Tracking Genes Across the Globe

November 30, 2006

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Theodore G. Schurr

Genes, Peoples, and Languages. Luigi Luca Cavalli-Sforza. Translated by Mark Seielstad. xii + 228 pp. North Point Press/Farrar, Straus & Giroux, 2000. $24.00.

In recent decades, the field of genetics has significantly expanded our understanding of human evolution and biological diversity. Through the use of population genetic models, we have gained insights into the evolutionary forces—genetic drift, mutation, migration and gene flow—that have shaped the patterns of genetic diversity observed in modern populations. In addition, using a powerful set of molecular biological methods, we have identified a great array of genetic markers that can be used to track human movements across the globe through time and space. The problem has been to figure out how to piece together these data with those from paleoanthropological and archaeological research to obtain the most comprehensive picture possible of modern human evolution.

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One of the first researchers to attempt to synthesize genetic, linguistic and archaeological data and infer the genetic history of modern humans from it was Luigi Luca Cavalli-Sforza. His pioneering work on the genetic diversity of European peoples set the stage for similar investigations of biological and linguistic diversity in human populations from around the world and continues to this day. Genes, Peoples, and Languages summarizes his lifetime of research into the origins and diversity of modern human populations and reflects his experiences in and perspectives on the field of human population genetics. Based on public lectures he gave at the Collège de France in the 1980s, which have been modified and updated for this English edition, the book effectively communicates complex ideas for a general audience without sacrificing the important technical details that underlie them; thus it should be of great interest to professional as well as lay readers.

The book is remarkable for its succinct yet thorough description of an enormous body of genetic data, most of which can be found in Cavalli-Sforza's 1994 book The History and Geography of Human Genes. That volume provides more detailed descriptions of the patterns of genetic variation observed in world populations, including maps of allele frequencies in different geographic regions, tables of genetic distances estimated from them and numerous figures illustrating the genetic relationships among these populations.

Genes, Peoples, and Languages opens with a general overview of the biogenetic basis of human diversity, which leads into a discussion of the difficulties of classifying humans into races. This is followed by descriptions of the statistical and phylogenetic techniques used to analyze genetic data and infer population relationships. These are quite accessible, even for the nongeneticist, and very nicely prepare the reader to evaluate the information presented subsequently.

Next Cavalli-Sforza examines the questions of when modern humans first appeared in Africa and when they dispersed into the rest of the world. As it turns out, these two events may be separated by quite a bit of time: The modern human lineage emerged perhaps 100,000 to 125,000 years ago in Africa, and the modern human expansion out of East Africa is now estimated to have taken place some 50,000 to 70,000 years ago. Interestingly, many of the data used to demonstrate this dichotomous process have been generated in only the past two or three years. During that time new hominid fossils have been discovered in Africa. Just as the latter discovery has revolutionized our understanding of the human phylogeny, molecular genetic data have helped us reconceive the way in which our species has evolved and migrated around the globe.

The author then shifts to the subject that has been the focus of much of his research over the past three decades—the elucidation of evolutionary and demographic processes from genetic data. He discusses the use of archaeological and linguistic information and genetic data to infer the settlement pattern of Europe and to test various hypotheses regarding the spread of Indo-European languages and agriculture in the region. This is the best summary of his work on the genetic and linguistic origins of Europeans that I have read.

In a chapter on genes and languages, Cavalli-Sforza demonstrates that one can trace human expansions in different parts of the world by comparing phylogenies of languages and genes and taking into consideration geographic, archaeological and geological factors that might have influenced their branching patterns. These constructs can further provide insights into the demographic changes that have affected the modern human gene pool and the distribution of languages across the world. This approach yields cogent explanations for the presence of linguistic isolates such as Ket and Basque, the expansion of Indo-European languages throughout Eurasia and the ubiquity of Bantu languages throughout sub-Saharan Africa.

The final chapter examines the nature of human culture and how cultural knowledge is transmitted in vertical (intergenerational) and horizontal (intragenerational) dimensions. Here Cavalli-Sforza explores the possible coevolution of biological and cultural traits—the ways in which cultural practices can constrain or shape patterns of biological diversity in human groups. A section considering linguistic evolution as cultural evolution is very thought provoking.

Genes, Peoples, and Languages is a thorough summary of the biogenetic data from modern human populations. But some of its interpretations don't reflect the complexity of recent findings. The putative Caucasoid ancestry of mummified individuals found in the 1970s in the Tarim Basin in Xinjiang Province in western China is a case in point. As Cavalli-Sforza notes, these mummies lacked typical mongoloid (northern Asian) cranial features, had greater stature and different hair color than other populations in the area and differed in their material culture from surrounding Asian groups. Some individuals were buried with tartans similar to those made in Scotland, Austria and Switzerland. It was hypothesized that the Tarim Basin people spoke an extinct Indo-European language (Tocharian) and were pastoralists from the Russian steppes who migrated from northern Europe into western China some 3,000 to 4,000 years ago.

Despite the fact that preliminary mitochondrial DNA (mtDNA) data suggested a possible European (Eurasian) origin for these people, a craniometric study by B. E. Hemphill published in 2000 (after Genes, Peoples, and Languages had presumably gone to press) indicates that the Tarim Basin populations had a more complex ancestry than was initially supposed. The earliest groups had their closest affinities with populations from the Indus Valley, and the later ones exhibited affinities with peoples of the Oxus River Valley of south-central Asia, with both groups being considerably divergent from one another. These results argue against a Russian steppe origin for the Tarim Basin peoples and indicate that further genetic research is needed to clarify their relationships to other Indo-European and South Asian populations.

Elsewhere in the book, Cavalli-Sforza states without further elaboration that the Negrito populations of Southeast Asia may represent the closest Asian relatives to the ancestral African immigrants that entered this region of the world some 40,000 to 60,000 years ago. However, various biogenetic studies of the Negritos have indicated their affinities with other Asian populations. In The History and Geography of Human Genes, Cavalli-Sforza himself found this same general association in data on multiple protein polymorphism loci from East and Southeast Asian populations. Thus, it is not clear which data set (or sets) from the Negritos has shown the deeper genetic linkages with ancestral African populations. Additional molecular genetic data from Negrito populations will undoubtedly help to clarify their exact relationship with other African and Asian populations.

But these are minor quibbles. In fact, they highlight the dynamic nature of the field of molecular anthropology and illustrate how various lines of anthropological evidence may reveal different kinds of population associations, ones that require a synthetic approach similar to Cavalli-Sforza's to resolve them. They also show how quickly new archaeological and genetic discoveries can fundamentally alter previously held models of population relationships.

Fortunately, the extensive work carried out by the author and his colleagues at Stanford has provided a firm foundation of genetic evidence with which to compare new molecular data. This is especially true with respect to the Y-chromosome data currently being generated by researchers in and from Cavalli-Sforza's laboratory, as these have been vital for defining paternally inherited genetic lineages in human groups and the migrations that have dispersed them. Given the rapid accumulation of new molecular data from mtDNA, Y-chromosome and autosomal loci, one can hope for a broader summary of both classical protein polymorphism and molecular genetic data sets in the near future.

Reviewer Information

Theodore G. Schurr is a postdoctoral scientist in the Department of Genetics, Southwest Foundation for Biomedical Research, San Antonio, Texas. He has spent the past 12 years investigating the genetic prehistory of Asia and the Americas through field and laboratory studies of mtDNA and Y-chromosome variation in Asian, Siberian and Native American populations.

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