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Syncing Up
Alone we can do so little; together we can do so much.
-Helen Keller
When mud is just the right consistency, it can stop time in its tracks. In 2007, near Kenya's Lake Turkana, archaeologist Jack Harris, who had worked for decades at the site, brushed away layers of sand to uncover traces of long-ago life. Indentations in the mud had hardened into stone. Among many tracks of birds and antelopes were some familiar oblong prints: ninety-seven impressions of human feet so well preserved that all the toes were visible. Carbon dating revealed, astonishingly, that these prints had been frozen in time for 1.5 million years. They were the oldest human footprints ever discovered.
These fossilized prints offered a tantalizing glimpse into the life of Homo erectus, who roamed the earth throughout the early Stone Age. While tried-and-true archaeological techniques could identify the species involved, they couldn't discern what activity the tracks reflected. They could have been left by a band of passing nomads, by one thirsty individual's repeated water runs, or by any number of other lakeside scenarios.
To plumb this mystery, young scientists flocked to the Turkana site to try new techniques that had been revitalizing the study of human origins. To experts on human anatomy and locomotion, minute structural details of the well-preserved prints spoke volumes. Neil Roach and Kevin Hatala used precise digital measuring tools and 3D modeling software to match prints from the same individuals and simulate the strides connecting them. In a behavioral science approach, they recruited volunteers of different ages and genders from the nearby Daasanach people to walk and run on some lakeside mud. From these reference points, they could identify the sex, age, and height of the paleo pedestrians, as well as their timing and pace. The tracks were created by a group of young men trekking quickly together-the earliest evidence of a human sex-segregated group activity. Tellingly, the group traversed the shore like hunters in pursuit of prey, rather than converging on it like seekers of water. The totality of the Turkana evidence points to an intriguing explanation-a hunting party stalking an antelope.
Archaeologists had long noticed antelope bones in erectus living sites, but had no clue how our forebears came by this bounty. Antelopes startle at the slightest sound and sprint away at sixty miles per hour. Not even Usain Bolt could catch one! This early species lacked any weapons that could close the distance. They invented only one tool in their million years on the planet: a handheld chunk of flint used for chopping, pounding, and smashing. This staggering technological stasis gave archaeologists the impression of erectus as little beyond apes in intelligence. Some surmised that they came by their occasional venison as scavengers (by scrounging carrion from the savanna after the big cats had had their fill).
But the Turkana tracks raised a more exalted possibility. The San people of the Kalahari Desert (the world's oldest tribe) can stalk antelopes through a technique called "persistence hunting." Several hunters work to separate an animal from its herd and then chase it, track it, and chase it again-repeatedly, until it overheats and collapses (evolution built antelopes to be sprinters, not marathoners). The key to this hunting method is coordinating on the same target; it does no good to chase different antelopes around all afternoon. The hunters must form a shared plan. Biologist and professional tracker Louis Liebenberg proposed that early humans like erectus could have collaborated this way to hunt antelopes. Roach and Hatala's findings provide supporting archaeological evidence.
The hunting explanation implies that erectus enjoyed a larger leap in brain evolution than previously assumed. Our closest ape relatives, chimpanzees, are impressively inventive but not collaborative. A chimp may figure out how to move logs to feast on the termites underneath, but never do two chimps work together to move a log that is too heavy for one of them. Nor do they ever point to inform a peer about something. This is one reason why they can't coordinate to hunt antelopes-they can't point out the intended target. We can't be sure exactly what the young male humans were doing that day long ago near Lake Turkana, but their tracks tell of a level of teamwork beyond what prior primates could muster.
At later sites, archaeologists find hints of coordinated activity in gathering as well as hunting. In Israel's Hula Valley, Naama Goren-Inbar excavated a lakeside site where a soil layer from eight hundred thousand years ago that shows extensive evidence of human inhabitation. Her team uses laboratory techniques to analyze microscopic bits of plants, bones, and stones. From the elevated presence of botanical fragments in this soil layers (relative to earlier and later layers), they infer the foraging of fifty-five different plant species. This harvest was not just low-hanging fruit, mind you, but also included nuts from tall trees, tubers from deep underground, and even water lilies that blossom far from shore. The sheer range and challenge of their foraging bespeaks coordinated work. The haul even included plants that are inedible unless cooked, suggesting that these erectus women may have also gathered fire-the glowing embers after a lightning strike that could be used to start cooking fires. In 2004 Goren-Inbar's microscope first discovered bits of charred wood and bone in soil samples, but her team couldn't be sure they weren't from natural fires. Three years later, they discovered "phantom hearths," rings of stone fragments scarred from repeated exposure to fire. A decade earlier, biological anthropologist Richard Wrangham had pondered the evolutionary paradox that erectus brains expanded while their jaws and guts shrank (i.e., an increased demand for calories, yet reduced capacity to supply them) and posited that the advent of cooking could be the explanation. Archaeologists at the time scoffed because no hearths could be seen at erectus sites, but Goren-Inbar's microscope revealed the smoking gun. We can't reconstruct these primeval barbecues in full detail, but we can bet that roasting an antelope was not a one-caveman (or cavewoman) job. Not even survivalists like Bear Grylls would try that. For the butchery, fire-building, basting, and browning, you'd want a cooking crew working in close coordination.
A final surprise about erectus involves language. Chimps use gestural signals to communicate fixed messages but lack grammar for combining these into sentences that express novel ideas (likewise, sign-language training experiments succeed in teaching them many words but not in teaching grammar). This shortcoming is understandable because chimps lack the specialized brain structures that we humans use for syntactic processing. This left-hemisphere circuitry is, neuroscientists believe, the reason that 90 percent of humans are right-handed. Interestingly, studies of erectus skeletal asymmetries reveal that they too were predominantly right-handed. Paleodental analyses of tooth wear concur: 90 percent stuffed their faces from the right side. This "righty" predominance is not present in chimps or in the australopiths of three million years ago (that immediately preceded the first humans). Its presence in erectus suggests the emergence of syntactic circuitry and capacity, likely a gestural language that built on preexisting gestural signals. Contrary to linguist Noam Chomsky's long-dominant theory that language evolved recently and discontinuously in a massive mutation, these findings about handedness (among other discoveries) now place it far deeper in the human cultural past.
Footprints, water lilies, fire scars, tooth wear. Separately, such clues are open to various interpretations. But together, they provide converging evidence that our primordial human predecessor has been grossly underestimated. Erectus was not the single-tool simpleton that archaeologists have long portrayed. It was the first hominin to operate in coordinated groups, a critical step toward tribal living. By coordinating-melding minds and meshing actions-erectus groups foraged more efficiently, fed themselves, and forged the solidarity that comes from working in concert. Its evolutionary breakthrough was not walking upright (as its name forever implies) but working as a team. Its great innovation was not the hand axe (that all the textbooks trumpet) but the hunting party, the gathering squad, the cooking crew-and the linguistic communication that made them all possible.
These revelations of erectus teamwork are part of a dawning scientific awareness that humans’ social smarts (and social lives) are more central to our ascent-from prehistory to the present-than previously understood.
Classically, the science of human origins was all about how the body evolved. A famed illustration, entitled The March of Progress, portrays it as a succession of silhouettes: Australopithecus on all fours, then knuckle-dragging Homo habilis, Homo erectus standing more straight, Homo heidelbergensis carrying a crude spear, Homo neanderthalensis with a more fine-tooled club, and then finally the upstanding profile of our own kind, Homo sapiens. It was progress in terms of posture, at least!
Yet even this triumphalist diagram can't hide that there were trade-offs involved. Across millions of years, our forebears gradually lost apes' thick fur, sharp fangs, crushing grip, and four-legged speed. This was a lot to lose, survival-wise. Big cats could catch them in one pounce. Cave bears could tear them to shreds. But while they lost their physical defenses, they gained brain size. Apes were already brainiacs compared to other mammals, but humans evolved brains three times larger. These freakishly large thinking organs are "gas guzzlers": 2 percent of our body weight that consumes 20 percent of our calories. Anything so resource-consuming must have been very helpful-somehow-to survival.
Thoughts don't leave fossils, so archaeologists didn't have evidence about how bigger brains helped. Most presumed that hominid brains ballooned to provide greater mastery over the physical environment. That is, chimps evolved big brains so they could navigate large terrains, devise ways of cracking nuts, and find trees with ripe fruit. Early humans developed even bigger brains to avoid quicksand, craft spears, and build shelters. This explanation seemed so self-evident that it remained untested through most of the twentieth century.
It took a scientist versed in both physiology and primatology to test it. For every primate species, from tiny spider monkeys to hulking mountain gorillas, Robin Dunbar calculated an "encephalization index"-brain size as a proportion of body size. Then he analyzed whether braininess is related to how these primate species live and survive-their range, diet, group size, mating habits, and so forth. The correlations revealed, surprisingly, that brainier species don't cover more territory, crack more nuts, or eat more fruit. But they do mate more judiciously, maintain longer "pair bonds," and cooperate in larger groups. Brainier primates have more complex social lives. Dunbar proposed a new theory: big brains evolved for mastery of the social environment, not mastery of the physical environment.
This "social brain hypothesis" gained further traction as behavioral scientists adopted neuroscience tools for imaging brain activity. A wave of revolutionary studies hooked people up to fMRI machines while they engaged in different kinds of thinking tasks. They found that social judgments (e.g., reading a person's intentions, or anticipating their feelings) are handled by different parts of the brain than judgments about the physical world. The forebrain regions that handle social thinking (e.g., the prefrontal cortex) are the ones that expanded most dramatically in the evolution from apes to us.
Then, in the mid-aughts, another offbeat study sealed the case for social aptitudes as our species' signature strength. An expert on both child cognition and primate cognition, Michael Tomasello decided to conduct the ultimate standardized test. His lab administered a broad battery of aptitude tests to three groups of test-takers: humans, chimps, and orangutans. The humans were preschoolers, so education wouldn't give them an edge. The tests were administered as puzzles with food treats as rewards. Perhaps this seems a foolish study-surely humans (with much bigger brains) were much smarter across the board? Surprisingly not. On standard tests of cognitive capacities relevant to physical things (e.g., object permanence, shape rotation), chimps didn't differ from humans, and orangutans were not far behind. But on tests of social cognitive capacities (e.g., inferring intention from an action, learning a skill from a demonstration), humans performed almost perfectly, whereas the chimps and orangutans floundered. For instance, after watching someone pop open the end of a plastic tube to access the treat inside, all of the human toddlers reproduced this method to solve the puzzle. Chimps and orangutans watched the same demonstration but somehow missed the lesson. When given their turn, they tried to break the tube or bite it open, grimacing with frustration at their inability to get the treat.
It's easy to take social inferences for granted because they come so naturally to us. We can read intentions and reproduce demonstrated actions effortlessly. But they are computationally complex problems for which our brains just happen to have dedicated chips. Our innate wiring for social cognition largely consists of several tribal instincts: the human-specific adaptations that evolved to enable our distinctive form of social living. As we'll see, three major tribal instincts evolved at different stages of the Stone Age. They helped our forebears learn the ways of their group and act upon them. In more academic terms, they encode group patterns and then enact these patterns. You can think of these systems as both a radar that continually scans the social environment and an autopilot that helps you steer safely through it.
Tribal instincts changed the human experience of group living in several ways. First, they accelerated the acquisition of learned skills. With tribal instincts, I don't have to rely on effortful trial-and-error experience, as I can pick up many skills through observation. If I see a peer knocking fruit from a high branch with a stick, my brain encodes this action so that when I am next in the situation I feel impelled to try it. If other group members witness me, they can pick up the skill in the same way, and so on and so on, until everyone in the group knows it. As a practice becomes common to a group this way, it takes on meanings and functions beyond its original instrumental value. It's what "we" do. It contributes to similarity within the in-group and distinctiveness from out-groups, heightening feelings of connection and loyalty. It also powerfully enables collaboration. When I learn a skill through observing groupmates, the code is tagged in my head as shared by the group. Because I know that they know it, I can anticipate their moves, understand their intentions, and make complementary contributions. The power of such known knowns has been discovered independently in many different fields-linguists call it "common ground," game theorists call it "common knowledge," cognitive scientists term it "second-order knowledge," and psychologists prefer "metacognition."
This transformative tribal wiring-honed by a million years of evolution-put our Stone Age forebears on the path to living in highly cultural and highly collaborative groups. You may have heard the well-worn twentieth-century notion that our forebears knew only small bands of close relatives who lived and foraged together (like a chimpanzee troupe) until the "agricultural revolution" of about ten thousand years ago enabled permanent settlements, surplus production, and the time for impractical symbolic activities (such as building the Stonehenge temples). The new science of the past decade has upended this story of social development-not just its chronology but even its phylogeny. Archaeologists have uncovered "princely burials" and temple-building that occurred tens of thousands of years before any agriculture. Temples paved the way for farms and settlements, not the other way around. Traces of large-scale hunts from hundreds of thousands of years ago bespeak the start of clan-level organizations. And fully a million years ago, our forebears already foraged with coordination far beyond that of other primates. These discoveries cast the human journey in a new and ennobling light. From almost the very start of humanity, we have lived as tribal animals.
