Acanceh in Mexico is home to many Mayans
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In the past, the quantity of zinc and other trace elements in human regimes has been largely determined by the levels in local soils. Now it has been shown that our ancestors have evolved to deal with local variations in micronutrient levels as they migrate around the world.
This could have led to dramatic side effects – it is possible, for example, that the short stature of certain peoples around the world is a by -product of adaptation to low iodine levels. It is also possible that these past adaptations require some people today to obtain too much or too much specific micronutrients.
“For most of the human evolution, the micronutrient composition of what you eat depends on the underlying soil,” explains Jasmin Rees at the University of Pennsylvania.
His team scanned nearly 900 genomes from around the world to find proof of adaptation to local levels of 13 trace elements, including iron, manganese and selenium. To do this, the team looked for signs of positive selection in 270 genes linked to the adoption of these elements-that is to say for variants of genes which have become more common in specific populations because they have provided an advantage.
The strongest proof has been in the genes linked to iodine among the Mayan peoples of Central America. Similar signatures have also been found in the Peoples Mbuti and Biaha in Central Africa, which have a shorter stature than most people.
In 2009, it was suggested that the short stature of the peoples of Mbuti and Biaka could be due to adaptation to a low iodine. Indeed, genes linked to iodine influence thyroid hormonal activity, which has an effect on growth. These peoples are also known to be less subject to goiter – the widening of the thyroid gland due to an iodine deficient diet – than neighboring groups.
Because the Mayan population is also very short, explains Rees, its results support the idea that adaptation to a low iodine affects size. She and her colleagues also point out that the soils of the tropical forest of the Mayan region are known to be weak in iodine.
“It’s very speculative,” she says. “We cannot say exactly what causes these short statures, but we see, at the very least, a coincidence where there seems to be a selection on the genes associated with iodine in these short -term populations.”
In the Uighur and Brahi peoples of Central Asia and the South, where the levels of magnesium of the soil are particularly high, there is a strong selection affecting two genes linked to the absorption of magnesium. Some of these gene variants were previously linked to low magnesium levels in the body, so the team suggests that these changes reduce the absorption of magnesium to prevent the toxicity of high levels in the environment.
These are only two examples – the team has found signs of positive selection linked to at least one micronutrient in almost all the populations of the world. “We see really widespread adaptation signatures,” explains Rees.
This study is only the beginning, she says. More work is necessary to pin the effects of the many variants of genes identified by the team. Now that food is exchanged worldwide, it could turn out that people with certain variants need more or less specific micronutrients. Rees compares to the way in countries like the United Kingdom, darker people are invited to take vitamin D all year round rather than winter.
“It would be important to know if individuals of particular populations are likely to be particularly necessary for additional micronutrient supplements,” explains Mark Stoneking at the Max Planck Institute for evolutionary anthropology in Germany.
“With regard to the identification of selection signatures from genomic data, they have done advanced work,” he said. “But much more work must be done to verify that these have been really subject to selection – some of them will inevitably prove to be false positive.”
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