930,000 years ago, the human lineage was on the verge of extinction. Our ancestors had begun to spread across the planet by walking upright and using simple stone tools. A few millennia ago there were about 100,000, which isn’t many today, but enough to keep going in a hostile world. However, something happened and the population collapsed to 1,200 individuals. 98% of the entire population is missing. Later, for 117,000 years, 1,170 centuries, this small population that would fit in a disco fended on the brink of extinction. Until 800,000 years ago the population gradually recovered and increased to 30,000 individuals.
The Paleolithic, in which this story takes place, was a time of change, with some animals replacing others in waves that swept the world from east to west with climatic shifts that could get our ancestors into trouble, to the point of almost ending with them ended. This demographic bottleneck increased evolutionary pressures on the few remaining humans and favored changes such as the merging of two chromosomes into one very similar to our genome. And it sparked the emergence of a new species, possibly the common ancestor of the Neanderthals and Denisovans, who are now extinct, and the Sapiens. This last human species outlived and perhaps contributed to the extinction of all others, and from these 1,200 survivors descended the more than 8,000 million humans of today.
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This would more or less replicate what happened in that distant time based on what an article published today in the journal Science suggests. In this work, which brought together researchers from China, Italy and the United States, the authors sought to shed light on the foggy stage at the end of the Lower Pleistocene, when humanity’s penultimate great cognitive leap took place. For reasons still unknown, almost no fossils have been found from this period, just over 900,000 years ago, with a few exceptions such as the skull pieces found in Gombore (Ethiopia) or the remains of Homo antecessor from Atapuerca in Spain.
a common ancestor
In the absence of bones, scientists used a technique called FitCoal, which makes it possible to draw conclusions about what happened to the population from which an individual descended by examining their genome. The researchers used the genome sequences of 3,154 people from around the world and with their analysis discovered the glaring bottleneck that led to the global population of our ancestors only having 1,280 individuals capable of reproduction. Because there were so few alternatives, inbreeding multiplied, and the effects of this loss of diversity are still felt today. However, the authors suspect that this moment of tribulation may have led to the emergence of a new species, perhaps Homo heidelbergensis, the common ancestor of Neanderthals, Denisovans, and Sapiens.
As almost always when attempting to travel back into such distant time, speculation covers part of the vast desert where millions of years are spoken of as if a millennium were not an eternity. As an explanation for the catastrophe, the signers of the Science article point to major climate changes that prolonged the ice ages and led to major droughts in large parts of the world. To justify the boom that began about 813,000 years ago when the population multiplied twentyfold, fire control is used, evidence of which was found in Israel as early as 790,000 years ago, or a less hostile climate with more game or vegetables to keep the to satisfy hunger.
Antonio Rosas, director of the paleoanthropology group at the National Museum of Natural Sciences in Madrid, acknowledges that the narrative emerging from the study published today appears “beautiful” but also believes it is “an ad hoc coincidence between the dates that…” They get and other paleontological data.” he. Furthermore, Rosas believes that maintaining such a small population for so many thousands of years is “hardly credible” because “it is outside the usual population dynamics.” “In such a long time, circumstances would have changed to allow recovery,” he says. “But in general it is true that the bottleneck coincides with a transitional phenomenon from the Lower to Middle Pleistocene, which is a phenomenon of faunal displacement at a planetary scale, particularly in Eurasia.” It is not a one-off phenomenon and the phenomenon that they watch happens there,” he concludes.
The results, published in Science, not only demonstrate a new technique for extracting information from the deep past, but also provide another building block for continuing the reconstruction of human evolutionary history. The hypotheses raised by the existence of this ongoing bottleneck will require the discovery of new fossils to complete this information, and although this is now impossible, there is always hope of recovering DNA from such ancient remains.
Antonio Salas, a specialist in population genetics at the University of Santiago de Compostela, emphasizes the interest of this work, but also reminds us of the limitations of these approaches. “It could be speculated that speciation phenomena may have occurred during the bottleneck period, leading to the emergence of the bird [último ancestro común] We shared them with the Denisovans, the Neanderthals, with whom we diverged between 765,000 and 550,000 years ago, but many of these phenomena have been irrevocably lost,” he warns. “Many methods for detecting positive natural selection rely on the resistance that exists between the selected genetic variant and those that have a dependent relationship with it, and the signals of dependence between genetic variants can be lost in 10,000 years,” adds he added. Through hundreds of thousands of years of evolution and the multitude of dangers humanity has faced, many chapters of our history will have been lost forever.
In addition to the explanations, the results also leave questions unanswered. Salas wonders “where this ancestral population lived,” whether “they were small groups of interconnected hunter-gatherers,” or had a different way of life, or “what really got into this bottleneck.” And also: “How much of what happened in the human lineage has been lost forever, in large part because of these demographic events.” The authors of the paper estimate that the bottleneck also increases the level of endogamy of our ancestors and such may have contributed to the loss of 65% of human genetic diversity. Later, other defining moments of humanity, such as the last departure from Africa 70,000 years ago, led to new bottlenecks and a further reduction in this diversity, eventually giving rise to a species in which we are all close relatives, descendants of a handful of pairs that have them miraculously survived.
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