For 700,000 years, our species’ ancient relatives in East Africa led rather stable lives, relying on an enduring set of skills and survival strategies. They made large, simple hand axes from nearby stones, perhaps using them to slice up prey, cut down branches, or dig for tubers.
But by 320,000 years ago—around the same age as the earliest fossil evidence of Homo sapiens—these early humans drastically changed their ways. They began crafting smaller, more nimble points that could fly through the air as projectiles, some made from obsidian gathered from many miles away. They collected red and black pigments—substances later humans frequently used in symbolic ways such as cave painting.
Now a new study in Science Advances suggests that one major reason behind this sudden shift in behavior lies underground: tectonic activity that fragmented the landscape.
Scientists have long pointed to changes in climate, such as the onset of wet or dry periods, as the key driving force behind the adaptation of our early ancestors. The new study puts this idea to the test by examining a detailed record of environmental changes over almost a million years, etched into a 456-foot-long core of sediment layers extracted from an ancient lake.
The geologic record of the lake reveals a cascade of ecologic change around the same time new technologies appear in the archaeological record. A variable climate and a shifting landscape caused once stable food and water resources to ebb and flow, likely throwing the lives of early humans in East Africa into turmoil and forcing them to innovate and adapt.
“Adaptability is, I think, really the crucial hallmark of our species,” says the study’s lead author Rick Potts, a paleoanthropologist at the Smithsonian National Museum of Natural History.
Homo sapiens’ flexibility in the face of new challenges may have been a vital reason the species outlasted other related hominins. What was once a gnarled family tree of different kinds of humans—Denisovans, Neanderthals, Homo erectus, and more—eventually dwindled to a single branch. “We’re the last bipeds standing, as I like to call us,” Potts says.
Of course, many mysteries still remain about the provenance of our species. As the study authors note, their work cannot pinpoint the origin of modern behaviors or the location where Homo sapiens arose. But the research does provide a strikingly detailed look at changes in the ancient landscape that set the stage for the development of some fundamentally human behaviors.
‘All hell broke loose’
The study’s reconstruction of ancient environmental conditions is set in a basin known as Olorgesailie, in southern Kenya, which was first excavated in the 1940s by British paleoanthropologists Mary and Louis Leakey. Potts began work at the site when he joined the Smithsonian in 1985. “I thought it was going to be about a three-year project,” he says. “It’s now in its third decade.”
At the time, advances in dating techniques revealed that Olorgesailie contained a rich array of hominin tools that dated as far back as some 1.2 million years ago. But as researchers carefully teased through the layers of sediment laid down over time, they ran into a problem. “We had 180,000 years that were missing,” Potts says.
Some 500,000 years ago, movement along big fractures remodeled the landscape. What was once a basin became a high point on the land, exposing it to wind and water that eroded a blank spot in the geologic record until some 320,000 years ago. During that interval, Potts says, “all hell broke loose, and something changed in the behavior of the hominins in a big way.”
Windows into our ancient ancestors’ lives at this time reveal they were surprisingly advanced for such an early date. Their tools were diverse and much more compact, including small triangular stone points that could have been used as projectiles. “Things start out big and clunky and they become small and portable,” Potts says of the shift. “It’s like the history of technology ever since.”
The world around these early humans also transformed. Before the leap in human behavior, large-bodied animals grazed the region—"lawn mowers of the Pleistocene," Potts says—including Palaeoloxodon recki, one of the largest elephants that ever walked the Earth; Theropithecus oswaldi, a supersize relative of living African baboons; and Hippopotamus gorgops, a hefty ancient cousin of hippos. But by 320,000 years ago, around 85 percent of the mammal species in this region disappeared, and a menagerie of modern critters with smaller bodies popped up in their stead.
To better understand the drivers behind the change, the team began in September 2012 to drill into a flat plain that once contained an ancient lake near their study site. “It was a lot of guesswork, but we guessed well,” Potts says of the drilling location.
They pulled up a 456-foot-long core of lake mud, which was riffled through with layers of volcanic ash that helped pin the segments to specific points in time. The core spanned a period between roughly a million years ago to 83,000 years ago—including the missing 180,000 years.
The missing time
With a battery of tests, the researchers untangled the complex history of the landscape, which is punctuated by tectonic unrest. The region is set in the East African Rift zone, where the continental tectonic plate is splitting into two chunks. As the land stretches, it thins and fractures, forming new valleys and hills where there was once a flat expanse—and a key period of this breakup started some 500,000 years ago, previous research suggests.
The newly formed smaller valleys exacerbated any changes in rainfall, according to the team's analysis, causing frequent swings between flooded and dry ground starting some 400,000 years ago. The greenery also swung wildly between woody and grassy as water availability yo-yoed. These ever shifting conditions are likely what jump-started the leap in hominin behavior, Potts says.
“Water availability is key to survival,” says Kristin Krueger, a paleoanthropologist from Loyola University who was not part of the study team, via email. “If that changes, organisms have three basic choices: migrate, adapt, or go extinct.”
With resources scarce or in flux, ancient humans were likely forced to travel farther away from familiar lands, she notes. “Did that lead to them discovering different types of resources? Did that lead to increased cooperation with other groups to survive? Did it lead to advances in technology to help them eke out a living?” Krueger wonders. “These are the questions this study is exploring.”
There are many tangles in the branches of the hominin family tree, and the connections between each seem to grow more complicated as researchers find more clues. One of the tricky parts of the story at Olorgesailie is the lack of human remains, says John Stewart, an evolutionary paleoecologist at Bournemouth University who also was not part of the study team. “You could have more going on than we can see,” he says.
Studying the ancient ecology in other locations throughout Africa would help researchers better understand the factors that could have driven hominin evolution over time, says Isaiah Nengo, a paleontologist with Stony Brook University's Turkana Basin Institute, who was not involved in the new study. Nengo is leading a team to look at how similar tectonic and climatic influences could have shaped evolution in northern Kenya, starting at a time before the first hominins had even taken shape.
“To have that fine-grained record of data is really cool,” he says. “It could turn out that they’re onto something.”