When our genes speak, they tell a story to say the least… eventful! This is how a team of researchers discovered that our very distant ancestors would have come close to extinction 900,000 years ago.
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[EN VIDÉO] The evolution of the population for 200,000 yearsToday, we are eight billion human beings on Earth: how did we get here…
We can say that we were hot! Because it seems that with some 1,300 individuals, the human line could well have died out. At least that’s what a new study reports. published in Science.
It all started around 930,000 years ago, when the Earth was undergoing a major climate change. We are then in a period commonly called the “Middle Pleistocene Transition”, which is characterized by a fundamental change in the duration of glacial cycles. Whereas the periodicity of the glaciations was previously 41,000 years, this one lengthens considerably to reach 100,000 years. The contrasts between glacial and interglacial periods are then much more marked. This evolution of the climate, which did not happen overnight, it must be specified, did it impact the lives of our distant ancestors?
A demographic drop of more than 98%!
Possible. In any case, this period seems to have been particularly dark for the human line. To the point that we would have even come close to extinction. This drama of the dawn of humanity was highlighted by a team of Chinese researchers who carried out a genomic analysis on 3,154 individuals around the world. Their results suggest that the human population fell sharply and dramatically around 900,000 years ago. From 100,000 individuals, the population would indeed have decreased to reach a critical value of around 1,280 individuals of reproductive age. That is a demographic loss of… 98.7%! Moreover, this situation would not have been transient and could have lasted for 117,000 years. The time to leave an indelible mark in our genome.
A loss of genetic diversity that is still seen in our genome
When a species undergoes a significant demographic reduction (we speak of a bottleneck), the genetic diversity decreases indeed inevitably and brutally. A change that is therefore inscribed in the genes of the descendants. You should know that this is however not the only bottleneck that humanity has known during its history, even if it was rather severe. Famine, wars, climatic crises…, many hazards have caused the human population to falter over time. The study of our genome has thus revealed that a significant demographic reduction occurred 7,000 years ago in the northern hemisphere. But going back 900,000 years in time was still a real scientific feat, the signal of these bottlenecks of course fading over time. The researchers have thus developed a new numerical method dedicated to the processing of this type of data and making it possible to observe the evolution and divergences of genetic lineages over time.
The loss of genetic diversity between 930,000 and 813,000 years ago thus appears to be more than 65%. According to our own current criteria, we would therefore have been classified as a critically endangered species!
What does not kill you makes you stronger
However, this event would have played a major role in the development of humans, in particular by accelerating the evolution of the brain. This bottleneck would also have helped shape our genome by causing the fusion of two chromosomes to form the hominin-specific chromosome 2. Indeed, while all species of hominids alive today (the great apes) have 24 pairs of chromosomes, we humans only have 23! It is therefore perhaps by being on the threshold of extinction that our distant ancestors would have acquired this specificity which makes us men.
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