Researchers have two main lines of attack in their quest to study the history of migration. Mitochondrial DNA is inherited from the mother and so tells us about the female side; the Y chromosome, which determines male sex and is inherited from the father, gives us information about male ancestry. Historically, mitochondrial analysis was first on the scene, leading to the announcement of 'Mitochondrial Eve' in 1987. She was thought to have lived in Africa about 200,000 years ago; we are all her descendants, although this does not mean that she was the only woman living at that time; it's just that the others have no surviving descendants.
Wells has a gift for making vivid analogies. He uses a culinary one to illustrate this idea.
Imagine a Provençal 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, because some families aren't lucky enough to have had girls. By the time we reach the present century we are left with only one surviving recipe— la bouillabaisse profonde. Why did this one survive? By chance …
The Y-chromosome DNA provides a finer degree of 'resolution' than does the mitochondrial DNA and is the most useful tool we have for studying how populations have evolved. This is partly because the Y chromosome has many sites on which variations (polymorphisms) can occur. Studies of this chromosome show that all modern humans were in Africa until at least 60,000 years ago— an astonishingly recent date. If we compress the evolutionary time scale down to a year, in which apes first appear on New Year's Day, "Modern humans wouldn't show up until around 28 December, and they wouldn't leave Africa until New Year's Eve!"
The second half of the book is mostly devoted to a fairly detailed discussion of the implications of the numerous Y-chromosome mutations or markers, which have been given a variety of M numbers (M45, M173, M175 and so forth). These can be quite difficult to keep clear in one's mind. There is a useful genealogical tree on p.181 that shows their interrelationships; it would have been even more useful if it had appeared earlier or had at least been referenced from time to time in the text. Incidentally, the focus on the Y chromosome means that the reference to Man in the title is correct, although Wells notes that the distribution of mitochondrial markers agrees with the Y-chromosome distribution.
Wells uses the distribution of these markers in different parts of the world to trace how our ancestors expanded their range after leaving Africa. At first they travelled east along the coast, eventually reaching Australia. There was little tendency to move north at first but later they did so, expanding east into Asia and west into Europe, where they produced the 'Great Leap Forward' as evidenced by their more sophisticated tools and impressive art. The Americas were populated by people who crossed the Bering Strait, which at the time was dry land owing to low sea levels caused by the last ice age.
Although archaeology and genetics are our main sources of knowledge for human migrations a third line of evidence is provided by cultural and linguistics changes. The attempt to identify cultural change with migration is regarded as old-fashioned today, Wells says, but he thinks this is regrettable. Genetic and linguistic patterns overlap in some cases and this is evidence for migration.
It seems that southern European populations experienced a greater influx of neolithic farmers from the Middle East, carrying lineages such as M172, than did northern Europeans. One possible scenario is that farming spread first around the Mediterranean, with Neolithic Middle Eastern immigrants favouring its climate, similar to that of the Levant. Only later did indigenous Palaeolithic Europeans take up agriculture in the interior, gradually spreading the culture— but only a small percentage of the genes— of the Neolithic throughout. The Cro-Magnons of northern Europe appear to have made a conscious decision to leave behind the Palaeolithic for a new Middle Eastern lifestyle with a small minority of Middle Eastern immigrants.
The migration question has also received a lot of attention in connection with the Indo-European languages. The existence of this linguistic group has been recognised since the late eighteenth century. It implies that there must have been a precursor, Proto-Indo-European, and there has been much controversy about who the speakers of this language were and where they lived. Wells has an interesting discussion of what our knowledge of Y-chromosome markers can contribute to the debate.
The book concludes by lamenting the disappearance of many ancient languages in the modern world. Another difficulty that faces population geneticists is the increasing tendency for people to leave their original homelands and settle elsewhere, often in large conurbations, which is leading to homogenisation and reducing the usefulness of genetic markers. And colonial exploitation in the past has led many indigenous people to be understandably wary of scientists and archaeologist and to be reluctant to participate in research.