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BOOK REVIEW

A Master Synthesizer

Oren Harman

A Genetic and Cultural Odyssey: The Life and Work of L. Luca Cavalli-Sforza. Linda Stone and Paul F. Lurquin. xiv + 227 pp. Columbia University Press, 2005. $45.

A young man of 18, having hesitantly prepared his registration documents for medical school, takes his place in line at the university to hand them in. While waiting, he notices that the line beside his is for registration in the natural sciences. Suddenly, an exciting thought enters his mind: Perhaps I should switch lines! But no, that would mean filling in an entire new set of registration forms. Better to stay put.

Geneticist L. Luca Cavalli-Sforza...Click to Enlarge Image

The place was Turin, the time 1940. Eventually all of the students at the university except those in medical school would be drafted to fight in World War II. The young man's laziness would thus prove to be a stroke of luck. His name? Luigi Cavalli—now L. Luca Cavalli-Sforza. After medical school and a brilliant stint as a bacterial geneticist, he would take up population genetics and demonstrate the all-important role of chance in the evolution of mankind. So despite his choice of lines in 1940, it would be in the natural sciences rather than in medicine that he would make his great contributions.

Now the first biography of this extraordinary man has appeared: A Genetic and Cultural Odyssey, by anthropologist Linda Stone and geneticist Paul F. Lurquin. It is most welcome.

Cavalli was born in Genoa in 1922. At age 20 he stopped calling himself Luigi and at the suggestion of a mentor renamed himself Luca. Then at age 27 he was officially adopted by his maternal stepgrandfather, Count Francesco Sforza, and changed his name to Cavalli-Sforza.

In 1948 he met the great English statistician R. A. Fisher, who offered him a job at the University of Cambridge applying statistical methods to the study of the genetics of bacteria. Cavalli-Sforza's involvement in figuring out bacterial conjugation (how bacteria mate) has somehow been omitted from most histories, but in fact he helped Joshua and Ethel Lederberg and William Hayes crack this fundamental problem. At the time, a geneticist remarked jocularly, "How can you believe this bacterial sex thing, knowing that it was discovered by a Jew, an Italian, and an Irishman!" But the evidence was incontrovertible: Bacteria do sometimes engage in genetic recombination. This seminal scientific discovery, Cavalli-Sforza's most important, opened up new avenues for genetic and medical research and transformed molecular biology—leading, among other things, to the operon model of gene regulation proposed by François Jacob and Jacques Monod.

After a decade of work on bacteria, Cavalli-Sforza decided that "humans are more charismatic than E. coli!" and turned to the study of what would ultimately earn him international fame: the genetics of human populations and its bearing on human culture and history. He wanted to know why there are genetic differences between populations of humans, such that a particular blood type, for example, may be found more frequently in Alaskan Eskimos than in the Blackfoot tribe of Native Americans. If all humans have a common origin, how did these variations come to be, he wondered. Could one study the factors responsible?

Honing his statistical skills, Cavalli-Sforza developed mathematical tools to trace the contours and causes of genetic variation, measured at first by diversity in human proteins and blood types, and later, following the development of mitochondrial and Y-chromosome techniques, by differences at the DNA level. These methods enabled him to distinguish among the four factors that traditionally influence gene frequencies in a population: natural selection, migration (gene flow or admixture), gene mutation and chance persistence of mutations that offer no selective advantage or disadvantage (referred to as genetic drift). His models showed that the last phenomenon played a crucial role in accounting for much of human genetic diversity: For example, during certain transitions (called "bottleneck events"), only a small proportion of a population might survive to reproduce—not because this group is necessarily more fit, but rather merely due to chance. The genes of this particular chance group will then dominate the entire gene pool of populations to come.

The role of drift is important. Deleterious or advantageous mutations to the gene pool that are under the control of natural selection (for example in genes for skin color, body size, hair texture and the shape of the nose—traits that have been used to categorize humans into races) can teach us a lot about the history of the environmental challenges faced by different populations of humans. But neutral or almost neutral mutations that constitute drift are more useful in tracing the history of the humans themselves because they are far more numerous. By showing that the unseen genetic diversity within groups of people is much greater than that which accounts for the clearly apparent differences between them, Cavalli-Sforza's work has constituted a scientific blow to the biological concept of race. Sadly, as Stone and Lurquin show, this has not shielded Cavalli-Sforza himself from unfounded accusations of racism.

Above all a master synthesizer, Cavalli-Sforza grasped that much of human evolution—biological, cultural and linguistic—can be illuminated by looking at how and where groups of humans have migrated across the globe. When people move from place to place, they take with them their genes, their diseases, their languages and their culture. He understood that to unravel humankind's complex movements out of Africa some 50,000 years ago and across the Earth since, the usual divides between the social and natural sciences would need to be felled—archaeological, linguistic, historical and anthropological comparisons would have to be marshaled alongside the genetic evidence.

Cutting across disciplines, Cavalli-Sforza met with much resistance: from linguists, who challenged the assumptions behind the neat fit he demonstrated between genetic and language maps of the world; from biologists and mathematicians, who critiqued his genetic and statistical techniques; and from cultural anthropologists, who considered his quantitative models of cultural diffusion to be oversimplifications based on naive analogies from genetic diffusion.

Some of Cavalli-Sforza's claims and models remain contested, and a more detailed analysis of the criticisms would have been welcome in this book, which was written, perhaps to a greater extent than it should have been, under Cavalli-Sforza's guidance. (For a more penetrating and hardheaded, albeit sympathetic, critique, readers might want to consult John Maynard Smith's Did Darwin Get It Right?, which contains an excellent review of Cavalli-Sforza and Mark Feldman's models of cultural diffusion.) Nevertheless, Stone and Lurquin do an excellent job of presenting Cavalli-Sforza's impressive iconoclasm and awesome intellectual breadth.

They report that Cavalli-Sforza remembers once asking anthropologist Claude Lévi-Strauss what he thought of cultural transmission and receiving this answer: "It is too complicated to study." Cavalli-Sforza chose to challenge that view. Theorizing about culture by analogy to biological evolution may or may not ultimately prove useful. But if we are ever to understand ourselves and our history better than we do today, we must attempt to integrate the traditionally separate compartments of human knowledge. This fascinating little book demonstrates that Cavalli-Sforza has blazed a path well worth following.

 

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