Remembering Dr. Luigi Luca Cavalli-Sforza, a pioneer in population genetics
Learning about human history has a way of charming our imagination; there’s something captivating in finding out who your ancestors were and how their meandering stories all collide with you. The same is true at a population level—for thousands of years, people couldn’t help but wonder why there’s so much variation in how different populations look, and which population of humans was the first. Only recently did we discover how to answer these questions, and it’s due in large part to the work of Dr. Luigi Luca Cavalli-Sforza, who died late last week at the age of 96.
Prior to the 1940s, we did not know much about human history beyond the written record. Fossils and other artifacts gave us a great amount of insight, but it was still incredibly hard to explain how modern human populations came to be. That’s not to say people didn’t try—elaborate stories and theories were constructed to explain how people came to inhabit all parts of the earth. These stories often revolved around naïve and sometimes nefarious ideas that one’s own society is the best society, and therefore they must be the original human beings. Such inaccuracies were laid to rest when scientists like Dr. Cavalli-Sforza figured out how to use genetic data to trace ancient human movements1.
Dr. Cavalli-Sforza was still a young scientist when he took up population genetics. This field of study makes use of both statistics and genetics to compare differences and similarities between various populations. Following World War I, scientists had learned that the prevalence of blood types (A, B, O, AB) varied across populations, and correlated with geography. This was a big finding because previously people relied on superficial traits, like skin color, to compare populations—a method that we now know is error-prone. With the discovery that populations could be characterized by genetic trait frequencies like blood types, the door opened for scientists to begin using genetics to study the dynamics of human populations2.
Dr. Cavalli-Sforza entered the field by studying the relationship among small populations of people in Italy using blood types. In the course of his studies, he noted the importance of genetic drift—the random change in frequency of DNA variants. To put it another way, genetic drift means that a populations traits aren’t always the result of natural selection; small populations may acquire different traits due to chance. Though he was not the first to describe it, genetic drift had largely been ignored in the study of human evolution until Dr. Cavalli-Sforza recognized its importance through his studies. Without an understanding of genetic drift, the description of human ancestry would be incomplete2.
One of Dr. Cavalli-Sforza’s most important achievements was the use of Principal Component Analysis (PCA) to trace human origins. This is a complex computational method that allows researchers to comprehend multidimensional data. To put it more simply, Dr. Cavalli-Sforza figured out how to apply PCA techniques to genetic data in order to find out how human populations are related. Using one of the first electronic computers in Italy to perform PCA analysis, Dr. Cavalli-Sforza and his team set the stage for a modern understanding of the history of human populations. Using genetic traits from diverse populations, they showed that humans likely originated in Africa and extended into the rest of the world from there3.
Supporting the idea that all humans are related, Dr. Cavalli-Sforza teamed up with linguistic experts to show that the evolution of language mirrored that of DNA, and both pointed to a single founding population of humans from which all languages and human populations are derived. As technology advanced over the next 50 years, he continued to refine this model. He was also a formidable and outspoken critic of racism. Throughout history, people have tried to justify prejudice and racial views based on pseudoscience. Dr. Cavalli-Sforza was relentless in his efforts to explain the lack of scientific justification for racism—often devoting segments of his books to addressing this topic1-3.
Those who knew Dr. Cavalli-Sforza describe him as an incredibly impactful mentor, a positive personality, and perhaps one of the most influential scientists of the 20th century. He inspired people to take an interest in population genetics and understand how human diversity came to be.
2Cavalli-Sforza, L. L., et al. The History and Geography of Human Genes. Princeton University Press, 1997.
3Manni, Franz. “Interview with Luigi Luca Cavalli-Sforza: Past Research and Directions for Future Investigations in Human Population Genetics.” Human Biology, vol. 82, no. 3, 2010, pp. 245–266., doi:10.3378/027.082.0301.
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