For hundreds of thousands of years, the ancestors of early humans in the East African Rift Valley could expect certain things of their environment. Freshwater lakes in the region ensured a reliable source of water, and large grazing herbivores roamed the grasslands. Then, around 400,000 years ago, things changed. The environment became less predictable, and our ancestors faced new sources of instability and uncertainty that challenged their previous long-standing way of life.
The first analysis of a new sedimentary drill core representing 1 million years of environmental history in the area shows that at the same time early humans were abandoning old tools in favor of more sophisticated technology and broadening their trade networks, their landscape was experiencing frequent fluctuations in vegetation and water supply that made resources less reliable. The findings suggest that instability in the surrounding climate, land and ecosystem was a key driver in the development of new traits and behaviors underpinning human adaptability.
In the Oct. 21 issue of the journal Science Advances, an interdisciplinary team of scientists describes the prolonged period of instability across the landscape in this part of Africa (now in Kenya).
“There was a perfect storm of tectonic events and changes in climate that made the environment extremely variable and unstable,” said coauthor Rachel Lupien, a postdoctoral researcher at Columbia University’s Lamont-Doherty Earth Observatory. “Our ancestors were forced to adapt.”
Members of the team, led by researchers at the Smithsonian Institution’s National Museum of Natural History and the National Museums of Kenya, documented the behavioral and cultural shift in 2018, based on artifacts recovered at an archaeological site known as Olorgesailie. Decades of study at Olorgesailie has shown that early humans there relied on the stone hand axes for 700,000 years. Their behaviors and strategies for survival were remarkably stable.
Then, beginning around 320,000 years ago, people living there entered what is called the Middle Stone Age, crafting smaller, more sophisticated weapons, including projectiles. At the same time, they began to trade with distant groups and to use coloring materials, suggesting symbolic communication. These changes were a significant departure from their previous lifestyle, likely helping them cope with their newly variable landscape.
“The history of human evolution has been one of increasing adaptability,” said lead author Richard Potts of the Smithsonian. “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 [has] ever existed.”
While some scientists have proposed that climate fluctuations alone may have driven humans to evolve, the new study indicates that the picture is more complicated. Instead, the team says that climate variability was but one of several intertwined environmental factors. Their analysis suggests that faulting on the land surface caused by tectonic activity, and disruptions in vegetation and fauna combined with shifting climate to drive humans to adapt.
In seeking to understand the major evolutionary transition uncovered earlier at Olorgesailie, Potts and his team had been frustrated by a large gap in the region’s environmental record. Erosion at Olorgesailie, a hilly area full of sedimentary outcrops, had removed the geologic layers representing some 180,000 years of time at exactly the period of the transition. To learn about how the region changed during that period, they had to look elsewhere.
They arranged to have a Nairobi company drill into the nearby Koora basin, extracting sediment from as deep as they could. The drill site, about 15 miles from the archaeological dig sites, is on a flat, grassy plain. With support from the National Museums of Kenya and the local Oldonyo Nyokie community, a 139-meter-deep core was removed. That cylinder of earth, four centimeters in diameter, turned out to represent 1 million years of environmental history.
Dozens of collaborators at institutions worldwide worked to analyze the core, which is now the most precisely dated African environmental record of the past 1million years. Charting radioisotope ages and changes in chemical composition and deposits left by plants and microscopic organisms through the different layers, the team reconstructed key features of the ancient landscape and climate across time.
They found that after a long period of stability, the environment became more variable around 400,000 years ago, at a time when tectonic activity fragmented the landscape. By integrating information from the drill core with knowledge gleaned from fossils and archaeological artifacts, they determined that the entire ecosystem evolved in response. As parts of the grassy plains were fragmented along fault lines, small basins formed. These areas were more sensitive to changes in rainfall than the larger lake basins that had been there before. Concurrently, elevated portions of the terrain allowed water to run off from high ground, contributing to the formation or drying of lakes.
At the same time, precipitation became more variable, leading to frequent and dramatic fluctuations in water supply. With these fluctuations, a broader set of ecological changes took place. Vegetation in the region changed repeatedly, shifting between grassy plains and wooded areas. Meanwhile, large grazing herbivores, which no longer had large tracts of grass to feed on, began to die out and were replaced by smaller mammals with more diverse diets.
“There was a massive change in the animal fauna during the time period when we see early human behavior changing,” Potts said. “The animals also influenced the landscape through the kinds of plants that they ate. Then with humans in the mix, and some of their innovations like projectile weapons, they also may have affected the fauna. It’s a whole ecosystem changing, with humans as part of it.”
Potts notes that while adaptability is a hallmark of human evolution, the study brings up the larger question of whether we are equipped to endure the unprecedented, rapid changes now being wrought by man-made climate change and biodiversity loss. “The question is, are we now creating through our own activities new sources of environmental disruption that [will] challenge human adaptability?” said Potts.
Among others, the study was coauthored by Lamont-Doherty scientists Peter deMenocal (now at Woods Hole Oceanographic Institution) and Kevin Uno.
Adapted from a press release by the Smithsonian Institution National Museum of Natural History.