New palaeogenetic study based in Mainz and London sees development of lactase persistence as the result of a natural selection process
The ability of adults to digest lactose and thus milk and milk products most likely played a decisive role in the evolutionary history of Europeans in the Prehistoric era. Researchers at Johannes Gutenberg University Mainz (JGU) and the University College in London have discovered that the genetic marker related to so-called lactase persistence hardly existed in Europe during the early Neolithic Period. "Our ability to easily digest milk as adults must have spread later through a process of natural selection," states Professor Dr. Joachim Burger of the JGU Institute of Anthropology. "It is quite likely that the ability to digest milk was even a decisive selective evolutionary advantage among sedentary farmers and stock farmers in Central and Northern Europe." The results of the study, which analyzed ancient genetic material (aDNA) obtained from archaeological skeletons, have been published in the journal Proceedings of the National Academy of Sciences.
The enzyme that helps the human body digest lactose is called lactase. In infancy, humans produce sufficient quantities of this enzyme, but after weaning only small quantities are produced so that adults have a reduced physiological ability to use milk. This holds true for much of the adult population around the world, with the exclusion of Europe and parts of Africa. It is generally Northern Europeans who produce the enzyme lactase still after infancy and this enables them to readily digest large quantities of milk as adults. A similar ability has been identified among a few African populations where this genetic trait, known as lactase persistence, developed independently of the European context.
About 70% of the population in Northern Germany, Scandinavia, and the Netherlands has the gene for lactase persistence. The frequency of this trait decreases the further south one looks, so that by the time Southern Italy is reached, it has virtually disappeared. Palaeogeneticists in Mainz have now shown that this dissemination first occurred in the last 8,000 years and that it was the result of a natural selection process. With the collaboration of an English colleague, the JGU scientists examined Mesolithic and Neolithic skeletons using molecular genetic techniques. The remains date back to the phase when humans made the shift from hunting and gathering to farming and livestock breeding. The anthropologists found that the marker for lactase persistence was hardly present in Europe during this period.
The high frequency of this trait today, especially in Northern Europe, must thus be the result of a process of natural selection that occurred after the early Neolithic period. Consequently, when the first domesticated goats, sheep, and cattle were introduced into Europe about 8,000 years ago, the majority of the early farmers were not able to digest milk. The small percentage of the population that was already lactose tolerant at the time were thus the ancestors of an evolutionary success story. Thanks to the minor small physiological/genetic variation they possessed, this minority was favored by evolution for thousands of years so that in the end the frequency of this marker rose from virtually nothing to more than 70%. "Milk is a drink rich in energy that also contains lots of calcium," explains Martina Kirchner, who examined fossil DNA at the trace laboratory in Mainz looking for this prehistoric lactase persistence. Anthropologist Joachim Burger, head of the Palaeogenetics Group at Mainz University, adds that "milk could combat the high rate of child mortality following weaning and it also served as a substitute form of energy in years with poor harvests."
The physiological ability to break down lactose must have been such an enormous genetic advantage during the Prehistoric period that researchers now speak of this gene as having the highest positive selection rate compared to all others in the entire human genome. "This is the first piece of direct evidence for positive selection among humans," emphasizes Mark G. Thomas, co-author of the study and Professor of Evolutionary Genetics at University College London. Indeed, this natural selection process must have occurred at an extraordinary rate because from an evolutionary perspective 8,000 years is a very short amount of time for such a mutation to spread. Socio-cultural factors may also have spurred on the biological evolutionary process. It is quite possible that the Neolithic farmers and cattle breeders who were lactase persistent were not only able to nourish more children thanks to their valuable milk and meat resources, but also enjoyed more prestige and power as a result. In turn, this affluence as well as the larger number of offspring became more and more entrenched among these animal and dairy farmers from generation to generation. About a year ago, Burger's team had already demonstrated that the early Neolithic farmers could not be the ancestors of today's Europeans. Now it seems quite likely that the origins of today's Northern and Central European populations can be traced back to 5,000 B.C. and a small group of milk-drinking farmers and stockmen who lived around that time.