How Far From the Tree? (original) (raw)
First Chapter: 'Genes, Peoples, and Languages'
By MARK RIDLEY
human being is a walking archive of historical evidence. Our ancestry as fishes shows up every time we choke. Our windpipes are in front of our throats, though our mouths are below our noses, and the absurd crossover from mouth to throat is evolutionarily descended from a sensible plumbing system in fish. But our genes are an even better source of historical evidence than our anatomies, and there is no better historical reader of our genes than Luigi Luca Cavalli-Sforza. In ''Genes, Peoples, and Languages,'' he says that in 1951 he began ''wondering whether it was possible to reconstruct the history of human evolution using genetic data from living populations.'' Since then he has invented a new way of studying human evolution.
Cavalli-Sforza's research makes use of the global map of human genes. For instance, the A, B and O blood groups are under simple genetic control, and their frequencies have been measured all over the globe. The O blood group has a frequency of about 100 percent in South American Indians, about 50 percent in Northeast Asia and 30-40 percent in Europe. There is a mass of evidence of this kind, collected over the past century. Cavalli-Sforza uses genetic differences among people from various places to reconstruct the ''tree'' of human evolution: a branching diagram of relations among different populations. He posits that populations that are genetically more similar probably share a more recent common ancestor than those that are genetically more distant. The results helped to establish a classic story: the common ancestor of all humans lived in Africa, about 70,000 to 100,000 years ago. Some Africans colonized Asia, and then Australia (55,000 years ago) and North America (maybe 30,000 years ago). About 40,000 years ago others from Africa, along with colonists from Asia, founded the European population of Homo sapiens and, probably, as they did so, eliminated the Neanderthals who had preceded them. Cavalli-Sforza, a geneticist at Stanford for many years, first produced a tree of human evolution in the 1960's. Indeed, he largely invented the idea, and it was not until the 80's that other scientists looked at the same question, using other kinds of genetic evidence. They have supported his main conclusions.
His second big project looked at the spread of farming in Europe. Before it originated in the Middle East 10,000 years ago, humans were hunters and gatherers. But farming soon spread across Europe, in a great wave, from 9,000 to 6,000 years ago. Archaeologists used to think farming spread by cultural copying, as people imitated successful neighbors. Cavalli-Sforza suspected it might have spread because the farmers themselves moved, as the new practice led to higher population densities and subsequent migration. He tested the two ideas with genetic evidence. If the farmers moved, they would have taken their genes with them; but if the farming habit alone was copied, it would have had no genetic consequences. What he found was a gradient of genes across Europe, fanning out from the Middle East, and the gene map almost exactly matches the archaeological map of the spread of wheat. The movement of people, rather than the copying of a practice, caused the spread of farming. Cavalli-Sforza says that when he first published the work in 1984, it ''was not immediately welcomed by Anglo-American archaeologists.'' They are more welcoming now.
Genes are a powerful source of historical evidence, but they are not bug-free. One problem is that humans form a genetic continuum. Evolutionary tree diagrams were invented to show relations between different species, like humans, chimpanzees and gorillas. A tree diagram for species makes clear sense; each branch in the diagram represents a distinct line of creatures. Within a species, the meaning of the branches is less clear and can even be bogus. If chimpanzees were a genetic mix, resulting from repeated interbreeding between gorillas and humans, it would be odd to represent their evolution by one branch leading off the human line of the tree. The reality would be a blurry mess. But that is exactly what ancestral relations are like within the human species. It is difficult to say what a tree means when its branches are within one species, and Cavalli-Sforza says little about that question.
He says more about the related question of human races. One misinterpretation of a human evolutionary tree would be that it shows the branching off of distinct races, with separate histories. A major achievement of human genetics has been exploding the theory that races are genetically distinct. They are genetically only skin-deep: races do differ in a small number of genes that influence superficial features like skin color. But the great majority of our genes are a mish-mash and do not fall into any discrete subcategories of human being. Cavalli-Sforza shows that the European population is the most genetically mixed-up on earth, being a mix of genes from Asia and Africa. He uses this to poke fun at Arthur de Gobineau, the 19th-century French author of the ''Essay on the Inequality of Human Races,'' which helped inspire German racism. De Gobineau, he says, ''would die of rage and shame at this suggestion since he believed that Europeans . . . were the most genetically pure race, the most intellectually gifted and the least weakened by racial mixing.''
Right on! But I should have liked Cavalli-Sforza to tell us more about what these mix-ups mean for the branches on his evolutionary trees.
The book is written for nonspecialists. It contains some technical terms, but they are well flagged. Readers can easily fast-forward through them. It is also a personal book, almost amounting to an intellectual autobiography. Cavalli-Sforza describes his own work on cultural evolution but ignores the work of other scientists. He describes his own research on the ''great diasporas,'' like the spread of farming, but ignores another method that has been used. He describes his own reconstruction of human evolution but does little more than mention complementary research that uses mitochondrial DNA. However, the personal style of the book makes it more readable than a balanced survey could hope to be. And Cavalli-Sforza has himself contributed so much to the subject in the past half-century that he is the ideal author for a personal account of it.
Mark Ridley is a lecturer in the department of zoology at the University of Oxford.