Three new studies suggest that early humans in East Africa started doing much more complex things—making more sophisticated tools, trading with neighboring groups for better stone, and maybe even using symbols to communicate—in order to survive rapid climate shifts 320,000 ago. Those findings may support the theory that bigger social networks, more complicated tool-making technology, and symbolic thinking helped drive early humans to evolve larger brains by the Middle Pleistocene, around 200,000 years ago.

But that kind of development doesn't just happen. Brains are expensive organs to maintain, in terms of the energy required to keep them nourished and oxygenated, and that size upgrade would have come at a cost. To succeed, bigger brains would have to offer enough of a survival advantage to outweigh the extra burdens they entail.

For that to be the case, humans' ability to survive and reproduce would have to depend on the things we might need such a big brain for, like communicating with lots of other humans in more complex ways or making and using more complex tools. That's why many paleoanthropologists have suggested that the kinds of cultural developments we see in Middle Stone Age sites in East Africa could have been responsible. Cultural development, in other words, drove the physical evolution of our brains in a really major way.

And according to one of the new studies, early humans were making smaller, sharper, more advanced stone tools and trading stone raw materials through social networks that spread across the landscape of Kenya’s Olorgesailie Basin by 300,000 years ago. That's enough time in advance of the Pleistocene brain-size jump to back up the idea that culture could drive it. The Middle Stone Age, in east Africa, shows up in the archaeological record as the moment when the large, bulky, relatively simple stone handaxes and cleavers of the Acheulean culture (1.2 million to roughly 500,000 years ago) are replaced by small, sharp bladelets made with the more advanced Levallois technique, which requires careful planning to chip flakes from a prepared core chunk of stone.

But pinning down exactly when the Middle Stone Age started has been difficult. Geochemist Alan Deino of the Berkeley Geochronology Center and his colleagues dated several sites around the basin by analyzing the ratio of argon isotopes in rock layers to tell when the material above and below a site had been deposited. That provides a minimum and maximum age for the artifacts in between. The dates point to the Middle Stone Age, with all its cognitively demanding tool-making and stone trading, kicking off by around 300,000 years ago.

Early humans didn’t always shop local

Alison Brooks of George Washington University and the Smithsonian Museum of Natural History and her colleagues focused their attention on a site called BOK-2. Here, in a layer of artifacts dating back to between 305,000 and 302,000 years ago, they found tools and flakes of obsidian—even though there's no obsidian anywhere near BOK-2. The nearest deposits are between 25 and 50km away as the crow flies, and in the rugged terrain of the Olorgesailie Basin, people would have had to walk much farther to dodge rifts and steep slopes. And X-ray fluorescence identified about 22 percent of the obsidian at BOK-2 as coming from even farther away than that.

At sites older than BOK-2, occupants made their tools out of local volcanic rock, with exotic materials like obsidian and green, white, or brown chert showing up less than eight percent of the time. But as archaeologists moved from deeper, older layers at BOK-2 to more recent ones, they watched the sources of stone get more diverse and more distant. By 305,000 to 302,000 years ago, 42 percent of the recovered tools were obsidian, despite the lack of a convenient nearby source.

Obsidian is a smooth volcanic glass that makes such sharp edges that it's still used in some modern surgical tools. The fact that early humans at BOK-2 were going to such great lengths to get it is a sign of very advanced planning. But Brooks says the presence of obsidian also means that the inhabitants of BOK-2 were probably trading with people closer to the obsidian deposits, not just walking for days to pick up some rocks and then walk home again.

The obsidian sources are also near good sites for human habitation—they’re home to pastoralists today, in fact—so people probably lived there during the Middle Stone Age. Although the inhabitants of BOK-2 brought home obsidian and could have offered salt or ochre from their own area in return, the trips might have been as much about maintaining the social bonds that enable trade as they were about swapping goods.

That means early humans had complicated, relatively long-distance social networks much earlier than paleoanthropologists previously thought (previous studies said humans didn’t learn these tricks until the Middle Pleistocene, 200,000 years ago, around the time of the big brain-size increase).

There are also indications that people at BOK-2 were preparing goods for trade. Among the finds at BOK-2 were several rounded lumps of soft, dark mineral that turned out to contain manganese, which provides a dark brown pigment. But not all the pigments found at BOK-2 were local. Some of the material found there seems to have come from farther west, at a site called GOK-1, where Brooks and her colleagues found two lumps of an iron mineral streaked with red. The lumps appeared to have been ground with a stone to produce powder, and one lump bore two holes that appeared to have been made by humans.

According to Brooks:

The choice of importing the ochre from a distance rather than using a more common local material which accomplishes the same purpose argues that having a red face or hair or clothing or weapons also carried a symbolic message of some sort... This may have been true of the Olorgesailie ochre, especially in view of the fact that it is not from the local area, that its red color is very bright, and that someone did a lot of scraping and semi-perforation to get it into powdered form.

Thriving under pressure

So the pieces—large, extended social networks, symbolic thought marked by red and brown pigment, and the latest in stone-knapping technology—were in place by 300,000 years ago, which is at least 140,000 years earlier than the first actual fossil evidence we have of Homo sapiens. Deino says the Middle Stone Age “likely constitutes the context in which the anatomical and behavioral characteristics of our species evolved.”

But what drove the early humans of Olorgesailie Basin to move past the big, simple tools and small social groups of the Acheulean and suddenly do more complicated things? According to a third study, led by paleoanthropologist Rick Potts of the Smithsonian Museum of Natural History, the answer might be that they were trying to survive a suddenly unstable world.

His team sampled the ancient soil of Olorgesailie Basin for fossilized particles of plant tissue called phytoliths. According to the carbon isotope ratios in those tiny plant fossils, life here was pretty stable through most of the Acheulean when the basin was a well-watered floodplain. But around 320,000 to 295,000 years ago, the local climate took a turn for the wildly unpredictable.

The environment had already been in the process of a slow shift toward more arid grasslands, but around the start of the Middle Stone Age, the climate started swinging back and forth from wet to arid without much warning. A source of food or water that early humans could depend on one season might be gone the next. And humans weren’t the only ones struggling. Large grazing animals died off, at least locally, around the end of the Acheulean, and Middle Stone Age archaeological sites mostly contain bones from their smaller relatives, who were better adapted to the drier, less-stable climate.

In modern times, hunter-gatherers like those of the Kalahari cope with similar challenges by moving around more and relying on extended social networks to get by.

Potts said:

Trade networks help to spread the risks of food scarcity—when survival resources are lacking where one group lives, it becomes helpful to be aware and have a relationship already built up with groups further away where things may be better... This is exactly what hunting-gathering peoples have done in recent times—their access to resources is not limited to where they typically live and forage, but also to the wider geographic network of places and habitats where others live.

And the early humans of Olorgesailie Basin faced an extra challenge. Eastern Kenya’s geological history is full of violent tectonic upheaval, the kind that reshapes landscapes in a matter of moments, and the Olorgesailie Basin lies in the shadow of a volcano, Mt. Olorgesailie. So when tectonic activity in the region increased around the beginning of the Middle Stone Age, that made life in the basin even more uncertain.

The lesson is clear: when the climate can’t make up its mind and even the ground under your feet isn't stable, it’s time to step up your stone-knapping game and get to know the neighbors. And that, according to Brooks, Deino, and Potts, is what early humans did.

These changes, in turn, probably provided the evolutionary incentive for bigger brains. So the story of human intelligence isn’t as simple as “big brains let us do more complex things.” Doing more complex things helped us evolve bigger brains, which then let us do even more complex things. By the late Pleistocene, 130,000 years ago, humans were making the first beads, producing paint and using it to draw complex geometric designs, fishing and collecting shellfish, and making arrow and dart tips.

We just needed a bit of motivation to get there.

Science, 2018. DOI: 10.1126/science.aao2216

DOI: 10.1126/science.aao2200

DOI: 10.1126/science.aao2646 (About DOIs).