The modern man – Homo sapiens – left Africa around 60,000 years ago, first reaching the Near East and the Middle East. But how did it then reach Europe? A series of scientific articles recently published in the journal Nature and coming from an international team led in Great Britain provides information which allows us to begin to reconstruct our family tree of the last 45,000 years. This research has historical value but also informs us, as we will see, about disease risks.
It is the genome sequencing of 1,600 of our European ancestors dating from 11,000 to 2,000 years ago, compared to the genome sequencing data of more than 400,000 contemporary Britons, which enabled this feat, in s also based on available archaeological data.
Hunter-gatherers, farmers and herders
It appears that Western Europe was colonized by Homo sapiens in three successive waves. First by hunter-gatherers who came from Asia, around 45,000 years ago, and who gradually inhabited the entire European area. Then came a population of farmers from the Middle East – particularly Anatolia – at the end of the Stone Age (Mesolithic), around 11,000 years ago. This is the beginning of agriculture. This population, while mixing with the previous inhabitants, gradually supplanted them.
Finally, at the beginning of the Neolithic era, around 5,000 years ago, a pastoral population of breeders, originating from the North of the Black Sea and the Caspian Sea, arrived and established themselves particularly in the north of the Europe. In Denmark, this population replaced the previous inhabitants – the farmers – in a very short time, perhaps violently, perhaps due to their weakening due to epidemics.
Our northern European ancestors had a stronger immune response
Comparison with genomic data and the traits of our contemporaries indicates that this population of breeders, the latest arrivals, is partly at the origin of the current Scandinavian population with which it shares a larger size and a lighter skin tone. than that of Southern Europeans. The characteristics of a population may reflect a selection of people who have an advantage: better adaptation to the environment, particularly nutritionally, or better resistance to infectious agents.
In the context of the considerable evolution of the living environment in the Neolithic and then in the Bronze Age – densification of the population, cohabitation with livestock and their parasites – it is very likely that these populations were confronted to the emergence of infectious scourges such as tuberculosis or the plague. The genetic analysis of our ancestors from northern Europe highlights a phenomenon of positive selection of an immune nature during this period, that is to say an increase in the frequency of individuals carrying genetic traits related to immune responses. This suggests selection by resistance to epidemics.
Among these traits is an “antigen” allele of the HLA histocompatibility system (human leukocyte antigen) whose determining role in cellular immunity is known. However, it turns out that this same allele is known as a risk factor for the occurrence of multiple sclerosis, a disease much more common in the North than in the South of Europe. Its prevalence is around 140 cases per 100,000 people on average in Europe, but is twice as high in Northern Europe… And is much lower in Asia and Africa.
Multiple sclerosis is an autoimmune disease that affects the nervous system by attacking myelin, the substance that surrounds and protects the nerves. The concordance between the higher prevalence of the disease in northern Europe and the presence of the risk HLA allele indicates that the population of breeders who arrived 5,000 years ago brought with them this risk factor for multiple sclerosis. . What was undoubtedly a protective factor against infectious diseases – thanks to a more effective immune response – has become for Northern Europeans a factor of vulnerability to multiple sclerosis. A turnaround in the situation that is undoubtedly not exceptional, the result of contemporary living conditions sheltered from most devastating infectious diseases!
Alain Fischer is president of the Academy of Sciences and co-founder of the Institute of Genetic Diseases.
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