You and I are descendants of chimplike creatures* who left the rainforest and moved to the savannah six or seven million years ago. On first glance it would seem like an odd decision for our ancestors to leave the trees, as there were virtually no predators that could hunt them successfully when they were in the forest canopy. Even superb tree climbers such as leopards don’t attack chimps in trees, as chimps are simply too fast and too dangerous when they are in their element. On the ground, however, chimps are easy prey. They are ungainly on two legs, comparatively slow on all four, and their small size makes them an easy meal for large cats such as lions, leopards, or the saber-toothed tigers that once roamed East Africa.
So why leave the trees? What compelled our ancestors to trade the safety and sheer exuberance of life in the canopy for a slow and clumsy existence on the ground? There is vigorous scientific debate on this question, but one widely endorsed theory is an updated version of the “savannah hypothesis.” This hypothesis was proposed by Ray Dart in 1925, when he published the discovery of Australopithecus africanus, or “the man-ape of South Africa.” After noting that humans were unlikely to have evolved in tropical forests because life there was too easy, Dart wrote, “For the production of man a different apprenticeship was needed to sharpen the wits and quicken the higher manifestations of the intellect—a more open veldt country where competition was keener between swiftness and stealth, and where adroitness of thinking and movement played a preponderating role in the preservation of the species.”
Dart was right that we evolved in the savannah, but in 1925 he had no idea what forces put us there. We now believe that tectonic activity along the East African Rift Valley is what split us from our chimpish ancestors. All the earth’s surfaces, including the landmasses that make up the continents and the bottoms of the oceans, sit on tectonic plates. These plates float around on an underlying mantle, which emerges as a viscous liquid when it flows from a volcano but is under so much pressure below the earth’s crust that it is more like pliable road tar. The heat emanating from the earth’s core creates incredibly slow but strong currents in the mantle, and these currents carry the plates around with them. Sometimes these plates ram into each other in super slo-mo, as is the case with India smashing into Asia, a by-product of which is the Himalayas (which continue to rise a few centimeters each year). Sometimes these plates tear apart and move away from one another. In Africa, the east side of the continent is slowly unzipping from the rest, starting at the Red Sea, in the north, and ending at the coast of Mozambique, in the south.
The tectonic activity along this geographic zipper created the East African Rift Valley and slowly and sporadically raised vast portions of Ethiopia, Kenya, and Tanzania to an elevated plateau. These changes in topography led to localized changes in climate, with the rainforests on the east side of the Rift Valley drying out one by one, to be replaced by savannah. So it turns out that we didn’t leave the trees after all—the trees left us.
Because our chimpish ancestors were so impressive in the trees and so unimpressive on the ground, the gradual replacement of the rainforest with savannah meant that they had to find a new way to make a living. The fruits, berries, and leaf buds they were accustomed to eating receded along with the trees, their opportunities to hunt for meat were greatly diminished by their slow speed on the ground, and, to top it off, enormous predators prowled the grasslands. So how did our ancestors respond to this double whammy of disappearing food and newly dangerous predators? No doubt many of our would-be ancestors perished, but some of them survived and eventually began to thrive, and their story is our own.
The Dik-Dik/Baboon Strategy
Our chimpish ancestors are not the only tree dwellers who ever tried out life on the ground, so scientists often look to the behavior of other species to see how chimps might have adapted to the grasslands. One analogue can be found in baboons. Although baboons are monkeys and not apes (and hence not as clever as chimps), they resemble chimps in many ways, and several baboon species reside on the African savannah. Savannah baboons live in large groups, which gives them the advantages of many eyes to watch for predators and many teeth with which to defend themselves. The “baboon solution” to savannah life isn’t a terrible one, as evidenced by the fact that there are still plenty of baboons, but it is stressful and fraught with danger. Baboons often meet an abrupt end in the mouth of a hungry lion or leopard.
In their confrontations with predators, baboons depend heavily on their massive incisors, which are larger than those of a chimp even though baboons themselves are smaller. If our chimpish ancestors had “decided” that biting was the answer to their savannah dilemma, our faces would likely be more doglike than they are today, with a protuberant jaw and much larger teeth. Our tiny jaws and pathetic canines indicate that the baboon solution doesn’t appear to have suited our ancestors, who took a different approach to life on the plains. Indeed, this decision was already evident by the time we had evolved into Ray Dart’s Australopithecus, whose jaw and teeth were halfway between a chimp’s and our own.
Because chimps are brainier than baboons, they take longer to reach adulthood, and their slower maturation rate means they demand more maternal care. As a consequence, chimps have an older age of initial reproduction and a lower rate of reproduction than baboons. This slower reproduction would have put our ancestors at a greater risk of extinction if they had been picked off at the same frequency as baboons. For this reason, our chimpish ancestors who survived this evolutionary pressure cooker were probably the ones who did their utmost to escape the notice of lions, saber-toothed tigers, and other predators rather than taking a more confrontational approach.
Indeed, hiding is the primary survival strategy for many herbivores. Consider the dik-dik, an antelope about the size of a house cat that also lives on the East African savannah. By virtue of their diminutive size, dik-diks have no defense against any predator larger than a poodle, so they spend their lives hiding. They are impressively quick and agile when chased, but not fast enough to survive being hunted on the open grasslands. As such, dik-diks blend into their surroundings, stay on the lookout for predators, and never stray far from heavy bush.
Our chimpish ancestors weren’t as quick as dik-diks, but they could climb trees. It’s likely they spent their day hiding, watching for predators, and scrambling up nearby trees for safety. When modern chimps are in the savannah, they adopt this sort of combined dik-dik/baboon approach, clustering together more than chimps do in the rainforest and cautiously avoiding open areas where there are no trees available for a ready escape. Perhaps even more interesting, savannah chimps exhibit two other unique behaviors: they fashion crude spears out of tree branches, which they use to poke into tree hollows to skewer and retrieve the monkeys hiding inside, and they are more likely than rainforest chimps to share with one another. Both these behaviors mimic changes shown by our ancestors after they left the forest (more on this later).
These data from savannah chimps and baboons suggest that greater watchfulness would have allowed our ancestors to eke out a living on the savannah, and probably played an important role in their survival for the first few million years after the disappearance of the forest. Unlike baboons and dik-diks, however, our ancestors were not content with this modicum of success. The savannah brought with it new opportunities for a clever ape whose hands were no longer required for locomotion. Change didn’t come overnight, but across the ensuing three million years, numerous adaptations to our minds and bodies suggest that we found entirely new ways to protect ourselves on the grasslands.
Throwing Rocks at Lions
What would you do if you were attacked by an animal that was too strong, too ferocious, and too fast for you to flee or fight off with your bare hands? In my case, it doesn’t take much imagination to answer this question. I grew up in a neighborhood that was inattentive to leash laws, and my friends and I were often chased by a German shepherd and Doberman pinscher that lived on our street. Even though I was a scrawny kid, and these dogs would still intimidate me today, by the age of seven or eight I had become pretty good at defending myself by throwing stones. Especially if my brothers or friends were with me, all we had to do was bend over to gather rocks, and the dogs running toward us would pull an immediate about-face. When I was alone, I took off for the nearest fence or tree, because I couldn’t throw rocks fast enough to do the job, but the addition of even one other person meant we could stand our ground.
These experiences suggest how our ancestors might have responded to the threat of predation on the savannah: by throwing stones, particularly if they could band together and throw lots of them. We can’t look back in time to see if that’s what they did, but we can look at differences between our bodies and theirs to see if this strategy is plausible. So, what does the evidence show?
Sure enough, a number of changes in the fossil record support the stone-throwing hypothesis. Most of these changes can be found at least partially in our ancestor Australopithecus afarensis (aka Lucy, who roamed East Africa three and a half million years ago and was a predecessor of Ray Dart’s Australopithecus africanus). Lucy wasn’t much brighter than a chimp, judging by the size of her brain, but she appears to have devised new ways to deal with predators beyond hiding and hoping not to be noticed. Compared to a chimpanzee, she had a more mobile hand and wrist, more flexibility in her upper arm, a more horizontally oriented shoulder, and more space between her hip and the bottom of her rib cage. This changing constellation of traits was likely a product of the fact that she was bipedal (she walked upright), a habit her ancestors evolved on the savannah. These new traits were also incredibly useful for throwing.
When you watch people toss a ball back and forth at the beach, you may get the impression that throwing is mostly a function of arm and shoulder muscles. If you want to learn to throw with power and accuracy, however, you need to watch baseball players, quarterbacks, or hunter-gatherers. Among experienced throwers, arms and shoulders are just a small part of the equation. Power throwing begins by stepping forward with the opposite-side leg (e.g., a left foot step for a right-hander), progresses through rotation of the hips, followed by rotation of the torso and then shoulders, and finally the elbow and wrist follow through.
These sequential motions take advantage of the fact that the combined forward and rotational forces of the body stretch the ligaments, tendons, and muscles of the arm and shoulder, which accelerate the arm forward at the very end of the throw, like the snapping of a rubber band. Chimps are stronger than we are, but they can’t generate this sort of elastic energy when they throw because their joints aren’t flexible enough and their muscles don’t line up in the right way. These changes to the hips, shoulders, arms, wrists, and hands are what made Lucy and her fellow Australopithecines much better stone throwers. These same changes also supported excellent clubbing,* which would have been useful whenever throwing failed to do the job.
Driving off a Doberman with rocks is one thing; driving off lions and saber-toothed tigers is another challenge altogether, especially when you weigh between sixty and a hundred pounds and stand three and a half to five feet tall, as Australopithecines did.* Nonetheless, throwing can be incredibly effective if you practice a lot. I first had my nose rubbed in this fact when I was in my late twenties and visited the Ohio State Fair with my girlfriend. One of the stalls had a pitching net with a radar gun, and I decided to impress her with my athletic prowess. I was pretty pleased with my fifty-mile-per-hour throws, and she seemed suitably awed—until a gangly twelve-year-old set up shop next to me. Without so much as breaking a sweat, this prepubescent eighty-five-pounder easily hurled ball after ball at sixty-plus miles per hour. Not wanting to lose this manly contest to a human twig, I threw my last ball as hard as I possibly could and was rewarded with a wildly inaccurate fifty-five-mile-per-hour pitch and excruciating pain in my elbow and shoulder. My girlfriend consoled me by suggesting that throwing was more practice than power—I think this was the moment when I first knew I wanted to marry her—and of course she was right.
Keeping in mind that practice makes perfect, we see that the throwing hypothesis is more plausible, particularly if throwing is taken up by an entire group. Consistent with this possibility, the historical record also indicates that throwing can be remarkably effective. There are numerous descriptions of encounters between European explorers and indigenous populations in which conflict ensued and the indigenous population was armed only with stones. The European explorers typically relied on guns and armor, but they often lost these skirmishes, sometimes badly. Consider these three historical accounts that anthropologist Barbara Isaac dug up for her wonderful article “Throwing and Human Evolution.”
In hardly any time at all they had so badly beaten us that they had driven us back into shelter with heads bloodied, arms and legs broken by blows from stones: because they know of no other weaponry, and believe me that they throw and wield a stone considerably more skillfully than a Christian; it seems like the bolt of a crossbow when they throw it.
—Jean de Béthencourt, 1482
The enormous stones hurled by the savages maimed one or other of our people at every moment . . . a shower of stones, so much the more difficult to avoid, as being thrown with uncommon force and address, they produced almost the same effect as our bullets, and had the advantage of succeeding one another with greater rapidity.
—Jean-François de Galoup de La Pérouse, 1799
Many a time, before the character of the natives was known, has an armed soldier been killed by a totally unarmed Australian. The man has fired at the native, who, by dodging about has prevented the enemy from taking correct aim, and then has been simply cut to pieces by a shower of stones, picked up and hurled with a force and precision that must be seen to be believed . . . the Australian will hurl one after the other with such rapidity that they seem to be poured from some machine; and as he throws them he leaps from side to side so as to make the missiles converge from different directions upon the unfortunate object of his aim.
—John Wood, 1870
These accounts highlight the potential deadliness of collective stone throwing, but they also highlight a crucial point: cooperation is the key to making this strategy a success with large animals such as lions and leopards.
The Psychology of Collective Action
Chimps are more likely to compete with one another than they are to cooperate, and thus it would have been difficult for our chimplike distant ancestors to act collectively to drive off large predators. A lone Australopithecus afarensis throwing stones (perhaps while other members of its group ran away) would have ended up in the belly of a slightly bruised predator, but many Australopithecines throwing stones could probably have driven off hyenas, saber-toothed tigers, and even lions. It was this need for collective action that brought about the most important psychological change that enabled us to thrive, rather than just survive, on the savannah: the capacity and desire to work together.
Modern chimpanzees cooperate loosely with one another when they hunt as a group and when they attack other chimps as a group, but their fundamental orientation toward group members who are not kin or close friends is competitive. Thus, it is likely that the first hundred, thousand, or even million times our chimpish ancestors were sneaking across the grasslands, they scattered for the nearest trees at the first sign of attack. But somewhere along the line, our ancestors banded together in their collective defense, at which point they all stood a better chance of survival.
Individuals in groups who learned to work cooperatively in this manner were at an enormous advantage, and would have easily outbred individuals in groups committed to a strategy of “every chimpish chap for himself.” Just as important, evolution would have favored any subsequent psychological changes that supported the quality of the group’s collective response. Our ancestors who liked to cooperate, and who could be counted on by others to be cooperative, reaped a great reward as a result.
Once Australopithecines learned to fend off predators by throwing stones, they would have soon discovered that they could also hunt via collective stone throwing. Collective stone throwing requires little advance planning or coordination, and thus was possible even with the limited cognitive abilities of our distant ancestors. Whenever a group of Australopithecines happened upon possible prey, they would likely have pelted them with rocks. Throwing would also have enabled Australopithecines to scavenge recent kills from other animals, as any lone creature that had made a kill would soon join its prey in the pot if it tried to defend its dinner in the face of hurtling stones.
Stone throwing not only massively enhanced the benefits of cooperation but also created new means to enforce it. The greatest challenge to cooperation is free riding, or the tendency to skip the hard work while sharing the benefits. Many of the Australopithecines in these early savannah groups would have been tempted to be free riders, running away at the first sign of predators while the rest of the group worked cooperatively to fend them off. No doubt our ancestors found such free riders frustrating, just as we do when members of our work groups don’t pull their weight but still nod wearily when our boss thanks the team for the all-nighter. But now our ancestors had new weapons at their disposal to ensure cooperation.
Their first weapon was the threat of ostracism. To be forced out of a group of apes in the forest was a bummer, but to be forced out of a group of Australopithecines on the grasslands was a death sentence. For this reason, our ancestors rapidly evolved a strong emotional reaction to the threat of being ostracized.* The Australopithecines who didn’t mind being tossed out of their group are not the Australopithecines who became our ancestors, so the threat of being ostracized soon brought free riders in line. Ostracism and rejection have remained important tools for enforcing cooperation through to the present, and as a result we still find social rejection incredibly painful and do whatever it takes to stay in our group’s good graces.
For repeat offenders who were difficult to ostracize (either because they stuck to the group like limpets or because they were aggressive and didn’t take kindly to ostracism), the threat of collective punishment likely worked wonders. The ability to kill at a distance is the single most important invention in the history of warfare, because weaker individuals can attack stronger individuals from a position of superior numbers and relative safety. Stoning was probably one of the earliest forms of punishment our ancestors meted out to peers who failed to do their part, and it has remained a common punishment through to recent times. For example, the Bible invokes stoning as retribution for a variety of sins, even though it was written when people could also hang, decapitate, crucify, or otherwise kill one another in terribly inventive ways. The safety* and effectiveness of stoning transgressors were not lost on the creators of these biblical laws.
Although throwing rocks is not exactly rocket science, this early collective action sparked the evolutionary processes that led to the extraordinary expansion of our mental capacities over the ensuing three million years. The decision to throw rocks at predators might not seem like a big deal, and it might not have helped all that much the first few hundred or thousand times, but when it finally worked, it changed everything.
Collective Action Brought About the Cognitive Revolution
Scientists once believed that we became so smart to take advantage of the opportunities to manipulate objects that are afforded by opposable thumbs. There is undoubtedly some truth to this possibility; after all, octopi are awfully smart, and their tentacles provide the same opportunities as opposable thumbs. A huge brain would also be of little use to a zebra, who couldn’t possibly make or wield any tools with its hooves.
Ultimately, though, dealing with fellow group members is a much greater mental challenge than manipulating objects. For this reason, many scientists have adopted the social brain hypothesis, which is the idea that primates evolved large brains to manage the social challenges inherent in dealing with other members of their highly interdependent groups.* This hypothesis has particular purchase with humans, and not just because we live in larger groups than other great apes. Rather, once our ancestors began reaping the benefits of teamwork, they laid the groundwork for all sorts of social innovations, most of which came one or two million years later (and are the topic of chapter 2). These social innovations required a larger brain to coordinate and achieve, and they put greater pressure on our ancestors to get smarter.*
Cooperation made our ancestors smarter, but cooperation also demanded numerous changes in the ways their minds worked. First and foremost, our ancestors began to benefit from information sharing. In their previously competitive lives, knowledge was power—it still is, of course—and sharing personally valuable information was highly unlikely. Once our ancestors started cooperating, however, they would have been much more effective when everyone was on the same page.
The first step toward getting on the same page is shared attention. If I’m competing with other members of my group, I don’t want them to know what I’m thinking, which means I don’t want them to know where I’m looking, either. Whether I’m eyeing a potential mate or a tasty fig, I’ll keep it secret so others don’t get there first. But if I’m cooperating with other members of my group, then I will want them to know where I’m directing my attention. If a tasty prey animal comes along and I spot it first, I want others to notice it too, so we can work together to capture it.
Our chimp cousins are good at assessing visual perspective; they can discern what their fellow chimps are able to see from their vantage point. But chimps have evolved to make it more difficult for their peers to gather this information by hiding the direction of their gaze with brown sclera (the part of the eye that surrounds the cornea). If you look at a chimp’s face, you can’t really tell where it’s looking without closely inspecting its eyes. In contrast, humans have evolved white sclera, which clearly advertise the direction of our attention. Following the gaze of a chimp, gorilla, or orangutan is no easy task, whereas the direction of our attention is readily available to others even when our faces and eyes point in a different direction.
The fact that we advertise the direction of our gaze in such a manner provides clear evidence that we typically gain more from others knowing what has grabbed our attention than we gain from keeping it a secret. Otherwise, our eye sclera would never have evolved away from those of the other apes. Some people have argued that such changes can occur because they benefit the group, and hence indirectly benefit the individual (as a member of the group). Such an argument is possible in principle, but the group benefit would need to be huge and the individual cost small for such a system to evolve. If the group benefits from knowledge that is costly to its individual members, in most circumstances the individuals still won’t share the knowledge. Individual success determines what genes get passed to the next generation, even if individual success comes at a cost to the group.
As a consequence, when group goals conflict with individual goals, individual goals win out almost every time. Chimpanzees are far more self-oriented and far less group-oriented than we are, which is why they struggle to work effectively as a group. But once we moved to the savannah and found that cooperation was the key to success, we had the good fortune that group goals and individual goals aligned for the first time in the great apes. In other words, our expulsion from the forest created a new niche for apes who cooperated more than they competed. This evolutionary alignment of our group and individual goals eventually brought us to the top of the food chain, despite the conspicuous absence of any biological weaponry beyond our large brain.
In this sense, our cognitive evolution over the last six million years can be seen as a process of unintentionally pulling ourselves up by our own bootstraps. Our cooperative solution to a local climate crisis created, for the first time on this planet, a social-cognitive niche, and we spent the next few million years evolving new capacities to exploit this niche more effectively.
The Social Leap That Made Us Human
When our ancestors chanced upon a social solution to the challenges of life on the savannah they set in place a cascade of events that eventually led to our human origins, which is why I describe our move from the rainforest to the savannah as the “social leap.” The leap from the trees to the grasslands was clearly metaphorical (and was really more of a shove than a leap, at any rate), but our leap to a social solution allowed us to move out of the shadows of the larger predators and set the stage for more complex social strategies.
Had we happened upon another solution to life on the grasslands (such as more effective burrowing, hiding, or running), I would not be writing this story and you would not be reading it. The choice our ancestors made was partially random but heavily constrained by the opportunities they had at their disposal.
Loss of our rainforest habitat could easily have been the end of us. If you replayed this vanishing rainforest scenario repeatedly, nine times out of ten I suspect we’d end up as timid versions of baboons at best, constantly looking over our shoulder for lions while keeping an eye on the nearest tree. Extinction or marginal existence was a much more likely outcome than our move to the top of the food chain. But some of our ancestors got lucky and found a solution to their existential crisis, and we are the beneficiaries of their resilience.
Our transition from the grasslands to Google was assuredly brutal and wildly inefficient, but that is the nature of evolution itself. Changes are continually wrought on this earth, and life either adapts or goes extinct. Indeed, humans would probably never have evolved at all if a massive asteroid had happened to miss planet Earth sixty-six million years ago. By smashing into the Gulf of Mexico and triggering global firestorms and climate change, that random bit of space junk eliminated all the enormous predators that had dominated our planet for over a hundred million years. We might have been able to drive off a lion or saber-toothed tiger by throwing stones, but our ancestors would have been tasty snacks for a T. rex no matter how many of us there were or how well we worked together. Our social leap was brilliant and seemingly prescient, but also highly dependent on a long series of events that just happened to go our way.
Most important of all, our social leap also transformed the evolutionary pressures on us. In response to the risks and opportunities inherent in our new life, we dramatically changed our mental proclivities and expanded our cognitive capacities over the next several million years. But that is the story for the next chapter.