In a paradigm-changing study, scientists have discovered that the genes in the “power stations” of our cells are expressed differently in Africans and non-Africans. In fact, mutations in these genes may have been crucial to human survival in the cold European climate, suggest scientists from Ben-Gurion University of the Negev, Be’er Sheva.
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These “power plants” in our cells, called mitochondria, are responsible for converting the food we eat into a molecule called ATP (adenosine triphosphate), which is like gasoline for the body. Mitochondria look like tiny blimps inside the cells and, uniquely among our cellular structures, have their own DNA, with 37 genes: the separate human DNA in the cellular nucleus is believed to have around 20,000 to 25,000 genes.
(Partly because of that independent DNA, and the fact that mitochondria reproduce on their own, the accepted theory is that they originated from bacteria, which somehow entered into a symbiotic relationship with one-celled animals some 2 billion years ago.)
All humans have the same basic mitochondria, with the same 37 genes. But in a startling study, scientists have now discovered that the regulation of mitochondrial genes in Africans and non-Africans differs. “All we discovered is that there is an effect to the mutations,” explains BGU’s Prof. Dan Mishmar.
“Among all African populations, genes encoded by the mitochondrial genome had lower level of expression compared with Caucasians,” he said, adding that there is no comparative data yet on Asian mitochondrial expression compared with African and Caucasian.
Until the BGU study, published in PLOS Genetics, genetic variation in the mitochondrial genome was considered neutral: that mutations to mitochondrial DNA, that didn’t kill the host, had no impact. It turns out they did, and may have been crucial to modern human survival in Europe.
The great human migration
All non-African human populations today, including Australian aborigines, are believed to have arisen from a single migration out of Africa, about 60,000 years ago. There had been earlier migrations of humans out of Africa, but they left no known survivors.
Since Africans and non-Africans differ in expression of mitochondrial genes, the mutations in regulation had to have happened after the humans left Africa.
“People migrated outward from Africa, for family reasons or whatever. But ultimately the ones who survived outside Africa were people who could contend with the cold climate in Europe,” said Mishmar.
“Mutations in the mitochondria, the power plant of the cell, are exactly the thing,” he added. So although the role of the mutations has yet to be investigated, Mishmar is willing to speculate that they could affect the body’s energy metabolism.
Given that in science we are driven by results, noted Mishmar, one conclusion that begs to be reached is that the change in mitochondrial expression helped the shivering humans-out-of-Africa survive in European conditions. Theoretically, a higher level of energy metabolism would have helped the migrants cope with cold.
How? We don’t know. The results are clear, though. “Our paper shows strong evidence that mitochondrial DNA genetic variants have a regulatory impact, and hence are functional and not passive. This implies that such variants likely played an active role as humans left Africa to populate the rest of the world,” said Mishmar, who investigated the topic with his graduate students Tal Cohen and Liron Levin.