The Journey of Man: A Genetic Odyssey Read Online Free PDF Page A

pluribus unum
    What we call the beginning is often the end
And to make an end is to make a beginning.
The end is where we start from.
T. S. Eliot, ‘Little Gidding’ (
Four Quartets
)
    The study of human diversity was, until the twentieth century, limited to variation that could be observed with the naked eye. The subject of countless studies by Broca, Galton and the biometricians in Europe and America, this era marked a ‘collection’ phase of physical anthropology – the early stages of a new field of scientific enquiry, when there is no unifying theory with which to analyse the data accumulated. There was only one problem with the growing mass of data on human morphological variation – there was no simple correspondence between the newly rediscovered laws of heredity and the characters being measured. While there is certainly a genetic component to human morphology, it is clear that dozens – probably hundreds – of separate genes control this variability. Even today, the underlying genetic causes have yet to be deciphered. Thinking of Broca’s craniometric studies, if a particular bump on the skull is found in two unrelated individuals, does it necessarily represent the same genetic change? Are the bumps really the same characteristic, and thus representative of a true genetic relationship, or do they simply resemble each other superficially – by chance? It was impossible to know.
    Genetic variation was critical for the study of human diversity because is it is genetic change that actually produces evolution. At its most basic level, evolution is simply a change in the genetic composition of a species over time. Thus in order to assess how closely relatedindividuals are – in particular whether they form a single species – it is important to know something about their genes. If the genes are the same, then they are the same species. What physical anthropology desperately needed was a collection of varying traits – known as polymorphisms, from the Greek for ‘many forms’ – with a simple pattern of inheritance. These could then be used to study human diversity in an effort to categorize it. Some traits like this were already known, particularly diseases like haemophilia. The problem with disease-causing polymorphisms was that they were simply too rare to be of any use in classification. Common, genetically simple polymorphisms were critical.
    These arrived in 1901, when Karl Landsteiner noticed an interesting reaction upon mixing the blood from two unrelated people: some of the time it clumped together, forming large clots. This coagulation reaction was shown to be heritable, and it constituted the first demonstration of biochemical diversity among living humans. This experiment led to the definition of human blood groups, which would soon be applied to transfusions all over the world. If your doctor tells you that you have type A blood, this is actually the name given by Landsteiner to the first blood group polymorphism over a century ago.
    Building on Landsteiner’s insight, a Swiss couple named Hirszfeld began to test the blood of soldiers in Salonika during the First World War. In a 1919 publication, they noted different frequencies of blood groups among the diverse nationalities thrown together by the hostilities – the first direct survey of human genetic diversity. The Hirszfelds even formulated a theory (accepted by some to this day) in which the A and B blood groups represent the traces of ‘pure’ populations of aboriginal humans, each composed entirely of either A or B individuals. These pure races later became mixed through migration, leading to the complicated patterns of A and B seen in their study. They failed to explain how the two races may have arisen, but given that group A was thought to have originated in northern Europe, while B was a southern marker at highest frequency in India, it seems that there must have been two independent origins of modern humans.
    In the 1930s an