Throughout Earth’s history, climate change has pushed animal and plant species into extinction. About 250 million years ago, global warming triggered by massive volcanic eruptions wiped out 96 percent of all marine species during the Permian period, as Science magazine details. Now, some researchers think climate change did the same to ancient humans.
A research team led by Pasquale Raia of the University of Naples Federico II in Italy cross-referenced nearly 3,000 archaeological records of human species with temperature, rainfall and other weather data over the past 5 million years. Their findings, published in One Earth, suggest that global cooling episodes influenced human evolution, driving three of modern human’s cousins to extinction.
Although some scientists think the fossil record for ancient humans isn’t detailed enough to draw such conclusions, Raia thinks his team’s research offers people today a warning from human evolution’s past.
“It is worrisome to discover that our ancestors, which were no less impressive in terms of mental power as compared to any other species on Earth, could not resist climate change,” he said in a press statement published by EurekAlert.
Modern humans, called Homo sapiens, are part of the genus Homo, which has existed for at least 2.8 million years. Several different species of Homo lived on Earth during that time, and the archaeological record shows that some had the smarts to control fire, establish social networks, make stone tools and create clothing. Although these signs reveal clues to technological and cognitive skills, only H. sapiens survived.
Why? So far, no one knows. The researchers note in One Earth that “no consistent explanation has yet been advanced, despite the enormous importance of the matter.”
Raia, an evolutionary biologist, teamed up with more than a dozen other scientists to investigate. They tapped a fossil database that contained 2,754 archaeological records to map where and when six different Homo species lived over time, as Sapiens reports. The researchers also used a statistical modeling technique called a past climate emulator to reconstruct ancient climate conditions at the locations where the six species lived, going back 5 million years.
Next, they analyzed the environmental conditions at particular timepoints and found that the climate changed suddenly for three species — H. erectus, H. heidelbergensis and H. neanderthalensis — just before their last known appearance in the fossil record, according to The Scientist. Specifically, it became colder for all three, wetter for H. erectus and drier for both H. heildelbergensis and H. neanderthalensis.
The researchers used techniques from the field of conservation biology to assess whether these species may have been vulnerable to climate change. For instance, H. erectus, which lived approximately 110,000 years ago on what is now the Indonesian island of Java, lived in warm, humid conditions. But according to the climate model, a glacial period may have created temperatures too cold for the species to adapt.
Their analysis shows that climate change claimed more than half of the H. erectus and H. heidelbergensis niche prior to their disappearance — along with a quarter of the H. neanderthalensis niche. Changing habitats and increasing cold likely limited food sources and threatened survival for those more accustomed to warmer climes.
The Domino Effect of Environmental Change
Debates rage over how and why ancient humans disappeared. Several researchers not involved in this study have pointed out that the fossil record for human evolution is sparse and less reliable the further back in time it goes. Others say the last appearance of fossilized remains at a given location may not mean the species went extinct at that time — it may simply mean that fossils from later dates have not yet been discovered, as The Scientist reports. Further analysis and studies of animals and plants from these same locations and timepoints may lend credence — or not — to Raia’s hypothesis.
In the EurekAlert press statement, Raia says he agrees with the comments from his colleagues but that his team’s main findings “hold true under all assumptions” and serve as a warning as the planet faces unprecedented climate changes. “We were surprised by the regularity of the effect of climate change,” Raia says. “It was crystal clear, for the extinct species and for them only, that climatic conditions were just too extreme just before extinction and only in that particular moment.”
Does this mean that modern humans could become extinct? According to the Smithsonian National Museum of Natural History, human evolution occurred during one of the most dramatic eras of climate instability in Earth’s history. Ancient humans faced many extreme challenges, including disease, injury and predation, and those species that adapted survived.
But for modern humans, time may be running out. The United Nations says the planet has fewer than 10 years to prevent irreversible damage. And even if humans can draw upon technological know-how to stay alive, the plants and animals they depend on for food could succumb to higher temperatures, more acidic oceans and other environmentals shifts, creating a domino effect that could make life on Earth increasingly more precarious, as researchers detailed in Scientific Reports.
“I personally take this as a thunderous warning message. Climate change made Homo vulnerable and hapless in the past, and this may just be happening again,” Raia says.