The mystery of the human intelligence explosion
Over the past 200,000 years since Homo sapiens evolved, something extraordinary happened. Somehow, we went from being clever monkeys with stone tools, to being insanely brilliant masterminds who use complex language, and control the planet with agriculture and technology. How did we start the intelligence explosion that anthropologists call the "human revolution"?  

Perhaps one of the biggest mysteries in our evolution is how humans developed symbolic culture, a very general catch-all term that includes things like art, language, personal adornment, weapons, and rituals like funerals. Symbolic culture is what sets humans apart from other species, and yet it also seems to pre-date the emergence of Homo sapiens. We find plenty of stone tools in Africa before 200,000 years ago, as well as evidence of fire pits and art-making. Our Homo erectus ancestors were throwing spears and making colorful paint from pigments thousands of years before we evolved.
The mystery of the human intelligence explosion
Photo by Patrick Gruban
So question is, when did these symbolic practices start to define humanity? There is no easy answer, and we may never gather enough evidence to come up with a definitive timeline. But there are two broad theories about how it happened. One suggests humanity made a great cultural leap forward roughly 80-60 thousand years ago, during the "revolution" when humans hightailed it out of Africa and into Eurasia. And the other suggests culture evolved gradually, in Africa, over Homo sapiens' entire species lifespan.

The Revolutionaries

The term "human revolution" is often attributed to anthropologist Paul Mellars, who wrote in 2007 that this event was an "accelerated episode of change" between 80-60 thousand years ago, right before the Middle Paleolithic (or middle stone age) transitioned into the Upper Paleolithic (or late stone age). For this reason, this change is also sometimes called the Upper Paleolithic revolution.

The mystery of the human intelligence explosionSExpand
There is also evidence that these new human behaviors included larger groups of humans living together, in social groups that were far more hierarchically organized than previous bands of hunter/gatherers. At the same time, these more structured groups were heading out of Africa into Eurasia, bringing their culture with them. Eventually, these revolutionaries absorbed or replaced the other human populations of Neanderthals, Denisovans and possibly others in Eurasia.
The mystery of the human intelligence explosion
What could have caused the revolution? Mellars is agnostic on this point, suggesting that it might have been a genetic mutation that spread swiftly through Africa. Or it might have been "stimulated by the economic and demographic pressures imposed by the rapid succession of climatic and related environmental changes." This was a period of rapid cooling, partly caused by the Mount Toba mega-volcano that erupted in Sumatra about 74 thousand years ago. People might have been innovating because of genetic changes, environmental ones, or both.
The mystery of the human intelligence explosion
Mellars compares the human revolution of 80 thousand years ago to a more recent one called the Neolithic revolution that led to the rapid development of agriculture and cities roughly 13-10 thousand years ago. He and prominent anthropologist Ofer Bar-Yosef believe that this parallel with the Neolithic is evidence that humans tend to develop in moments of cultural acceleration. We go through phases of rapid innovation, punctuated by periods of relative stability and even devolution.

Given that this is the case, they argue, it seems likely that symbolic behavior was the result of a revolutionary change. That also helps to explain why the fossil record shows an abundance of art and complex tools in the Upper Paleolithic, all over the world.
The mystery of the human intelligence explosion

The Gradualists

The problem with the "human revolution" account of events is that the past few decades have yielded more and more evidence that humans in Africa had symbolic culture long before the so-called acceleration. Anthropologist Sally McBrearty explores this evidence in an essay called "Down with the Revolution," (this builds off her earlier essay with Alison Brooks called "The Revolution That Wasn't") where she fleshes out many of the assertions you see in the figure above.

Based on new digs in Africa and the Middle East, we now have evidence that humans were making paint and burying their dead in symbolic ways over 180 thousand years ago, long before that "acceleration." Long-distance exchange of items and shellfishing also appear to have emerged before 100 thousand years ago. Of particular interest to McBrearty is the fact that ostrich eggshell beads, used as personal adornments, date back 75 thousand years.
In addition, as Alison George notes in a great article in New Scientist, there's now genetic evidence that a gene associated with human communication, FOXP2, might have spurred human symbolic thought 170 thousand years ago. Archaeologist Johan Lind told George that the "modern mind" might have actually emerged as early as 500 thousand years ago, with our ancestor Homo erectus. That would make sense, if we consider that Homo erectus used tools and fire.
The problem, as McBrearty and many others point out, is that evidence for human behavior in Africa is almost non-existent before 130 thousand years ago. At that time, sea level changes overtook some of the most widely-studied coastal human settlements in South Africa and swept away anything that had been deposited before that.
The mystery of the human intelligence explosion
Related to that, evidence of the human revolution advocated by Mellars is found mostly in Eurasia, not Africa. McBrearty suggests that we have to integrate the very early evidence we do have from Africa into the story of symbolism, which gives us a very different picture of gradual development over hundreds of thousands of years, rather than accelerated change over a few thousand.
There are also questions about whether what happened in Eurasia should properly be called a revolution at all, since the sudden shift in the anthropological record is the result of an influx of Homo sapiens immigrants. "The word revolution implies a home-grown development," McBreaty writes. "It does not imply that the changes were the product of a population replacement." In other words, it's not a revolution if a bunch of outsiders come in and convert the locals to a new way of life. Those outsiders are just continuing to build on a culture that started long ago, in Africa.
The mystery of the human intelligence explosion

Or maybe they weren't just in Africa, after all. Gorgeous and strange paintings discovered at El Castillo cave in Spain suggest that the local Neanderthals were engaging in pretty sophisticated symbolism 40 thousand years ago, long before Homo sapiens arrived in that part of Europe. The cave was occupied for thousands of years, and its many chambers are decorated with animal drawings, handprints, and intricate, abstract designs made of dots.[Answer: those weren't Neanderthals but Africans that crossed at Gibralter...]

It's possible that our ancestors had the capacity for symbolic thought long before Homo sapiens evolved. The hominins who left Africa before modern humans did, like the Neanderthals' ancestors, may have been developing symbolic behaviors in Spain, while Homo sapiens did it in South Africa. That would explain El Castillo cave, as well as why Homo sapiens was able to integrate the local Eurasians into their cultures so easily.
But to return to the question we began with, the answer may ultimately lie somewhere in between the revolutionaries and the gradualists. It's likely that humans were gradually developing symbolic behavior over 200 thousand years or more. But it's also possible that there were several thousand years during which this symbolic activity was popularized around the world, and became more sophisticated through cultural exchange with Neanderthals, Denisovans, and other hominin groups who found each other in various parts of Eurasia.

Annalee Newitz is the editor-in-chief of io9, and the author of Scatter, Adapt and Remember: How Humans Will Survive a Mass Extinction.