The Journey of Man: A Genetic Odyssey by Spencer Wells

lengths to collect samples of human placentas (an abundant source of mtDNA) from many different populations – Europeans, New Guineans, Native Americans and so on. The goal was to assess the pattern of variation for the entire human species, with the aim of inferring something about human origins. What they found was extraordinary.
    Cann and her colleagues published their initial study of human mitochondrial diversity in 1987. It was the first time that human DNA polymorphism data had been analysed using parsimony methods to infer a common ancestor and estimate a date. In the abstract to the paper they state the main finding clearly and succinctly: ‘All these mitochondrial DNAs stem from one woman who is postulated to have lived about 200,000 years ago, probably in Africa.’ The discovery was big news, and this woman became known in the tabloids as Mitochondrial Eve – the mother of us all. In a rather surprising twist, though, she wasn’t the only Eve in the garden – only the luckiest.
    The analysis performed by Cann and her colleagues involved asking how the mtDNA sequences were related to each other. In their paper they assumed that if two mtDNA sequences shared a sequence variant at a polymorphic site (say, a C at a position where the sequences had either a C or a T), then they shared a common ancestor. By building up a network of the mtDNA sequences – 147 in all – they were able to infer the relationships between the individuals who had donated the samples. It was a tedious process, and involved a significant amount of time analysing the data on a computer. What their results showed were that the greatest divergence between mtDNA sequences was actually found among the Africans – showing that they had been diverging for longer. In other words, Africans are the oldest group on the planet – meaning that our species had originated there.

    Figure 2 Proof that modern humans originated in Africa – the deepest split in the genealogy of mtDNA (‘Eve’) is between mtDNA sequences from Africans, showing that they have been accumulating evolutionary changes for longer.
    One of the features of the parsimony analysis used by Cann, Stoneking and Wilson to analyse their mtDNA sequence data is that it inevitably leads back to a single common ancestor at some point in the past. For any region of the genome that does not recombine – in this case, the mitochondrion – we can define a single ancestral mitochondrion from which all present-day mitochondria are descended. It is like looking at an expanding circle of ripples in a pond and inferring where the stone must have dropped – in the dead centre of the circle. The evolving mtDNA sequences, accumulating polymorphisms as they are passed from mother to daughter, are the expanding waves, and the ancestor is the point where the stone entered the water. By applying Zuckerkandl and Pauling’s methods of analysis, we can ‘see’ the single ancestor that lived thousands of years ago, and which has mutated over time to produce all of the diverse forms that existtoday. Furthermore, if we know the rate at which mutations occur, and we know how many polymorphisms there are by taking a sample of human diversity from around the globe, then we can calculate how many years have elapsed from the point when the stone dropped – in other words, to the ancestor from whom all of the mutated descendants must have descended.
    Crucially, though, the fact that a single ancestor gave rise to all of the diversity present today does not mean that this was the only person alive at the time – only that the descendant lineages of the other people alive at the same time died out. Imagine a Provencal village in the eighteenth century, with ten families living there. Each has its own special recipe for bouillabaisse, but it can only be passed on orally from mother to daughter. If the family has only sons, then the recipe is lost. Over time, we gradually reduce the number of starting recipes,