When we were one step away from extinction
Recently an international study has been published in Science. According to the study, for over 110 thousand years our species was at risk of extinction due to climate change.
Today, climate change is frightening due to the overheating of the Earth and the effects this can have on human living conditions. But, on the contrary, in the distant past it was a long glaciation that really put the presence of man on the planet at serious risk.
Today the world has exceeded 8 billion human beings but looking at the very distant period between approximately 930 thousand and 813 thousand years ago, the numbers are totally different.
Then, the extinction of our species was truly one step away, because, for 117 thousand years, the human population of reproductive age dropped to just 1,280 individuals of reproductive age.
More or less the number of pandas today.
The genetic bottleneck
The drastic reduction of the human population, equal to approximately 98.7% of the beings then present on the planet, was linked, according to the reconstruction of the scholars, to the glacial phase, the so-called "middle-Pleistocene transition", which, together with the changes in temperatures, also caused severe aridity in Africa and the extinction of other species of large mammals, in many cases used by humans as food.
Glaciation and the loss of genetic diversity
To deepen knowledge on the size of the Pleistocene population, the group developed a methodology based on human genetic variability, allowing its reconstruction backwards in time.
Starting from the complete genomic sequences of 3,154 current individuals from 50 different human populations and combining them with information on climate and fossils, it was thus possible to calculate a composite probability, from which was estimated that in a span of over 110 thousand years, the serious demographic crisis could cause the cancellation of approximately 65.85% of the genetic diversity existing at the time.
One step away from extinction
Over the millennia, there have been numerous fluctuations in the number of human beings on Earth. The scholars point out that "the history of population size is essential for studying human evolution."
In addition to major climate changes, the decline would also have coincided with subsequent speciation events. And the struggle for survival may thus have played a role in the formation of modern human chromosomes.
The bottleneck, the study continues, would be “congruent with a substantial chronological gap in the available African and Eurasian fossil record.”
In other words, the genetic results are confirmed by the lack of human fossils for around 300 thousand years, in almost perfect coincidence with the period of demographic collapse that emerged from the study.
The step towards Homo heidelbergensis
The recovery of paleoanthropological evidence began again around 650 thousand years ago, with Homo heildelbergensis.
“During a bottleneck, the normal ecological and genetic balances are upset, increasing the probability that unexpected genetic variants will be established, contributing to the emergence of a new species,” explains Giorgio Manzi.
The numerical reduction of man could therefore have been fundamental for the emergence of this new human species, defined by Fabio Di Vincenzo as "a true last common ancestor, that is, the human form that spread from Africa into Eurasia, giving rise to the evolution of three different species: Homo sapiens in Africa, the Neanderthals in Europe and the Denisovans in Asia”.
The publication "Genomic inference of a severe human bottleneck during the Early to Middle Pleistocene transition" is available at https://www.science.org/doi/10.1126/science.abq7487
