Published on : 15/04/2022 – 08:56
Paris (AFP) – What if human evolution was only a matter of celestial mechanics? For two million years, major climate changes linked to variations in the Earth’s orbit have guided the migrations of the first humans, according to a study published in Nature.
Erectus, Heidelbergensis, Neanderthal, Sapiens… These different lineages of the genus Homo, of which only the last has survived, have traveled across Africa and Eurasia over hundreds of thousands of years, succeeding each other, crossing each other, sometimes mingling. But paleontologists are struggling to reconstruct the spatio-temporal map of these ancient settlements, for lack of human fossils.
A solution to overcome this: delve into the climatic past. Because by modifying terrestrial ecosystems, the climate has necessarily influenced population movements. But there too, the geological data describing the environmental variations (polar cap, lake, oceanographic or cave sediments, etc.) are very sparse.
A study published in Nature could help complete the puzzle, by showing how, over a very long period of two million years, climate change has affected the distribution of human species and their dispersal around the world.
Everything depends on the Earth’s orbit around the Sun, according to the main author of this study published on Wednesday, climatologist Axel Timmermann, from the University of Busan in South Korea. This movement describes an ellipse, the shape of which varies every 100,000 to 400,000 years. And approximately every 20,000 years, the axis of the Earth relative to its orbital plane undergoes oscillations.
“A Pendulum”
This long-term celestial mechanics plays on the level of solar radiation that our planet receives, causing ice ages like the Pleistocene (between 2.6 million years and 10,000 years ago) and alternating dry and wet, like the episodes of “Green Sahara”.
Professor Timmermann compares this dynamic to that of a “pendulum which ultimately determines where to find food, and is therefore linked to the survival of a species, its adaptation to an environment, and its migration”, he explains. he in the study.
His team relied on more than 3,000 fossil and archaeological data, combined with climate models. A supercomputer then simulated how the climate reacts to the astronomical clock.
The researchers then developed a model calculating the probability that a species could have inhabited a particular place on the planet, over periods of 1,000 years spanning between 2 million years and 30,000 years ago.
The model transports us to the beginning of the lower Pleistocene, a dry and cold period which succeeded, 2.6 million years ago, to that of the Pliocene, wetter and warmer. She sees African groups such as Homo habilis and ergaster settled in environments with “low climatic variability, corresponding to low variability of the Earth’s orbit”: sorts of “niches” of habitat confined to the south and east of the continent.
“World Wanderers”
This behavior changes towards the end of the Pleistocene: the vegetation changes, opening “corridors” towards northern Africa, the Arabian peninsula and Eurasia. Allowing Homo erectus and Homo Sapiens to become these “global wanderers”, able to adapt to a wider range of climatic conditions. A flexibility that could explain the survival of our species, according to the study.
The climate model also suggests a pivotal role played by Homo heidelbergensis, a human group identified in Germany at the beginning of the 20th century, which would have lived between 800,000 and 160,000 years ago. The climatic disturbances that occurred in southern Africa 300,000 to 400,000 years ago would have influenced the evolution of its population, which would have separated into a Eurasian line with Neanderthal, and another African from which the oldest Sapiens would come.
The hypothesis should be debated among paleontologists, who are very divided on how to reconstruct the phylogenetic tree of human evolution.
“This study brings together an exceptional amount of environmental data over a long period of time. The model developed will certainly have applications for understanding human movements”, commented to AFP Antoine Balzeau, paleoanthropologist at the National Museum of Natural History, who n did not participate in the work.
On the other hand, he is more skeptical about the interpretations of the study on the differentiation of species. In particular because several of them, like the Denisovans, are excluded from the model.
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