How ancient humans responded to climate change reveals a "hallmark" of our species

In the East African Rift Valley, there is a sedimentary basin called Olorgesailie.

For 700,000 years, the early humans living there used stone hand axes. These large, clunky, oval tools were the longest-used tools in human history. Today, with new iPhones launched annually, it can be difficult to imagine technology remaining so static, for so long.

But some 320,000 years ago, ancient humans broke the mold. Sophisticated, smaller tools and weapons entered into Ologresailie’s record, but the reason why seemed lost to the sands of time. Now scientists propose a new theory to explain the sudden innovation — one that underscores one of Home sapiens' most admirable traits.

In a study released Wednesday in Science Advances, scientists show how adaptability, as well as our long, ambivalent relationship with nature, may have led ancient humans to switch up their tools.

Paleoanthropologist Rick Potts is founder and director of the Human Origins Program at the Smithsonian’s National Museum of Natural History and the study’s lead author. In the summer months, he’s typically found at the Olorgesailie dig site. This year, coronavirus-related travel restrictions meant he took part in a virtual field season instead. The challenges of the present forced innovation on him and his team, Potts tells Inverse — and that fits neatly with the theme of their new study.

The paper offers an analysis of a 139-meter sedimentary drill core, representing 1 million years of environmental history. The drill site is situated some 15 miles from the dig site of Olorgesailie.

By analyzing the different plants and microscopic organisms found in the core's layers, Potts and his team discovered a record of ancient environmental disruption — an “ecological cascade of interacting factors," as Potts describes it, which took place 400,000 years ago.

It was a chaotic time. At that point in ancient human history, there was a massive shift in both the climate and the availability of rainfall. The earth itself would have been riddled with fractures due to earthquake activity, and volcanic eruptions blanketed the landscape.

But despite the peril this instability placed our ancestors in, ancient humans appear to have developed new behaviors and tools to help them survive and thrive, the study suggests.

“I think what we observed gets to the heart of how the human species adjust to environmental disruption,” Potts says. “We’re seeing this capacity for being adaptable, that goes way back in time, even occurring right before the earliest fossil evidence we have of Homo sapiens.”

Explaining a shift — Potts originally proposed the idea that the shifts in tool technology observed at Olorgesailie were driven by a changing climate in the 1990s. Potts theorized “instability could be the driver of evolution by natural selection,” and that adaptability could be selected for in those unstable times.

“Adaptability, the facility to adjust to new environmental conditions, is a hallmark of our species,” Potts says.

Looking at a core of the earth around the dig site to do this work was "very unusual" for Potts, he says. But incorporating the technique, used to study climate change, has paid dividends in filling in details of our human story. Drilling revealed that that the instability early humans experienced was not just a disruption of climate, but disruption to all the factors that matter for an organism’s survival — vegetation, food, and water.

It also explained what traditional archaeological techniques could not. Erosion at Olorgesailie has removed geologic layers representing 180,000 years of time — a period that lands right in middle of the evolutionary transition between ancient technology. This gap in the environmental record had long been a frustration, Potts says.

The shift from hand axes — big, clunky, cutting tools — to the small, sophisticated, and sharp implements that replaced them in the Middle Stone Age was critical to our species' development, he explains. What emerged was essentially "a more mobile toolkit" which included scraping tools and projectiles, he says.

“You had things that could fly through the air for the first time, and the world has never been the same,” he notes.

The use of pigments also emerged during this time, hinting at a concurrent emergence of complex, symbolic communication. Obsidian also emerges as a tool material for the first time — it is the sharp, glassy rock that’s produced when volcanic lava cools rapidly.

Whether or not the emergence of obsidian as a material is related to the increase in volcanic activity observed in the cores is unclear. Interestingly, large pieces of the obsidian found at Olorgesailie appears to have come from nearly 60 miles away, the researchers say.

“That’s trade, and it’s completely different behavior from anything else we see in the archaeological record before that time,” Potts says.

Humans of past, present, and future — Homo sapiens fossils have not yet been found at the Olorgesailie — only their “calling cards,” as Potts puts it — the tools they left behind.

The earliest Homo sapiens fossils found were discovered in Morocco and date to about 300,000 years old. Homo sapiens fossils found in eastern Africa, meanwhile, date to about 200,000 years ago. But the human behaviors paleoanthropologists observe here do appear to date back to 320,000 years ago, with the driver of them — the instability of the environment — going back to 400,000 years ago.

As is often the case with human history, the timeline here is messy. Scientists still debate whether it’s a change in behavior — a new way of life, a new way of adaptation — that is what instigates a new species, Potts points out. In this case, the new species could be us.

“What we’re proposing here is that behaviors were coming before the earliest known faucets of Homo sapiens, just by a little bit. They’re from around the same time, but just a little bit ahead,” he says.

The parallel between these ancient humans' experience with environmental uncertainty and our own is not lost on Potts. When he considers the current demise of different species, he can’t help but be reminded of what happened between 500,000 and 320,000 years ago — a time of mass extinction, climate uncertainty, and ecological disruptions.

A key difference now is that we are driving the climate crisis. Another is that our social web is global, linked through advanced technology.

“The Middle Stone Age might have some lessons for us, but there’s the question of whether our ability to innovate, this time, will be ultimately beneficial rather than harmful,” Potts says.

“Can those elements of how human beings respond to environmental disruption come forward to the present day to assist? A different challenge is we’re not just small bands of wandering people, but nation-states. So we’ll see.”

Abstract: Although climate change is widely considered to have been a large-scale driver of African human evolution, landscape-scale shifts in ecological resources that may have shaped novel hominin adaptations are rarely investigated. We use well-dated, high-resolution, drill-core data sets to understand ecological dynamics associated with a major adaptive transition recorded in nearby archeological sites. The core is ~24 km from outcrops that preserve evidence of the replacement of Acheulean by Middle Stone Age (MSA) technological, cognitive, and social innovations dated to between 500 and 300 thousand years ago (ka), contemporaneous with large scale taxonomic and adaptive turnover in mammal herbivores. Beginning ~400 ka, tectonic, hydrological, and ecological changes combined to disrupt a relatively stable resource base, prompting increasingly large and frequent fluctuations in freshwater availability, grassland communities, and woody plant cover. Interaction of these factors offers a resource-oriented hypothesis for the evolutionary success of MSA adaptations, which likely contributed to the ecological flexibility typical of Homo sapiens foragers.