How did Homo sapiens come to rule the planet?
It’s a question that many archaeologists have struggled to precisely answer, but new research offers fresh support to a longstanding theory: Ancient climate shifts may have played a pivotal role in shaping human evolution. The findings were published Wednesday in the journal Nature.
“The study shows an intimate relationship between climate, environments, and human evolution,” Michael Petraglia, director of the Australian Research Center for Human Evolution, tells Inverse. Petraglia also wrote a related article on the new study.
What’s new — The study analyzed the impact of climate on six ancient hominin groups, including early African Homo (combining Homo habilis and Homo ergaster), Eurasian Homo erectus, Homo heidelbergensis, Neanderthals, and, yes, Homo sapiens.
The study identified three overarching conclusions about how climate shaped where and when ancient hominins settled, helping determine the evolution of our species.
First: The researchers conclude that Milankovitch cycles — astronomical shifts in Earth’s axis and orbit — impacted early human habitats as a result of changes in temperature, rainfall, and vegetation patterns. In other words: Climate helped determine where and when humans settled.
Second: Major species shifts, such as the transition from Homo heidelbergensis to Homo sapiens “correlated with massive shifts in climate” Axel Timmerman, the lead author of the study and director of Pusan National University’s Center for Climate Physics, tells Inverse.
The findings help explain why early humans became “global wanderers”
Finally: The researchers conclude that early Homo sapiens likely evolved from the hominin Homo Heidelbergensis in southern Africa some 300,000 years ago due to evidence of overlapping habitats between the two species. According to the study, climate stress and a decline in suitable habitats in this region led to the decline of Homo Heidelbergensis and the rise of Homo sapiens. Similarly, the findings suggest Neanderthals evolved from Homo Heidelbergensis in Europe.
Humans, Homo sapiens that is, in contrast to our predecessors, were able to make adaptations to our climate that allowed us to traverse and settle in a wider range of drier habitats. In short: The findings help explain why early humans became “global wanderers” according to Timmerman.
Why it matters — There are competing theories of human evolution, but the new research offers significant support for the variability selection hypothesis, which proposes that early hominin evolution and speciation were influenced by significant shifts in temperature and precipitation levels.
“The idea that climate played a role in human evolution is old,” Timmerman says. But until now, there’s been little quantitative data to support the theory.
The recent study “does not set up new evolutionary theories, per se,” explains Petraglia.
Rather, Petraglia says the research provides hypothetical models about where humans and their predecessors — such as Homo erectus and Homo heidelbergensis — may have settled and how environments impacted species transition, including the evolution of Homo sapiens. The research provides clues about the kinds of adaptations humans would have needed to develop in order to cope with climate shifts, such as fire or sophisticated weaponry.
But the study is not without controversy, as researchers will likely still debate whether the evolution from Homo heidelbergensis to Homo sapiens took place in southern Africa as the findings suggest. It’s possible that limited archaeological data from other parts of Africa could be impacting the study’s conclusions.
“Though they argue that habitats in southern Africa demonstrate that this was the place for a speciation between H. heidelbergensis and H. sapiens, I am not as convinced,” Petraglia says.
The study’s findings indicate that species shifts in ancient hominins were more likely to occur in southern Africa, lending support to the idea that Homo sapiens originated from a single area of Africa as opposed to multiple regions of the continent.
Whether humans originated from a single region or multiple regions of Africa is a “highly controversial” debate, according to Brian Stewart, curator of the Museum of Anthropological Archaeology at the University of Michigan. Stewart was not involved in the recent study.
Stewart says that the genetic evidence favors a single, southern African origin for our species, and the study indicates climatic factors that could help “investigate the timing and drivers behind the disappearance of Homo heidelbergensis and appearance of Homo sapiens across that wider region” of southern Africa.
How they made the discovery — According to Timmerman, researchers used the world’s “longest climate simulation model,” which generated more than 500 terabytes of data on the Earth’s history over the past two million years. For comparison, the standard external hard drive you buy online probably stores one or two terabytes of data.
“It ran non-stop on our supercomputer for six months,” Timmerman says.
The researchers then linked this climate data with a new and extensive database of human fossils and archaeological artifacts, also spanning the past two million years. From there, researchers created maps from habitat suitability models, showing where ancient hominin species likely settled and overlapped.
According to Stewart, this new research falls into a larger trend of paleoclimate scientists and archaeologists using climate simulation models, in conjunction with large-scale datasets, to connect ancient climate trends with the human fossil record. The habitat suitability models could potentially help scientists fill in gaps in the archaeological fossil record of human history, Petraglia says.
While studies like these are “often ambitious and can be valuable” Stewart says, they can also have “serious drawbacks,” including geographic research biases influencing what we consider to be “suitable” habitats” and relying on limited variables — like Earth’s orbital variations — to indicate climate change.
It can also be hard to use these models to simulate ecosystem changes in complex environments where early humans lived. Stewart says that “often such models do not match and sometimes outright conflict” with local data from paleoenvironmental archives.
Similarly, Petraglia says the habitat suitability models are a welcome tool, but they are just that — a single tool. When it comes to understanding human evolution, scientists still have “a lot of work to do on the ground, recovering not only fossils and archaeology but reconstructing the environments in which hominins found themselves,” he adds.
What’s next — According to Timmerman, the research team is already hard at work on developing human genetic models that could build on this recent study to determine “whether past climate change played a role in human genetic diversification.”
Timmerman also hopes the existing research will impact the field of archaeology by clarifying a time period that anthropologists refer to as the “Muddle in the Middle.” This murky time period in early human history occurred during the late-middle Pleistocene era between 150,000 and 400,000 years ago. The research provides a model for how other archaeological researchers can use climate data to shed light on the evolution of our own species.
But you don’t need to be an archaeologist to appreciate the findings, experts suggest, especially as the climate crisis forces humans to adapt to a changing environment.
“The study has relevance to us today, indicating that there is an intimate, and delicate balance between us and our natural world,” Petraglia says.