
patrimoine
in our Provencal village, and the details of lineage extinction and diversification that went on with the soup recipes also apply to this piece of DNA. It is known as the Y-chromosome.
    Now wait a minute, you might be saying – what’s going on with all of this maternal and paternal lineage gibberish? I thought that the whole idea of sex was to mix the mother’s and father’s genomes in a 50 : 50 ratio to produce the child? Why do we have these oddities that break the rules? For the mitochondrial DNA the answer is easy – it is actually outside of what we think of as the human genome, an evolutionary remnant of a time when it was a parasitic bacterium living inside the earliest cells. The story for the Y is a bit more complicated.
    One of the quirky features of sexual reproduction is that the chromosomes that actually determine our sex – the so-called sex chromosomes – are exceptions to the 50 : 50 sexual mixing rule. The double layout of our genomes, with two copies of each chromosome, fails us when we get to these chromosomes. This is because of the way in which sex is determined in most animals, through the presence of a mismatched sex chromosome. In the case of mammals, it is the male that is mismatched, with one X and one Y-chromosome. In females, the X-chromosome is present in two copies, like the other chromosomes, allowing normal recombination. In males, however, the Y only matches with the X in short regions at either end, which serve to align the sex chromosomes properly during cell division. The rest of the Y-chromosome, known as the