2023-09-01 16:22:34
by TELMO PIEVANI
Research based on genetic evidence. It is also signed by two Italian scholars. There was a population collapse of 98.7%. An unthinkable catastrophe befell our ancestors
Humanity was in serious danger of extinction, with a dramatic demographic collapse that reached 98.7%. An almost unimaginable catastrophe befell our ancestors between 930 and 813 thousand years ago, leaving only 1,280 fertile individuals alive, fewer than the pandas currently existing in the wild. It was a slow tragedy, lasting 117 millennia. The culprit, as often in these cases, was climate change: the alternations between glacial and interglacial cycles began in that period to widen up to intervals of 100,000 years and became increasingly extreme, leading to a wave of extinctions of large mammals in Eurasia and long periods of severe dryness in Africa. And our ancestors also had a bad time.
The surprising discovery of this drastic bottleneck in human evolution, never even hypothesized so far, was published today in Science by a group of Chinese scientists who collaborated with two Italian paleo-anthropologists, Giorgio Manzi of the Sapienza University of Rome and Fabio Di Vincenzo of the Museum of Natural History of the University of Florence. The research is also an example of how science can contribute to dialogue between peoples, given that the Chinese scientists involved come from both sides of the Taiwan canal.
For such sensational news, robust evidence is needed, which is not lacking. Starting from international gene banks, the complete genomes of 3,154 modern individuals belonging to 50 different human populations were analysed. With an innovative bioinformatics method, going back in our genealogical tree up to times that long precede the appearance of our species, the now very weak genetic traces left by our ancestors have been identified and from these it has been possible to calculate the demographic consistency of the populations of the past. In practice, without the need to extract ancient DNA from fossils, current human genetic variation is projected back in time to estimate the size of populations at specific times in the past. It is thus possible to discover ancient migrations, expansions and reductions of populations. Well, even comparing independent data sets, the substance does not change: around 900 millennia ago there was a generalized catastrophic collapse, which also fits very well with the fossil evidence and explains an old mystery of human evolution.
When the Homo genus appeared in Africa, between 2 and 2.5 million years ago, it left many archaeological and fossil traces for a long time. They are the signs of an archaic humanity called Homo ergaster, great walkers, probably the first to leave Africa and give rise to Homo erectus in Asia. It was a promising but vulnerable humanity. Then, just starting from 950 thousand years ago, an apparent silence falls: few remains that can be dated with certainty, as if almost all of them had disappeared. Even Europe, starting from 1.1 million years ago, seems to be completely depopulated of human beings due to a particularly cold period. We will have to wait 300 millennia to find the abundant fossil traces of a new humanity, with a larger brain and unprecedented anatomical characteristics, called Homo heidelbergensis. How do you explain such a hole in the documentation?
When such gaps or sudden accelerations are seen in evolution, the blame is usually placed on the fragmentary nature of the fossil record: those upheavals didn’t really happen, just a problem of lack of data and uncertain dating. It seems that in this case it is not so. The fossils perhaps told the truth. There really was almost extinction and at the exit of the bottleneck we find a new humanity. This is not surprising because, as paleontologists Stephen J. Gould and Niles Eldredge have taught, when a population is drastically reduced in evolution due to environmental causes, genetic changes tend to accumulate rapidly which can lead to the punctuated birth of new species. Bottlenecks then reduce genetic variability, which remains very low even among modern humans. Furthermore, in the same period we know that in the genus Homo two ancestral chromosomes fused together, generating chromosome 2 and bringing the count to 46.
Although some paleoanthropologists such as Chris Stringer still have doubts about the true extent of this bottleneck and the central role of Homo heidelbergensis, it is likely that there was a major transition in human evolution at that time. A transition that also affects us. In fact, starting with the survivors of the 900,000-year catastrophe, the engine of evolution returned to full speed. Groups of Homo heidelbergensis grew rapidly and spread from Africa throughout Eurasia, eventually giving rise to Neanderthals in Europe and Denisovans in Asia. Hand in hand with their arrival, the oldest evidence of the systematic and controlled use of fire and more advanced lithic technologies appear. In short, the world returned to be populated by well-organized humans.
Some time later, between 200 and 300 millennia ago, the Homo sapiens species was also born from the African descendants of the same Homo heidelbergensis, which, leaving its continent of origin, met its Neanderthal and Denisovan cousins, interbreeding with them. So we are literally the children of the few thousand survivors who managed to pass through that very narrow needle’s eye, to resist in a few shelters the inclement environmental conditions that lasted for tens of millennia.
We owe our existence to a climate catastrophe, which is doubly impressive if we think that now the climate is changing much faster than then and, this time, because of us. Of course, today we are much better equipped than 900,000 years ago, but this unexpected discovery teaches us how dramatic the cost of a climate crisis can be. The planet is much stronger than us.
September 1, 2023 (change September 1, 2023 | 16:22)
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