A new drill core, preserving a million years of environmental history in the East African Rift Valley, is helping untangle the complex relationship between human evolution and climate change.
For hundreds of thousands of years, our ancestors lived in this predictable environment. With reliable access to water from freshwater lakes and meat from large grazing herbivores, humans could maintain a remarkably stable lifestyle, with no need to change their behaviour or survival strategies.
But 400,000 years ago, according to new paper published in the journal Science Advances, everything changed.
A fluctuating climate brought frequent changes in vegetation cover and water supply. This boom-and-bust period threatened the stability humans were used to.
The study suggests that this ecological instability was a key driver in a major behavioural and cultural shift, leading to the traits underpinning human adaptability.
“The history of human evolution has been one of increasing adaptability,” says Richard Potts, lead author of the paper and director of the Human Origins Program at the Smithsonian’s National Museum of Natural History.
“We come from a family tree that’s diverse, but all of those other ways of being human are now extinct. There’s only one of us left, and we may well be the most adaptable species that may have ever existed on the face of the Earth.”
The researchers combined two main lines of evidence from studies of the Rift Valley: archaeological analysis of artefacts from a site called Olorgesailie and a sedimentary drill core that is now the most precisely dated African environmental record of the last million years.
The artefacts, excavated and studied over decades, document behavioural innovations.
For 700,000 years, humans at Olorgesailie relied on simple stone handaxes, but around 320,000 years ago, they suddenly began to craft smaller and more sophisticated weapons. They also began to trade resources like obsidian with other socially-connected groups and use colouring materials, which potentially indicates symbolic behaviour.
At the same time, the landscape was undergoing dramatic changes, according to a 139-metre-deep cylinder of earth drilled 24 kilometres from Olorgesailie.
By chemically analysing the core’s layers, the research team reconstructed a timeline of the key features of the ancient landscape and climate. The results tallied with the evolutionary transition apparent in the artefacts: after a long period of stability, the environment became more variable around 400,000 years ago.
Tectonic activity fragmented the local landscape, opening up small basins and changing the terrain, which in turn influenced water movement and contributed to the drying out of lakes. Coupled with changing rainfall patterns, this created frequent, dramatic fluctuations in the region’s water supply.
The ecosystem also changed on a broader scale, shifting between grassy plains and wooded areas. Over time, many large herbivores disappeared, and smaller mammals with more diverse diets increased.
The significant behavioural changes of humans at this time – shedding old tools in favour of new ones and broadening their networks – were likely evolutionary responses to the newly variable landscape and fauna. By adapting, our ancestors survived.
Previous studies have suggested that climate change alone drove the development of this major evolutionary shift, but this new research points to a more complex picture, involving the interrelations between climate, tectonic activity, and ecological disruptions in the fauna and vegetation.
Intriguingly, other now-extinct hominin species were also present in Africa at this time, including Homo heidelbergensis and Homo naledi, but neither species is associated with a similar technological shift.
Flexibility and adaptability seem to have been the key to survival for Homo sapiens.
However, Potts points out that this doesn’t necessarily mean we are equipped to face the dramatic ecological changes of today.
“We have an astonishing capacity to adapt, biologically in our genes as well as culturally and socially,” he says. “The question is, are we now creating through our own activities new sources of environmental disruption that will continue to challenge human adaptability?