Most people lose the ability to digest the milk sugar lactose after weaning, but some populations retain high levels of an enzyme called lactase, which allows them to break down lactose in adulthood. In a study published March 13th in the American Journal of Human Genetics, researchers identified genetic factors associated with lactase persistence in African populations and found that this trait became more prevalent in recent history in conjunction with the introduction and spread of cattle domestication in Africa. The findings provide strong evidence that lactase persistence evolved in human populations as a dietary adaptation.
"To date, there has never been a large-scale study of lactase persistence that included such a large set of geographically and ethnically diverse populations," says study author Sarah Tishkoff of the University of Pennsylvania. "Our study sheds light on both the genetic basis and evolutionary history of a biologically relevant trait in humans and the origins of pastoralism in Africa."
Individuals with northern European ancestry, as well as African, Arabian, and Central Asian pastoral populations with a tradition of fresh-milk production and consumption, retain high levels of lactase into adulthood. DNA sequence variations linked to lactase persistence have been identified in European populations, but until recently, little was known about genetic factors associated with this trait in African pastoral populations.
In the new study, Tishkoff, along with Alessia Ranciaro, also of the University of Pennsylvania, and collaborators around the globe sought to address this gap in knowledge. The researchers performed a large-scale sequencing analysis of all of the genomic regions thought to influence the activity of the lactase-encoding LCT gene in 819 individuals from 63 diverse African populations and in 154 non-Africans from nine different populations in Europe, the Middle East, and Central and East Asia. They identified several single-nucleotide variants--DNA sequence variations affecting a single nucleotide--associated with lactase persistence.
Moreover, their genetic analysis revealed strong evidence of recent positive selection affecting several variants associated with this trait in African populations, most likely in response to the cultural development of pastoralism. The origins of these variants coincided with the introduction of cattle domestication in Africa about 10,000 years ago and its subsequent spread through pastoral migrations.
Tishkoff will present an overview of genetic variation in Africa and discuss these findings as well as research about the genetic basis of complex traits, such as short stature in Pygmies, at the Cell Symposium "Evolution of Modern Humans: From Bones to Genomes," which will take place March 16-18 in Sitges, Spain. The symposium will bring together world-class researchers from a variety of fields, including genetics and anthropology, to discuss the current picture of the evolution of modern humans and formulate the most exciting questions for future research.