Key adaptation helps nomadic people survive in extreme desert

Cornell researchers have contributed to a multi-institutional study of how the nomadic Turkana people of northern Kenya - who have lived for thousands of years in extreme desert conditions - evolved to survive, showing humans' resilience in even the harshest environments.

In the study, published in Science on Sept. 18, a team of researchers from Kenya and the U.S., working with Turkana communities, identified eight regions of DNA in the genomes of the Turkana that have evolved through natural selection in the last 5,000 to 8,000 years. One gene in particular showed exceptionally strong evidence for recent adaptation: STC1, which helps the kidneys conserve water and also may protect from waste products in a diet, like the Turkana's, that is rich in red meat.

Cornell researchers helped to identify when and how the adaptive variant of STC1 emerged and to link it to changes in the environment, finding that the Turkana's ability to thrive with less water emerged around 5,000 to 8,000 years ago, at the same time Northern Kenya went through a period of aridification.

"The project really looks at it from all these different angles and comes up with this quite coherent story which sets it apart from other studies," said Philipp Messer, associate professor of computational biology in the College of Agriculture and Life Sciences.

Four years ago, lead researchers from the project - from the University of California, Berkeley; Vanderbilt University; the Nairobi-based Turkana Health and Genomics Project (THGP) and others - had already, after extensive discussions with Turkana elders and community members, sequenced 367 whole genomes and identified the STC1 gene. But they wanted to better understand how the adaptive variant of this gene evolved.

That's when they connected with Messer and then-graduate student and second author Ian Caldas, Ph.D. '22, who had developed a method using machine learning and simulations to infer how and when an adaptation emerged and how quickly it spread through a population.

"They wanted to know how this adaptation came about. Was it a new mutation? Did it already exist in the population previously and then become more widely prevalent as it became adaptive?" Messer said. "And Ian had developed this really cool new method to infer those parameters from genomic data."

Messer and Caldas found that the STC1 adaptation had likely already been present in the population at a low frequency long before it began to increase between 5,000 to 8,000 years ago. In another population in East Africa, the Daasanach, researchers found that the adaptation arose independently at around the same time.

"This made a lot of sense because that's when a lot of aridification happened in the region," Messer said. "We were also able to measure how strong selection was at this locus, and it's very strong."

They calculated that the selection coefficient is around 5%, which means Turkana with the adaptive variant of the gene, on average, had 5% more offspring than those without it. "It might seem like a small number, but if you have enough individuals, then it becomes statistically significant, and that adaptation is very likely to spread through the population," Messer said. "Five percent is in line with the strongest other examples of recent adaption in humans that we know of."

The study provides a uniquely robust link between the environment, genetic adaptation and the human phenotype and experience of Turkana: Over the course of years of blood and urine samples, the research team found that 90% of participants were technically dehydrated but otherwise healthy. Turkana get an estimated 70 to 80% of their nutrition from animal products such as milk, blood and meat, but gout, which can be caused by a buildup of waste products related to the body's processing of red meat, is rare in the community.

The research underlines humans' ability to survive and adapt to harsh environments - which is particularly germane given the impending impacts of climate change, the authors write. The study also has a direct impact on modern Turkana communities; as more in their population transition to urban environments, their genetic makeup may turn from beneficial to detrimental, a phenomenon called evolutionary mismatch. The broader research team found that Turkana living in cities are more prone to chronic diseases such as hypertension and obesity.

The team is currently working on a podcast, in the native language, to reach Turkana communities and pass on the knowledge gleaned from the study.

First author of the paper is A.J. Lea from Vanderbilt and THGP, and the senior author is J.F. Ayroles from U.C. Berkeley and THGP. Additional authors include researchers from Vanderbilt, THGP, UC Berkeley, Princeton University, the University of California, Santa Barbara, Arizona State University, the National Museums of Kenya, Stony Brook University, University of Nairobi, the National Institutes of Health and the Kenya Medical Research Institute.

Funding for the study came from Princeton, the John Templeton Foundation and the National Institutes of Health.