Science: Explaining human evolution through climate change

Fossil of a leg of Homo heidelbergensis on display at the Museum of Human Evolution on November 22, 2013 in Burgos, Spain. (©AFP/Archives/César Manso)

What if human evolution was merely a matter of celestial mechanics? For two million years, major climate changes linked to variations in Earth’s orbit have guided the migration of the first humans, according to a study published in Nature,

erectus, heidelbergensis, Neanderthals, sapiens … These different lineages of the genus Homo, of which only the last remains, have traveled through Africa and Eurasia over hundreds of thousands of years, crossing paths one after another, Sometimes mix. But paleontologists are struggling to reconstruct a spatio-temporal map of these ancient settlements due to the paucity of human fossils.

digging into the climate past

One way to overcome this: delve into the climate past. Because by modifying terrestrial ecosystems, climate has essentially affected displacement of the population. But even there, geological data describing environmental variations (polar caps, lakes, oceanographic or cave sediments, etc.) are sparse.

a study published in Nature could help to 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 spread around the world.

Everything depends on Earth’s orbit around the Sun, according to climatologist Axel Timmerman of Busan University in South Korea, lead author of the study published on Wednesday. This movement describes an ellipse, whose shape changes every 100,000 to 400,000 years. And every 20,000 years or so, the Earth’s axis undergoes oscillations relative to its orbital plane.

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“a pendulum”

This long-term astronomical mechanism plays into the level of solar radiation that our planet receives, leading to ice ages such as the Pleistocene (between 2.6 million years and 10,000 years ago) and changing arid environmental conditions. And humid, like the episodes of “Green Sahara”.

Professor Timmermann likens this dynamic to “the pendulum that ultimately determines where to find food, and is therefore linked to the survival of a species, its adaptation to the environment and its migration”, in the study he explained.

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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.

A Homo heidelbergensis skull on display at the Museum of Human Evolution in Burgos, Spain in May 2016.
A Homo heidelbergensis skull on display at the Museum of Human Evolution in Burgos, Spain in May 2016. (©AFP/Archives/César Manso)

The researchers then developed a model calculating the probability that a species might have lived at a particular location on the planet over a period of 1,000 years spanning between 2 million years and 30,000 years ago.

The model takes us to the beginning of the Lower Pleistocene, a dry and cold period that succeeded 2.6 million years ago, the Pliocene, wet and warm. She sees African groups such as Homo habilis and Ergaster inhabited by “low climate variability, consistent with low variability of the Earth’s orbit”: types of habitat “niches” confined to the south and east of the continent.

“world wanderer”

This behavior changes at the end of the Pleistocene: vegetation changes, opening “corridors” towards North Africa, the Arabian Peninsula and Eurasia. Allowing Homo erectus and Homo sapiens to become these “global wanderers”, able to adapt to a wide range of climatic conditions. A resilience that could explain the existence of our species, according to the study.

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Climate models also suggest an important role played by Homo heidelbergensis, a human group identified in Germany in the early 20th century, which lived between 800,000 and 160,000 years ago. Climate disturbances that occurred in southern Africa between 300,000 and 400,000 years ago may have influenced the evolution of its population, which would have separated along the Eurasian line with Neanderthals, and another African from which the oldest sapiens would come.

The hypothesis should be debated among paleontologists, who are very divided over how to reconstruct the phylogenetic tree of human evolution.

“This study brings together an extraordinary amount of environmental data over a long period of time. The developed model will certainly have applications for understanding human movements,” commented AFP Antoine Balzo, paleontologist at the National Museum of Natural History, who did not participate in the work.

On the other hand, he is more skeptical about the interpretations of studies on species differentiation. Especially because many of them, like Denisovans, are excluded from the model.

Source: © 2022 AFP

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