March 14, 2014
Cattle Domestication Helped People Develop Tolerance To Lactose
redOrbit Staff & Wire Reports - Your Universe Online
While most people lose the ability to digest the milk sugar lactose after infancy, some populations retain high levels of an enzyme called lactase, which allows them to continue to reap the nutritive benefits of milk – and now the authors of a new study believe they have discovered the genetic origins of this particular trait.
In the study, an international team of experts detail how they investigated lactase persistence in geographically diverse populations of Africa and how their findings support the notion that the ability to digest milk sugar evolved as a dietary adaptation in groups that raised cattle and consumed those creatures’ fresh milk.
The research was led by professor Sarah Tishkoff and postdoctoral fellow Alessia Ranciaro of the Department of Genetics at the University of Pennsylvania’s Perelman School of Medicine. The study is published in Thursday’s edition of the American Journal of Human Genetics.
Previously published studies had demonstrated that people of northern European ancestry, as well as those living in that region and those hailing from Africa, the Arabian Peninsula and Central Asia continue to express the lactase enzyme into adulthood – likely due to a history of fresh milk production and consumption.
Some of that research had linked the genetic origin of this trait in Europeans to a specific mutation which regulates the expression of the gene responsible for coding for lactase. In 2007, a team led by Tishkoff and Ranciaro examined African populations and discovered three previously unidentified genetic variants that were also associated with lactase persistence. However, those variants still did not fully explain why some Africans could digest milk.
In this new study, the duo joined forces with colleagues from France, Italy, South Africa, the Sudan and elsewhere in an attempt to close the gap in knowledge. They preformed a large-scale sequencing analysis of all genomic regions believed to impact the activity of LCT, the lactase-encoding gene, in 819 individuals from 63 diverse African populations and in 154 non-Africans from nine different European, Middle Eastern and Asian populations.
According to the university, the study authors identified multiple single-nucleotide DNA sequence variations affiliated with lactase persistence. In addition, their analysis uncovered substantial evidence of recent positive selection impacting several variants associated with this trait in African populations – probably a response to the rise of pastoralism and cattle domestication in the continent approximately 10,000 years ago.
“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,” Tishkoff said. “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.”
One of the variants they discovered, T-13910, was associated with lactase in Europeans and is believed to be between 5,000 and 12,300 years old. A second variant, G-13915, was found in the Arabian Peninsula, northern Kenya and northern Sudan, and dates to approximately 5,000 years ago – roughly the same time that camels were domesticated in that region, according to the study authors.
A third variant, G-13907, was identified in the northern parts of Kenya and the Sudan, as well as in Ethiopia. The study authors believe that this mutation could have originated in the Cushitic populations in Ethiopia and spread as those peoples migrated to the other nations over the past 5,000 years. The fourth variant, C-14010, was found in southern Africa, Kenya and Tanzania and is thought to have arisen between 3,000 and 7,000 years ago.
“We're starting to paint a picture of convergent evolution. Our results are showing different mutations arising in different places that are under selection and rising to high frequencies and then reintroduced by migration to new areas and new populations,” Tishkoff said.
Even so, the researchers said that there were still some groups that had the ability to digest milk but lacked any of the identified genetic causes for that trait. That leads the researchers to believe that there may be additional variants yet to be discovered, perhaps in parts of the genome that have not yet been analyzed, or that some types of gut bacteria could be helping digest lactose – a hypothesis that the team says they are currently looking into.