The branch on the tree of life that is the genus Homo is a gnarled, twisty one with many offshoots and dead ends. Of at least six species who have walked the Earth within the past few million years, all but one have gone extinct, for reasons that are largely a mystery.
Now, an extensive modeling study based on paleoclimatic data and nearly 3,000 archaeological records suggests that global cooling episodes were a major driver of the extinction of the Neanderthals as well as the long-gone Homo species H. erectus and H. heidelbergensis. To the authors of the research—published today (October 15) in One Earth—the findings are an ominous warning from the past with regard to our own future in a rapidly warming climate. But some other experts note that the fossil record isn’t reliable enough to draw such conclusions about past hominin extinctions with certainty.
“[The authors] did the best job I think they could possibly do with the data there are available,” remarks anthropologist Jessica Thompson of Yale University who wasn’t involved in the new study. “This is an interesting contribution, mostly to the Neanderthal debate [on how they went extinct] and a [valuable] set of hypotheses that people working earlier in time can look to if they’re trying to frame the work that they’re doing.”
A few years ago, evolutionary biologist Pasquale Raia of the University of Naples Federico II in Italy discovered a database of paleoclimatic data and decided to team up with archaeologists and paleoclimate modelers to investigate how different Homo species responded to the continuous oscillations of Earth’s climate between cold glacial periods and warmer stretches of time. To find out, they used a paleoclimatic emulator that models temperature, rainfall, and net primary productivity for the past 5 million years.
We know that climate change [can] be that bad to species—even to those which were, cognitively speaking, the biggest fish in the tank.—Pasquale Raia, University of Naples Federico II
They aligned those climate data with the locations and estimated ages of fossils for six Homo species—habilis, ergaster, erectus, heidelbergensis, neanderthalensis, and sapiens—to characterize the range of climatic conditions each species was able to occupy throughout its existence. The team compared that to the environmental conditions the species were experiencing at particular timepoints. Those comparisons suggested that for three species—neanderthalensis, erectus, and heidelbergensis—their climatic niches appeared to suddenly contract just before their last known appearance in the fossil record.
Further analyses using techniques typically employed by conservation biologists to assess how sensitive species are to current global warming suggested that those three Homo species were indeed highly vulnerable to changes in climate. That affirmed the idea that global cooling episodes played a key role in their demise, although in the case of Neanderthals, that was exacerbated by competition with Homo sapiens. “Their resource base was [likely] shrinking faster than they could find . . . other new resources,” Raia says.
What H. neanderthalensis, erectus, and heidelbergensis have in common, Raia adds, is that their disappearance represents evolutionary dead ends, and therefore a true extinction. By contrast, H. ergaster and H. habilis probably represent transitional hominins who evolved into other human species.
The last known survivors of H. erectus are thought to have existed roughly around 110,000 years ago on the Indonesian island of Java, long after the species had given rise to other hominins such as H. heidelbergensis and migrated out of Africa into Eurasia. In Java, H. erectus representatives were geographically distant from other known hominins at the time, making it unlikely they left any descendants, the authors write. The species—which likely thrived under the warm, moist southeast Asian climate—entered its final chapter with the arrival of the last glacial period, which the team’s model suggests must have brought on the coldest temperatures the species had ever experienced.
Like H. erectus, H. heidelbergensis also evolved in Africa and later ventured into Eurasia. The species is often thought to be an early ancestor of ourselves and Neanderthals, although skulls and other fossils found in southern China, India, and Germany are younger than the earliest known emergence of Neanderthals and H. sapiens–like hominins. That suggests some lineages of H. heidelbergensis did not branch off into other species and instead went extinct after those new taxa had evolved. H. heidelbergensis too succumbed to cold temperatures in southern Eurasia some 200,000 years ago, according to Raia’s research.
As for the Neanderthals, which went extinct in southern Europe around 40,000 years ago, competition with modern humans—who by that time had joined them on the European continent—almost certainly played a role in their demise. But “they were nonetheless still vulnerable [to climate change],” Raia says. There are a number of factors that may explain why our species survived when Neanderthals did not, including cultural and linguistic capabilities, and the fact that H. sapiens had a vast reservoir of individuals in Africa that could colonize Europe anew, Raia adds.
While the Neanderthals are physically extinct, some of their DNA continues to live on in people today.
The study’s findings on Neanderthals contribute to a lively debate about the relative importance of cold spells and the arrival of anatomically modern humans in Europe in the extinction of Neanderthals. That’s where the paper draws on the richest and most reliable fossil and dating records, notes Chris Stringer, a paleoanthropologist at the Natural History Museum in London who wasn’t involved in the new research. Yet looking further back in time towards the other species, the fossil record becomes less reliable. “Personally, I think that climate change probably was a significant factor in some of those extinctions,” but the quality of the fossil record makes such ambitious investigations difficult at this stage, he writes to The Scientist in an email.
It’s unclear how much a relatively sparse record for most early Homo species actually tells us about their historic distributions, due to biases in how well fossils preserve in different places and where people tend to look for them, Thompson notes. Nor is it clear whether a hominin’s last appearance in the fossil record actually represents when it went extinct. Making matters harder, many fossils are poorly dated and archaeologists don’t have a good handle on the chronology of specimens for some species such as H. erectus. For instance, Stringer notes that the authors based the last appearance of H. erectus on dates obtained from fossils at a site known as Ngandong in Java, “but we can’t be sure whether other less well dated sites . . . might be younger still.”
He adds that the authors made particular choices that in his view could influence the strength of their results, such as deciding to include fossils identified as another hominin species, H. rudolfensis, as part of the H. habilis lineage, and their interpretation of certain specimens where there is disagreement about their classification. Raia points toward several analyses he and his colleagues conducted to test various assumptions they made, including different taxonomic attributions. “In the end, the results still hold,” he says.
For three species—H. neanderthalensis, erectus, and heidelbergensis—their climatic niches appeared to suddenly contract just before their last known appearance in the fossil record.
“I actually think these authors do a pretty tidy job of arguing for why they chose the particular taxa that they did,” Thompson says. “They really went above and beyond trying to justify” their decisions, as is evident in the 110-page supplement that accompanies the paper.
The study may not provide definitive answers about the history of our genus, but it’s valuable in that it provides a hypothesis that could inform future archaeological research, she adds. “It might change where you might want to look for sites. . . . You might find out that maybe the animals represented in the fossils there are reflective of a climate that really supports the model, or you might find otherwise,” she adds.
In paleontological terms, it’s not uncommon for climatic changes to push species to the brink. But to Raia, the fact that even some of the most technologically advanced and intelligent creatures of their time weren’t immune to climate change serves as a warning to our species today. Both Raia and Thompson agree that Homo sapiens is well-endowed to survive climate change. But survival as a species isn’t the goal—rather, it’s survival and wellbeing of the human population as a whole, as well as the living flora and fauna that support our existence on Earth.
“When you think of what is happening right now, with all of the change we are causing because of our exploitation of fossil fuels, it makes thinking into the future even more worrisome, because we know that climate change [can] be that bad to species—even to those which were, cognitively speaking, the biggest fish in the tank,” Raia says.
P. Raia et al., “Past extinctions of Homo species coincided with increased vulnerability to climatic change,” One Earth, doi:10/1016/j.oneear.2020.09.007.