From tropical Africa to icy Siberia: how Homo sapiens adapted to colder climates

If Homo sapiens evolved in Africa, in regions with a predominantly tropical climate, how can Yakut people live in Yakutia, an area of Siberia where low temperatures can reach -70°C? This question was the starting point for a University of Bologna study investigating the genetic basis of the biological adaptations developed by populations living in subarctic regions in response to environmental stresses.

The study – published in Communications Biology – shows that, in the genomes of Yakut individuals, it is possible to observe combinations of genetic variants that help to increase their heat production and their cells’ response to insulin. The Yakuts have developed a distinctive metabolic profile that enables them to meet the high energy demands of survival in an exceptionally harsh climate.

"Human populations have been able to adapt biologically and culturally to the most diverse environments, characterised by stresses that are very different from those experienced by ancestral Homo sapiens populations", explains Marco Sazzini, professor of Evolutionary Anthropology at the Department of Biological, Geological and Environmental Sciences - BiGeA of the University of Bologna, who coordinated the study. "We wanted to investigate a striking example of this extraordinary ability to adapt: the human populations that colonised the regions of Eurasia close to the Arctic Circle, whose climate is characterised by long and extremely harsh winters".

Yakutia - officially the Republic of Sakha, in Russia - is located in north-eastern Siberia and is one of the most remote and inhospitable places ever settled by humans. The earliest archaeological evidence of Homo sapiens in the region dates back around 45,000 years.

For their investigation into the genome of Yakut people, the scholars did not focus on the search for individual genetic variants capable of altering, on their own, the expression of a particular biological trait, but instead investigated the evolution of complex adaptive traits.

"These biological traits are modulated by the synergistic action of a very large number of genetic variants across multiple genes, each of which individually has a negligible impact but proves important when combined with the effect of the others", says Giulia Ferraretti, the study’s first author. "Adaptations made possible by a polygenic mechanism of this kind are evolved by populations over much faster timescales than those mediated by individual mutations, and it is therefore hypothesised that they may have played a key role during the process by which our species dispersed across the planet".

Using different analysis methods, including machine-learning algorithms, the researchers were able to identify certain combinations of genetic variants peculiar to the Yakut population (in addition to others shared with Russian populations), capable of positively regulating physiological functions associated with exposure to cold.

The results show that these polygenic mechanisms have modified, for example, the metabolic pathways activated by thyroid hormone and insulin signalling, the differentiation of brown adipose tissue, and the use of lipid metabolism products in thermogenesis processes: all of these functions are linked to the regulation of energy metabolism and heat production.

And in all this, Neanderthals also play a part. Some of the genetic variants found among the Yakut people are not observed in the vast majority of modern human populations, but are present in the genomes of the Neanderthal individuals sequenced to date.

"These results tell us that the introgression of Neanderthal genetic material into the gene pool of the ancestors of present-day Eurasian populations favoured, and presumably accelerated, the process of biological adaptation to the cold of those groups of Homo sapiens that progressively pushed to ever higher latitudes", explains Sazzini. "Genetic mixing with Neanderthals may have been a crucial factor in the successful spread of a 'tropical' species like ours across the Eurasian continent".

During their evolution, Neanderthals lived in geographical areas characterised by a marked alternation between glacial and interglacial phases. Natural selection had presumably already been acting for several hundred thousand years, enabling them to develop genetic adaptations to particularly harsh climates. The encounter between Neanderthals and Homo sapiens enabled our species to take advantage of these beneficial combinations of genetic variants during the expansion into the territories of northern Eurasia.

The study was published in the journal Communications Biology under the title “Adaptive evolution at thyroid hormone, insulin and glycerolipid pathways improved energy metabolism in high-latitude Eurasian populations”. The research was coordinated by Marco Sazzini, a professor at the Department of Biological, Geological and Environmental Sciences - BiGeA at the University of Bologna and affiliated with the Alma Climate Interdepartmental Centre. Research fellow Giulia Ferraretti was the paper’s first author, and contributions also came from Stefania Sarno, Marta Alberti, Rosita Cicolini and Sabrina Pognant Viù.