We Homo Sapiens are just one of six different human species that have populated the world through history. And while evidence abounds regarding what prompted the demise of our closest relative, Neanderthals, what caused the downfall of the other four species remains largely a mystery.
New data suggest these ancient humans may have met a very modern end: climate change.
In a new study published Thursday in the journal One Earth, scientists present a new analysis of the fossil record using climate modeling. They propose a new theory to explain these hominids' extinction: that the disintegration of ancient humans' climatic niche was a huge driver in their subsequent eradication from Earth.
"They tried hard; they made for the warmest places in reach as the climate got cold, but at the end of the day, that wasn't enough."
Previous studies of ancient human extinction have largely focused on Neanderthals' competition with early Homo sapiens, but there is relatively little research into what may have spurred the downfall of other, more ancient hominids, including H. erectus and H. heidelbergensis.
"[T]he fate of our ancestors is made even more important by the current, ever-increasing pressure that rapid and extreme climate change will continue to put on our own species," the scientists write in the study.
Climate change is very much a modern problem, but in the ancient Pliocene and Pleistocene — the periods when these human species roamed — Earth's climate also experienced dramatic temperature swings. Notably, the last Ice Age falls right between these two eras.
By comparing both the fossil record of these ancient human species to climate models of the time, the researchers found clear evidence that extreme changes in climate were linked to these ancient humans' demise, Pasquale Raia, the study's first author and associate professor at University of Naples, said in a statement.
"[D]espite technological innovations including the use of fire and refined stone tools, the formation of complex social networks... past Homo species could not survive intense climate change," Raia said. "They tried hard; they made for the warmest places in reach as the climate got cold, but at the end of the day, that wasn't enough."
The fossil record — To understand what the world may have looked like for these ancient humans, Raia and his colleagues used a high-resolution climate emulator, capable of calculating the rainfall, temperature, and other data from the past five million years. They combined this massive data set with 2,750 archaeological records. By pairing these two different streams of data, the researchers were able to reconstruct what a "typical" environment looked like for these ancient humans, as well as the dramatic climatic changes that would have been cause for alarm.
Homo erectus, for example, was at home in tropical areas of South East Asia, so the group was well-adapted to warmer climates. This species' extinction took place during the last glacial period, which would have created a much colder climate than that in which they evolved to thrive in.
"Climate change made Homo vulnerable and hapless in the past, and this may just be happening again."
By tracing the overlap between when a specific species went extinct and dramatic climate events, the researchers saw a clear pattern: extreme changes in climate were closely linked to subsequent extinction.
"We were surprised by the regularity of the effect of climate change," Raia said. "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."
The researchers also discovered that Neanderthals were not immune to such effects on climate. Rather, they suggest that extreme climate, as well as competition with Homo sapiens, may have led to their extinction, too.
An ancient warning — While the environments of our ancient relatives were different to our environment now, the researchers note this climate history serves as a stark warning for our own future.
"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," Raia said. "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."
Abstract: At least six different Homo species populated the World during the latest Pliocene to the Pleistocene. The extinction of all but one of them is currently shrouded in mystery, and no consistent explanation has yet been advanced, despite the enormous importance of the matter. Here, we use a recently implemented past climate emulator and an extensive fossil database spanning 2,754 archaeological records to model climatic niche evolution in Homo. We find statistically robust evidence that the three Homo species representing terminating, independent lineages, H. erectus, H. heidelbergensis, and H. neanderthalensis, lost a significant portion of their climatic niche space just before extinction, with no corresponding reduction in physical range. This reduction coincides with increased vulnerability to climate change. In the case of Neanderthals, the increased extinction risk was probably exacerbated by competition with H. sapiens. This study suggests that climate change was the primary factor in the extinction of Homo species.