Scientists discover the human genes involved in adaptation to desert living

In northern Kenya, where the ground is arid, the sun is hot, and water is a daily grind, the people have created a life that has endured for centuries. Their territory extends into Uganda, South Sudan, and Ethiopia, and most of it is some of the toughest country on the planet. And yet this population has persisted, using livestock and tradition to survive in a world that would crush most.

New science reveals that their survival is written in their DNA. By studying hundreds of whole genomes, researchers discovered how the Turkana adapted to survive chronic dehydration, high-protein diets, and harsh desert environments. The findings reveal both the human body's resistance and the hidden costs of gene adaptation to an environment in transition.

Water access is a persistent issue for the Turkana. The researchers found that 76% of those interviewed indicated spending hours daily just collecting water, and nearly all indicated that it never was sufficient. Medical screening found that 89% of the group was dehydrated by definition, and more than a third were severely dehydrated.

Food is also affected by the environment. Farming is not possible in most areas, and hence livestock provides nearly everything. Milk, meat, and blood constitute most of the diet, with earlier studies showing that animal foods provide 70% to 80% of the daily calories. Milk alone contributes about 62% of the daily energy.

This dependence on cattle raised a fundamental question: had the Turkana evolved genetic adaptations that allowed them to live on such a demanding diet even as they were chronically water-starved?

A new analysis led by UC Berkeley and Vanderbilt University, conducted in collaboration with Kenyan researchers and the Turkana community, undertook one of the most intensive genetic studies of an African pastoralist community. Jointly funded by the Kenyan-American research project, the study entailed the sequencing of 367 full genomes and comparative analysis with neighboring societies such as the Maasai, Samburu, and Pokot.

The research revealed over 7.7 million genetic variations. Using strong statistical methods, the researchers identified eight regions of the genome that carried strong signatures of natural selection. While some had been seen in the Maasai before, most were new to the Turkana. Even some linked up with heart disease risk characteristics and markers for Alzheimer's disease.

“This study highlights how working with transitioning populations can lead to new models for understanding how present-day environments interact with past adaptations to potentially impact modern day disease risk,” added Amanda Lea, co-principle investigator of the ongoing study and an assistant professor at Vanderbilt University.

One of them was different, though: STC1. It regulates kidney function and is responsive to antidiuretic hormone, which inundates the body when it is thirsty. In experiments using animals, STC1 activity in the kidneys can increase eightfold after dehydration, sparing fluid and guarding against stress injury.

Two overlapping regions near STC1 had the most significant evidence for selection in the Turkana. Human kidney cell experiments confirmed that STC1 is induced upon exposure to antidiuretic hormone. Population blood provided two genetic markers in this region that were linked with urea variation, a significant measure of kidney function.

The finding suggests that STC1 plays a critical role in helping the body cope with dehydration and protein-rich diet—two characteristics of Turkana life.

When did the adaptations begin? Genetic simulations estimate that natural selection on STC1 began around 7,000 years ago, when East African pastoralism started developing and the climate in the region became arid.

The power and speed of this selection were remarkable. The selection coefficient was put at approximately 0.041, comparable to high-profile human adaptations like malaria resistance and lactase persistence to adulthood.

Incorporated, the same adaptation in STC1 was found. amongst the Daasanach, an additional pastoralist community dwelling to the east of Lake Turkana. The two populations both stemmed from the Nile Valley prior to bifurcating thousands of years ago. Shared adaptation indicates that natural selection influenced multiple populations under the same desert stress.

While STC1 was the most evident sign, researchers also looked at larger genetic changes. Their results suggested that the Turkana evidence polygenic adaptation—when many small genetic changes add up to have an impact on traits.

Adaptations arose in biomarkers related to metabolism and kidney function, including cholesterol, triglycerides, blood glucose, uric acid, and cystatin C. As a package, the results show strong adaptation in large genes and weak modifications across the genome.

But genes that once produced desert survival may no longer be that valuable. Turkana more and more are abandoning traditional pastoralism, either staying in rural villages with no herds or moving to cities and towns for waged labor.

That change comes with new diets and dangers. While over 90% of traditional pastoralists continue to consume blood, milk, and meat regularly, city dwellers consume little or none of these foods. They, on the other hand, live on processed foods bought in markets.

Scientists warn that this change can lead to what's called evolutionary mismatch. Survival genes can subsequently make for disease-prone long-term survival. Earlier studies showed that Turkana who took up urban lifestyles registered more cardiovascular markers. Kidney function in this study also differed substantially depending on lifestyle, with the differences in serum urea and creatinine levels.

"As more and more people migrate from rural to urban settings, so are we seeing a change in the patterns of disease," said Prof. Elijah Songok, Acting Director General, Kenya Medical Research Institute (KEMRI).

The Turkana Health and Genomics Project grew out of consultation with community elders, leaders, and families. Partnership guaranteed that the work was being conducted ethically, and results were returned directly to people.

“Working with the Turkana has been transformative for this study,” said Sospeter Ngoci Njeru, deputy director at KEMRI’s Centre for Community Driven Research. “Their insights into their environment, lifestyle and health have been essential to connecting our genetic findings to real-world biology and survival strategies.”

To make this conversation available, the group is developing a podcast in Turkana that explains the findings of the study and offers relevant health advice as lives change.

The Turkana lessons reach far beyond northern Kenya. The study shows how human DNA has adapted to specific environments, and how the very same adaptations can become disease causes when switched to new environments. As the population of the world moves more from traditional to urban environments, evolutionary mismatch may be the cause for the rise in diseases such as diabetes and heart disease.

The research also demonstrates the value of collaborating with indigenous peoples whose wisdom provides critical perspective to genomic science.

As global warming speeds up, that wisdom could indicate new ways of managing health in a warming, drying world.

Research findings are available online in the journal Science.

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