The climate has always had a decisive influence on mankind. Two studies have now used a combination of climate data and fossil finds to reconstruct how climate change determined early human migration. Accordingly, an extreme cooling 1.1 million years ago ended the first colonization of Europe by Homo erectus. Using climate models from the last 400,000 years, the second study shows that the initially separate distribution areas of Neanderthals and Denisovans began to overlap due to warm phases - and thus crossbreeding between the two early human species became possible.
For decades, research into our early human ancestors has focused on the fossil record of skeletal remains and artifacts. More recent research approaches also include information about environmental conditions and can thus reconstruct the conditions under which the early hominins lived. Findings about past climate changes enable conclusions to be drawn about the regions in which the combination of temperatures and precipitation were probably suitable for early human life and where the conditions led to early human settlers migrating or even becoming extinct. Two studies now provide new insights into the history of hominins.
Climate data from deep-sea boreholes
A team led by Vasiliki Margari from University College London has reconstructed the temperature in southern Europe from 800,000 to 1.8 million years ago using a deep-sea sediment core taken off the coast of Portugal. "The oldest known hominin remains in Europe were found on the Iberian Peninsula," explain Margari and her team. "They date from 1.5 to 1.1 million years ago and it was previously assumed that the hominin populations continued to exist once they became established." However, for the period between 1.1 million years and 900,000 years fossils and artefacts. The new research results now provide an explanation for this for the first time.
"Based on our analyzes of the deep-sea sediment core, we found that the climate during this period was not as mild as previously assumed, but that there were significant fluctuations," reports the team. "Around 1.145 to 1.123 million years ago there was a cold period comparable to the most extreme events of the most recent ice ages." By analyzing the remains of pollen and tiny algae in the sediment core, Margari and her team were able to draw conclusions about the environmental conditions at that time. "Rivers and winds carry tiny pollen from the adjacent land into the ocean, where it sinks and is deposited in the deep sea," explains Margari. "According to our analysis of the pollen from the sediment cores of the ocean, the cooling of the North Atlantic transformed the western European vegetation into an inhospitable semi-desert landscape." The surface temperature of the sea off Lisbon fell to six degrees Celsius during this period.
Ice Age eliminated early European settlers
In order to estimate the effects of the cooling on the then population of Homo erectus on the Iberian Peninsula, the researchers fed the new data into a climate simulation and compared it with archaeological and fossil findings. "The results show that the climate around the Mediterranean Sea became too hostile for archaic humans 1.1 million years ago," says co-author Axel Timmermann from the Center for Climate Physics in Busan in South Korea. Because early hunter-gatherer groups likely had limited ability to effectively protect themselves from the cold through fire, clothing, and shelter, the research team says the most likely scenario is that early human settlers in Europe died out during the Ice Age.
"According to this scenario, Europe could have been resettled around 900,000 years ago by more resilient people from Asia who, through evolutionary or behavioral changes, were able to survive under the increasingly intense glacial conditions," explains co-author Chris Stringer of the Natural History Museum in London. This interpretation agrees well with the previous fossil finds. "More fossil finds with the most accurate dating possible are needed to further support this hypothesis," writes Emily Beverly of the University of Houston in Texas in an accompanying commentary, also published in the journal Science.
Climate made early human interbreeding possible
The climate also had a formative influence in the further course of human history. This is shown by a study by a team led by Timmermann's colleague Jiaoyang Ruan. The research team modeled the climatic conditions of the last 400,000 years in Eurasia and combined the results with assumptions about the preferred environments of the early human species Neanderthal and Denisova. Thus, the Denisovans were adapted to cold environments characterized by boreal forests and tundra, while the Neanderthals preferred temperate forests and grasslands.
"This means that their preferred habitats were geographically separated, with the Neanderthals typically preferring southwest Eurasia and the Denisovans preferring the northeast," explains Ruan. However, climate changes ensured that the climate in northeastern Eurasia also became milder, so that the Neanderthals were able to expand their range until it finally overlapped with that of the Denisovans. Indeed, interbreeding between the two early human species did occur, as evidenced by fossils of common offspring.
"Both studies have the advantage that the models used can be rerun as more fossils and climate records are added," Beverly writes. "Furthermore, the rapid advances in neural networks and machine learning over the past five years are likely to provide new methods to answer unanswered questions about hominin evolution and migration."
Sources: Vasiliki Margari (University College London) et al., Science, doi: 10.1126/science.adf4445;
Jiaoyang Ruan (Center for Climate Physics, Busan, South Korea) et al., Science, doi: 10.1126/science.add4459