The artifacts were uncovered at Kenya's Olorgesaile Basin, a dry, shrub-strewn expanse in the African Rift Valley that has been a cradle of humanity for more than one million years.

In the early days, this basin contained a lake, and the red, rocky landscape was traversed by grazing animals, migrating birds and early human ancestors called homo erectus. A century of excavations by some of the most famous names in paleoanthropology, including Mary and Louis Leakey, has uncovered a wealth of artifacts from this period: a fossilised hominin forehead, butchered animal bones and scores of stone hand axes. These tools were heavy and powerful, the signature of an era of ancient hominin technology known as the Acheulean.

But then the environment began to change. Earthquakes shook the basin, causing the lake - the area's most reliable water source - to evaporate. Digging through layers of sediments spanning 800,000 to 300,000 years ago, the researchers saw that the climate swung wildly from wet to arid and back again. Rivers sloshed through the basin, then vanished entirely. The animals that inhabited the landscape shifted, too: huge grass eaters, including giant baboons, hippos, zebras and elephants, went extinct and were replaced by smaller species with more varied diets.

There is a gap in the archaeological record during this shift. But when traces of hominins reappear, about 320,000 years ago, they look very different. Instead of bulky hand axes, the tools scattered throughout the site are small, finely crafted points carved from black volcanic glass called obsidian. But obsidian doesn't naturally occur in the basin - it had to have been carried from many kilometres away.

This curiosity has stood out to Potts since he began working at Olorgesaile 34 years ago. Finally, in the early 2000s, he and George Washington University archaeologist Alison Brooks began to take a closer look.

Brooks, an expert in the phase of prehistory called the Middle Stone Age, immediately recognised the stone points as characteristic of the period. The tools were produced by gradually chipping away at a piece of stone, preparing the core so a single, final blow resulted in a flake of exactly the right shape and size. Some of the points were shaped into scrapers and awls; others bore a mark where they may have been attached to a shaft and used as spears.

"All of these things suggest complex weapons, complex technology, and ... a more elaborate thought process to make them," Brooks said.

Analysis of the stone revealed that nearly half the tools were carved from obsidian that originated 25 to 50 kilometres away across rugged terrain. But the flaked-off bits aren't scattered across the landscape - instead, tens of thousands of shavings are concentrated in the basin.

To Potts and Brooks, this is a signature of trade. Just as modern-day truckers wouldn't sate their hunger by eating the food they're transporting, the hominins who brought this obsidian to Olorgesaile must have carried it whole, then left it for the basin's inhabitants to cut into usable pieces.

An early excavation uncovered another peculiarity: chunks of rich red rubble unlike the surrounding rock. One had narrow holes on each side, as if someone had been trying to bore through it. Close analysis with a high-power microscope revealed that the rocks had been scraped, likely to make ocher pigment.

This, too, must have been transported from far away. Consulting with the local Masai, Brooks concluded that there was nowhere to get this particular rock in Olorgesaile.

If the ocher really was used to make paint, it would be the earliest evidence of such behaviour among humans and our close relatives.

"Colouring material is generally seen as evidence of complex symbolic behaviour," Potts said. "If you think of the ways we use colour ... it is usually a way of marking skin or hair or directions or something."

Why would the ancient inhabitants of the Olorgesaile Basin have needed such markers? It goes back to trade, Potts said.

"If you're expanding the geography of your survival, running into other groups and realising they have this valued type of stone, and negotiating for it in some way or another ... if it's advantageous to create these social group interactions and maintain them, then what better way than some symbolic means of communication?"

Paired with the climate data, a portrait of this period began to emerge. The insecurity created by an unstable climate would have pushed the Olorgesaile people to be better, smarter and more opportunistic hunters. And, just as humans have done for millennia since, they would have relied on the goodwill of their extended social networks to help them get by when times were tough.

"In essence, what we're dealing with here is the origin of adaptability," Potts said.

It's a quintessentially human trait. While natural selection has pushed other creatures to become exquisitely well-suited to their ecological niches - producing finches whose beaks precisely fit the seeds they eat, moths whose wings perfectly match the colour of the trees on which they hide - homo sapiens have evolved to fit any environment in which we find ourselves. We are the only creature that has made a home on every continent on the planet and one space station off of it. (Well, except for maybe some microbes.)

No hominin fossils from the Middle Stone Age have been found at Olorgesaile, so the researchers can't say exactly who was doing all this innovating. The earliest known fossils from homo sapiens are 300,000-year-old bones uncovered in a Moroccan cave.

There probably will be scrutiny about how socially advanced the people at Olorgesaile really were. Long-distance social networks are "a key unique feature of modern humans,"Curtis Marean, an archaeologist at Arizona State University in Tempe, told Science. But the evidence for such networks is debatable, since the stone for tools came from less than 50 kilometres away.

Brooks emphasised that this research is just a beginning. Olorgesaile represents a few pages from the multimillion-year history of humanity; she hopes further excavations in the basin and at other sites will add to the story.

"There were probably long periods of experimentation moving from Acheulean to the Middle Stone Age," she said. "I doubt we're going to find a single moment when suddenly everything changed."

Washington Post