Impatient for your next smartphone upgrade? Just be glad you weren’t born hundreds of thousands of years ago, when the key technology for survival—stone hand axes—stayed almost exactly the same for 700,000 years. Researchers have long debated the reasons behind this long period of stasis. Now, a study of unusually detailed environmental data from an ancient lakebed in Kenya suggests a turbulent mix of climate change, tectonic activity, and rapid shifts in animal populations about 400,000 years ago forged new social and technological adaptations, including smaller obsidian blades and long-distance trade networks.
It’s an intriguing idea, says Nick Blegen, an archaeologist at the University of Cambridge who wasn’t involved in the study. But he cautions it’s impossible to draw broad conclusions about the motivations behind human technological advances from environmental conditions at a single site.
About 1.2 million years ago in Kenya’s Olorgesailie Basin, early members of our genus Homo began to make roughly hewn stone axes with flaked edges. These hand axes were a relatively sophisticated advancement over an even older, cruder stone tool technology. Ancient humans used their versatile, improved tools for a variety of tasks, including butchering animals, scraping hides, sawing wood, and digging up edible tubers. Their new stone technology, known as the Acheulean industry, persisted more or less unchanged for about 700,000 years.
Over that time, the toolmakers’ environment was remarkably stable, with abundant freshwater lakes and vast grasslands that nourished large animals like giraffes, buffalo, and elephants. But about half a million years ago, the picture gets murky, says Rick Potts, a paleoanthropologist at the Smithsonian Institution’s National Museum of Natural History, who led the new study. Large-scale erosion wiped out the archaeological record in the region from 500,000 to 320,000 years ago. By the time fossils and tools reappear at the end of this period, Potts says, it’s clear things had dramatically changed.
The new toolmakers upgraded to smaller, portable, obsidian blades capable of far more precision than the crude hand axes, Potts and colleagues reported in a series of papers published in 2018. “It’s kind of like the history of technology, and it’s been that way ever since,” Potts says. “From big and clunky to small and portable.” The blades could also attach to wood to create spears and projectiles. And because the closest sources of obsidian were dozens of kilometers away, it was likely the site’s occupants traded for it, Potts says. Yet the stubborn gap in the archaeological record thwarted researchers’ attempts to pin down the factors behind these technological and social innovations.
Potts had an ace in the hole, however—a hole that happened to be 139 meters deep. With the help of a Nairobi, Kenya–based drilling company, he and colleagues extracted in 2012 a core of sediment from the bottom of an ancient lakebed in the nearby Koora Basin. The 139-meter core covered about 1 million years’ worth of sediment buildup.
By looking at microscopic features in the sediment, Potts and colleagues, including scientists from the National Museums of Kenya, puzzled out a rough timeline of the region’s climate and ecology. The presence of diatoms and algae told scientists about the lake’s water level and salinity, for example, and leaf wax helped them conclude whether the surrounding environment was woody or grassy. For the core’s first 600,000 years or so, the environment was stable. Then, about 400,000 years ago, “things go a little bit haywire,” Potts says. Freshwater supplies started to wax and wane; the environment rapidly cycled between grasses and woodlands. Between 500,000 and 300,000 years ago, the lake over Koora Basin dried up eight different times. The fossil record reveals that around the time the grasslands became patchy, large grazing herbivores were replaced with smaller, more agile creatures like gazelle, springbok, and kudu.
From past studies, the researchers knew that about 500,000 years ago, the region was rocked by volcanoes that fractured the landscape, draining large lakes and creating smaller basins prone to flooding and drying events. In short, the humans in the region faced an enormously unstable environment, the researchers report today in Science Advances. That instability may have jolted them out of their Acheulean complacency and spurred them to develop tools to hunt smaller and faster prey, wider transport networks, and more complex methods of communication, Potts says.
Blegen is not yet convinced that the obsidian at Olorgesailie is evidence of trade networks. It’s likely, he says, that humans were just venturing farther from home and returning with the precious stone. Even so, the environmental instability seen in the sedimentary core could have influenced the emergence of trade. Long-distance travel, he says, may have fostered contact with other groups and the extended social networks thought necessary for developing systems of trade.