Turkana pastoralists in northern Kenya live in one of the driest habitats on Earth. They can survive chronic dehydration and not have kidney damage.

DNA was taken from over 5000 people and whole genome sequencing from 308 people took place. To serve as a control, DNA was also sequenced from 59 genomes from other indigenous communities that live in the same region.

Whole genome analysis provided evidence of recent selection. One section of DNA in particular stood out. There was an enhancer region for the STC1 gene that caused significant upregulation.

STC1 is a gene that is active in kidney cells when the body becomes dehydrated.

When the blood water potential decreases, the osmoreceptors of the hypothalamus in the brain detect this stimulus and cause more ADH to move from hypothalamus cells to the posterior pituitary gland to be released by exocytosis into the blood (neurosecretion). ADH acts on the distal convoluted tubule and collecting duct of the kidneys. More aquaporins are inserted into the cell surface membrane of cells lining the lumen of the collecting duct. More water then moves by osmosis down the water potential gradient out of the lumen and back into the blood. As more water is reabsorbed, it means less water is excreted in the urine.

Researchers found out that ADH also causes gene STC1 to switch on. Also, that the concentration of the protein SCT1 was linked to the urea concentration. A diet high in protein is digested into amino acids but excess amino acids cannot be stored. So, the liver cells carry out deamination of amino acids to ammonia then the ornithine cycle to convert ammonia to urea. It is thought that STC1 protects against large concentrations of urea. Normally this type of diet would cause gout – a painful health condition but the people of Turkana rarely get gout.

During dehydration, STC1 leads to reduced water loss by further concentrating the urine. It also protects the kidneys from high salt concentrations that occur during dehydration.

The analysis showed that the mutation giving rise to the useful variant caused an upregulation of the STC1 gene so it acted as a ‘survival gene’ and the adaptation was to conserve even more water and protect against damage to the kidneys.

The useful variant was present in the genome before the selection pressure of dry conditions occurred. There is evidence that it was present at low levels over 5500 years ago but then selection acted to increase its frequency. It was at this time period that aridification (the drying of the landscape) started to intensify.

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