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How did we become the smartest creatures on Earth? The story of our ancestors’ mental leaps can be found in stones – a modern-day flint-knapper is our guide

SPARKS fly as stone meets stone, and shards of rock ricochet off the furniture around me. Each strike makes me flinch, but Bruce Bradley is the picture of cool concentration as he chips away at his axe head. It is, after all, a skill he has been honing since before he can remember. “I was a natural born flint-knapper. Laugh at that if you want, but I’ve got video to prove it.” As a baby, he says, he was often seen banging two rocks together in his parents’ garden. Then, when his family moved to Arizona, he developed his talents by copying the Native American arrowheads scattered across the desert.

Decades later, Bradley makes stone tools spanning the breadth of human history. His workshop at the University of Exeter, UK, is full of this handiwork. Piles of rocks line the walls, and to one side a deerskin with a dark stain hangs on a wooden frame. It was butchered using some of his team’s handmade tools, he tells me. “We’ve got a freezer out there full of dead parts – you could eat them if you wanted.”

My interests lie elsewhere. The stone tools on the table in front of me are not just useful, they tell the story of our journey from simple ape to thinking human. Previous attempts to trace the history of the mind have relied on speculation as much as hard evidence but, over the past three years, Bradley’s project has taken a more precise approach to looking inside the heads of the people who made these tools. Combining findings about stone-tool construction with neuroscience, psychology and archaeology, we can now estimate the origins of distinctly human mental abilities, such as when we first began to order our thoughts and actions, when our visual imagination blossomed, when we started to think about the past and future, and when we first played make-believe. There are even hints about the emergence of our capacity for patience, shame and suspicion – and the nature of our ancestors’ dreams.

People have long sought a “secret ingredient” unique to human intelligence that could explain our extraordinary cognitive abilities. Most recently, the spotlight has fallen on size – the idea that a big brain is the key. However, it is becoming increasingly clear that there is no secret ingredient. Instead, our peculiar way of thinking results from a reorganisation of the different brain regions, as much as from their expansion (see “Size isn’t everything”, above). What’s more, this began long before we diverged from chimps, around 6 million years ago. Indeed, comparable but more modest changes can be seen in many of our nearest relatives. “In a way we’re just an extreme great ape,” says at Stony Brook University, New York, who last year .

So what accelerated this evolution in our ancestral line beyond what was happening in other apes – and how did this give rise to new ways of thinking? Only by re-examining the archaeological record can we map out that path (click on timeline below). And that’s why I am in Bradley’s workshop.

He pauses in his work to show me three stone tools. The first and crudest of them is a jagged rock that signals perhaps the first landmark moment in that journey. Aside from walking on two legs, our earlier ancestors were distinctly ape-like, and like chimps and other primates, they probably had limited tool use, picking pebbles off the ground to crush nuts. But things change about 2.6 million years ago, with Australopithecus garhi. Rather than just using nature as they found it around them, they began to modify it, wielding one stone to chip the end off another and using the resulting sharp edge to butcher meat.

The idea of using one tool to create a more useful implement is itself a conceptual leap. But just as important is the fact that . This includes coordinating your limbs so that one hand is doing a different job from the other. “You need one hand for support, one for striking,” says Nada Khreisheh, Bradley’s colleague – movements that chimps struggle to master even with training. If such bodily control seems like more of a hop than a leap, consider all the new opportunities it opened up – including the creation of better tools – that would reward increased intelligence, accelerating our evolution compared with the other apes. “I’m willing to bet there would be no consciousness on this planet if we didn’t have flakable rocks,” says Bradley.

“I’m willing to bet there would be no consciousness on this planet if we didn’t have flakable rocks”

Even with that trigger, our ancestors were slow to progress. Things didn’t begin to take off until Homo erectus, about a million years later. H. erectus is significant for many reasons. As well as having broadly similar bodies to modern humans, they lived in bigger social groups than their predecessors. Successful communal living requires cooperation and the ability to detect and punish cheats who try to get something for nothing. According to Eva Jablonka at Tel Aviv University in Israel, those challenges may have spurred the evolution of complex emotions such as shame and embarrassment, which would help individuals toe the party line. “We became emotionally modern before we became cognitively modern,” she says. But what really marks out the thinking of H. erectus is encapsulated in the second tool of the three in front of me, an exquisite leaf-shaped object known as an Acheulean hand axe.

Better by design

We do not know what inspired this revolutionary design – it may even have come in a dream (see “Sweet dreams“). The first attempts, which date from around 1.5 million years ago, were fairly crude, but over the following 1 million years Acheulean axes became thinner and more symmetrical as they began to embody a more systematic style of working. Bradley’s demonstrations show that to achieve the more sophisticated designs, you need to prepare the surface of the rock – working away smaller chips to create an angle before striking off the larger, flatter flakes. “,” he says as he chips away at his own rock. Breaking down a goal into a series of smaller actions in this way shows the beginnings of hierarchical thinking. Chunking and sequencing our actions seems so central to the way we operate today – whether we are making a cup of tea or running a bath – that it’s almost impossible to imagine our minds working in any other way. But the refined Acheulean axes offer some of the first signs that our ancestors were beginning to develop the ability to organise their thoughts in these more complicated ways.

This innovation in axe design has been linked to another milestone in human cognition: language. It is such a complex system, dependent on many different thought processes, that its origins are sometimes described as evolution’s biggest mystery, but there is some evidence suggesting that tool-making could have been a catalyst. Bradley’s collaborator, Dietrich Stout at Emory University in Atlanta, Georgia, points out that articulate vocalisation requires precise movement of the lips and tongue. Chimps and other primates are unable to achieve these, but in our ancestors tool-making drove the development of the brain areas involved in motor control that could later be co-opted for speech. Stout also notes that the sequential thinking needed to create the leaf-shaped hand axes is similar to the thinking that allows us to understand and construct sentences.

that people making replicas of the Oldowan tools have greatest activity in areas associated with the motor control needed to speak, while brain activity in those making the Late Acheulean tools shows an overlap with that normally associated with linguistic grammar. That includes the inferior frontal gyrus along the bottom of the frontal lobe – an area that expanded rapidly in the human line compared with other apes.

Language is, arguably, our only unique feature, and its emergence set us on a road that led away from every other animal. Unfortunately, this turning point in our journey is virtually invisible in the archaeological record: Bradley can show me no tools that definitively signify the first words. But there are hints that our ancestors had begun speaking by the time of Homo heidelbergensis, thought to have evolved from H. erectus at least 600,000 years ago.

H. heidelbergensis was certainly more human in other respects. Its brain, at about 1200 cubic centimetres, was just a shade smaller than ours, providing a cognitive power that is evident in the variety of tools it used, including refined hand axes, cleavers and spearheads. To envisage an amorphous lump of rock transformed into these different shapes and styles would have required good spatial cognition, perhaps signalling the birth of the visual imagination. H. heidelbergensis also revisited certain places again and again, sometimes scattering hand axes across the ground. Some read this apparently inexplicable waste of good handiwork as an early attempt to signpost sites of significance. That is skirting close to the mindset needed for symbolism. Crucially, H. heidelbergensis also possessed refinements in its vocal anatomy. For instance, traces in bones indicate they had more nerves linking the brain and tongue than their predecessors, and their voice boxes seem to lack a balloon-like appendage that constrains vocalisations in other apes. Both of these changes would be needed to produce eloquent sounds.

Whenever it emerged, language brought a whole new set of mental challenges. “When I tell you a story, I can frighten you very easily,” says Jablonka. “And you have to control this fear.” It’s easy to take that ability for granted, but chimps fail to make a good distinction between symbols and real things – they go wild when they see a picture of a banana, for instance. In a similar way, our ancestors may have struggled at first to understand the mental images conjured up by language. To deal with their immediate visceral reactions, Jablonka says, they must have developed greater control over their emotions – and they would have learned to be more sceptical and suspicious of others in the process. They also needed a better verbal memory, so that they could remember what others had told them, to differentiate it from what happened in their own lives. Out of that emerged the ability “to tell my own story, the autobiography”, she says. If Jablonka is right, language contributed to our sense of self.

Beautifully crafted

Our ancestors were probably still navigating these difficulties as the human mind approached the last stretch of its journey. To demonstrate this final mental leap, Bradley draws my attention to the third object on the table in front of us. The beautiful Levallois tool is carved from shiny black stone. With dimples lining its edge, it looks a little like an oyster shell. Bradley tells me the tool is little more use for cutting and scraping meat than the cruder hand-axes – its value was probably aesthetic, rather than practical. To make it, a base stone had to be fashioned into a circle before the “lid” was removed with one strike. That craftsmanship takes great skill and patience, as Bradley and Khreisheh’s modern apprentices discovered. “People like making hand axes,” says Khreisheh, “but they hate making Levallois tools.” Since the process comprises many different stages, and the apprentices often need specific instruction, the mind that originally created this tool was probably capable of advanced hierarchical thinking and complex communication for tuition. These intricate objects first appear at least 300,000 years ago, but although they are found among the remains of our own species, they are most commonly associated with the Neanderthals.

Levallois tools provide some of the best evidence that Neanderthals shared much of the cognitive toolkit possessed by humans living at the same time. And herein lies the mystery. “Whatever the Neanderthals’ cognitive leap was, it stopped; it didn’t continue,” says Bradley. So why did we develop more ambitious inventions and rich artistic cultures, while they hit a dead end? Answer that question, and you get a glimpse of the final stage in the evolution of the human mind.

Some think the solution is child’s play – literally. Since our ancestors first diverged from the other primates, childhood has continued to get longer, giving the brain more time to develop outside the womb. From the remains of bones and teeth, it seems that early human children took longer to develop than Neanderthal ones. Child psychologist Alison Gopnik at the University of California, Berkeley, argues that the ” – the ability to consider how things might be, not just how they are. That allowed them to imagine the environment in more creative ways, giving them greater control over their surroundings, she says. As a result, they could do things that might not have occurred to earlier humans, like inventing new tools and building shelters.

“Time spent playing may have helped develop ‘counter-factual thinking’”

Frederick Coolidge and Thomas Wynn at the University of Colorado in Colorado Springs see a more dramatic trigger. They argue that our last cognitive leap was down to a chance mutation that increased our ability to hold several ideas in mind and manipulate them. Even in modern humans, this “working memory” is limited to about seven items. However, a small increase would have had huge consequences. An improved ability to remember what had just been said would have increased the sophistication of conversation, allowing more complex grammar with many different clauses. That means you can think and plan more hypothetically, using “what if” and “if, then” statements, for instance.

Working memory is also associated with creativity and innovation, because it allows you to mentally explore different solutions to a problem. Wynn and Coolidge also point to research suggesting that because it provides a bigger mental “blackboard” on which we can assemble the details of our past experiences, and draw on them to work out the best way to proceed with the task ahead.

A recipe for success

This hypothesis has been strengthened in recent years by a wave of circumstantial evidence. For example, Lyn Wadley at the University of the Witwatersrand in Johannesburg, South Africa, has looked at the steps involved in making glues used to stick spearheads to poles. Earlier humans had simple adhesives such as plant gum, but she has found that in Sibudu cave, South Africa, about 70,000 years ago, they began to cook up the tree sap with red ochre and beeswax to produce a superior glue that doesn’t break on impact or dissolve in water.

When , she found that she had to pay attention to many different factors, including the temperature of the fire, the moisture and different proportions of ingredients depending on the quality of the tree gum. “It took a lot of coordination to ensure success,” she says. That’s only possible with an enhanced working memory to keep all the different elements in mind at once.

Further clues come from the food these people ate. Around this time, early modern humans began to hunt small game, such as small deer species and rodents. Former army survival expert Klint Janulis, now at the University of Oxford, tried the methods they used and found he needed to place 10 to 15 traps to capture enough food to make it worthwhile. “Within a couple of hours you can set enough traps to feed yourself, and maybe another person, for a day,” he says. But that requires forethought, and keeping track of their locations needs just the kind of advanced cognition that Wynn and Coolidge suggest.

The timing of these advances at 70,000 years ago is particularly significant because they come just after the eruption of the Toba supervolcano in Indonesia, which plunged the world into a mini ice age and caused a human population crash in Africa. Any beneficial mutations within the small remaining population could spread quickly, leaving a permanent mark on their descendants. “All extant humans are descendants of those 2000 or so humans,” says Coolidge. If he and Wynn are right, then the explosion at Toba marked the beginning of the home stretch to modern thinking. Armed with this slightly superior thinking, we left Africa and took over the globe, while the Neanderthals and our other evolutionary cousins became extinct.

Of course, our journey isn’t over and it is tempting to speculate how the human mind will evolve in the future. Wynn wonders if we will see further changes in working memory. “It’s variable within populations,” he says. “We suspect it may still be under evolutionary change.” And it is possible that advances in technology could substantially change the mental challenges we face, just as stone tools did in the past. Claims that the internet is making us stupid have so far proven to be unfounded but the way we interact with one another is certainly changing and so are the mental skills associated with success.

Bradley is more interested in the past than the future. The air is now thick with flint dust as he hands me the finished axe. There are still many questions left to answer, he says, as we try to fill in the gaps between the known landmarks of cognitive evolution. “From our point of view it’s just scratching the surface of what could be done.” But he has already achieved one of his goals – he wanted to teach a new generation of flint-knappers the skills he has been refining since childhood.

There’s also a chance that his handiwork will find a place next to the artefacts he so admires. The Smithsonian Institution in Washington DC, he says, is interested in collecting his life’s work to demonstrate the progression of a modern day flint-knapper. “My body could even be a permanent exhibit there too, when I shift off this mortal coil,” he jokes. It would be a fitting place of rest for the “natural born knapper” who has spent his life trying to understand how we learned to be human.

This article appeared in print under the headline “The story in the stones”