Sharp Blue: Genes, Peoples and Languages

There is a desert in humanity’s past. Here, on the shores of the future, there are cities buzzing with commerce and industry, linked by highways and airliners and sea-lanes plied by gigantic cargo ships. Satellites race overhead, data streaks across global networks. A dizzying array of factories process raw materials into progressively more complex arrays of products. Start walking in the direction of the past and things slowly change. For the first few centuries of our trek, the cities are much the same but they become smaller and dirtier and less interconnected. Then suddenly, we reach the pre-industrial hinterland. For the next thirty or forty centuries, we’re in a rolling countryside of small towns and scattered farms and the occasional sea full of sailing ships. Occasionally empires rise and fall. Clothing and the products of industry becomes simpler. Here and there are picturesque pyramids. Iron and steel disappear, then writing. For another seventy centuries, there are farming villages and guys riding around on horses with bows and stone axes, and the occasional small town of stone or timber. Then, even the farms vanish and we reach the desert. Hunter-gatherers live here. They are few in number, but look like us, only dirtier, uglier, and not wearing jeans and t-shirts. This desert stretches off to the horizon, spreading itself over hundreds of centuries of uncharted wastelands. The Palaeolithic Desert and its Neolithic borderland are so far from the shores of the future that they’re strange and exotic and mysterious places. We know little of what happens there, of its peoples’ histories and cultures and customs. The trackless desert covers most of the time for which anatomically modern humans have inhabited the Earth.

The traditional way of looking into prehistory is through archaeology. This studies the artifacts left behind by the people of the past: tools, paintings, campsites, buildings. Unfortunately, archaeology can’t answer all the questions we’d like to ask. Archaelogical artifacts are the products of culture, and it’s not clear if the spread of distinctive classes of items is a sign of the diffusion of culture or the migration and expansion of peoples. The population geneticist Luigi Luca Cavalli-Sforza has contributed as much as anyone to the understanding of these issues, and in his popular science book Genes, Peoples and Languages he explains his findings. The central idea is that the ancient migrations and diversifications and interbreedings of peoples have left traces in modern genetic diversity, and by studying these traces we can reconstruct the largest scale structure of the history.

After an introductory section on genetics, genetic dating, mutation and genetic drift, Cavalli-Sforza describes how genes from populations across the world can be used to construct a tree showing progressive divergences in the expansion out of Africa, through the Middle East, along the south cost of Asia, into Europe, from south Asia northwards, and from north-east Asia into the Americas. There follows a tour-de-force in which he outlines his work on the principal component analysis of genetic diversity in Europe. Principal component analysis separates out independent contributions to modern diversity. Each principal component corresponds to an expansion or contraction of an ancient population. When plotted on a map of Europe, the contours of the first principal component almost exactly coincide with the contours of the spread of agriculture from the Middle East. It thus seems that populations of farmers carried agricultural society across the continent rather than the idea of farming spreading by cultural diffusion. The second principal component contours trace the repopulation of Europe after the last ice age. The third component displays an expansion from the steppe to the north of the Black Sea, and supports the hypothesis that Indo-European nomads originating from that region brought their languages and culture to Europe. The fourth principal component shows the expansion of the Greeks along the shores of the Mediterranean. The fifth component shows the contraction of the Basques. That such features of history and prehistory can be read so clearly from genes seems to me remarkable.

The following chapters describe the reconstruction of linguistic trees and their comparison with genetic trees, and the mechanisms of the spread of culture. The section on linguistics is interesting and lucid but much more familiar than Cavalli-Sforza’s own work. Unfortunately, the last section, on culture, is neither lucid nor interesting. Indeed, it feels tacked on, vague and incoherent. Nevertheless, Genes, Peoples and Languages is well worth reading, and it’s made me want to eventually read his technical work The History and Geography of Human Genes.