Record-breaking simulation suggests how the climate shaped human migration

The early human species Homo heidelbergensis (skull shown) could have been able to spread across the Earth because wetter, more migration-friendly weather conditions emerged, according to a climate model.Credit: Javier Trueba / MSF / Science Photo Library

A colossal simulation of the last two million years of the Earth’s climate provides evidence that temperature and other planetary conditions influenced early human migration – and possibly contributed to the emergence of the modern human species about 300,000 years ago.

The finding is one of many that have emerged from the largest model to date to examine how changes in Earth’s movements have affected the climate and human evolution, published in Nature1 Today. “This is another brick in the wall to support the role of climate in shaping human ancestry,” said Peter de Menocal, director of the Woods Hole Oceanographic Institution in Falmouth, Massachusetts.

The idea that climate can play a significant role in human evolution has existed since at least the 1920s2, as scientists began to discuss whether drier conditions had caused the early human ancestors to begin walking on two feet to adapt to life on the savannah. But so far, scientists have struggled to provide strong evidence that the climate played a role in shaping humanity.

Orbital influence

In the latest study, Axel Timmermann, a climate physicist at Pusan ‚Äč‚ÄčNational University in South Korea, and his colleagues ran a climate model on a supercomputer for six months to reconstruct how temperature and precipitation could have shaped what resources were available to humans in currently. few million years. Specifically, the researchers investigated how long-term fluctuations in the climate caused by the Earth’s astronomical movement could have created the conditions to spur human evolution.

Push and pull from other planets changes the Earth’s climate by changing both the planet’s inclination and the shape of its orbit. Over 41,000-year cycles, the Earth’s slope fluctuates, affecting the intensity of the season and changing the amount of rain that falls over the tropics. And over 100,000-year cycles, the Earth goes from having a more circular orbit – which brings more sunlight and longer summers – to having a more elliptical orbit, which reduces sunlight and can lead to periods of glacier formation.

Timmermann and his colleagues used a simulation that incorporated these astronomical changes, then combined their results with thousands of fossils and other archaeological evidence to find out where and when six species of humans – including the early Standing man and the modern Homo sapiens could have lived.

Movements and mixing

The study pumped out a staggering amount of data, and Timmermann says several interesting patterns emerged. For example, the researchers’ analysis showed that an early human species, Homo heidelbergensis, began to expand its habitat about 700,000 years ago. Some scientists have thought that this species could have given rise to a number of others around the globe, including Neanderthals (Homo neanderthalensis) in Eurasia and H. sapiens somewhere in Africa.

The model suggests that the distribution of H. heidelbergensis across the globe was possible because a more elliptical orbit created wetter climatic conditions that allowed the species to migrate wider. The simulation also showed that the most habitable regions in terms of climate changed over time, and the fossil record followed them.

“The global collection of skulls and tools is not randomly distributed in time,” says Timmermann. “It follows a pattern” that overlaps with climate change driven by the Earth’s movement. “This is amazing to me – here’s a pattern that no one has been able to see so far.”

Part of this pattern can provide fresh insight into where and how our own species originated. Some genetic studies of modern hunter-gatherer groups in sub-Saharan Africa – which tend to be genetically isolated – suggest that H. sapiens is the result of a single evolutionary event in southern Africa. But other studies point to a more complex history in which humanity began as a hotchpotch of many different groups of ancient Africans who together evolved into the people of today.

Timmermann and his colleagues say that their climate structure favors the hypothesis of a single evolutionary path. The model suggests that our species evolved when H. heidelbergensis in southern Africa began to lose viable habitats during an unusually hot period. This population could have evolved into H. sapiens by adapting to the warmer, drier conditions.

But this finding is unlikely to end the debate. “Arguing that a particular climate event led to a species-forming event is really difficult,” in part because of gaps in fossil and genetic records, says Tyler Faith, a paleobiologist at the University of Utah in Salt Lake City.

The same goes for many of the other patterns reported in the newspaper. “The people who have spent a career studying this will either violently agree or disagree with the proposals here,” de Menocal said. However, the model is a “phenomenal achievement in itself” and “gives you a template to ask these questions”.

Most researchers who have spoken with Nature say that more evidence will be needed to prove that astronomical cycles affected the trajectory of human origin. “If the solution to the mystery of climate change and human development could be addressed in a paper, it would have been done 40 years ago,” Faith says.

Therefore, Timmermann and his colleagues plan to run even larger models, including those that integrate genetic data.

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