My sleepless night in a chimpanzee nest

One night in the sparse forests of eastern Tanzania, I stood in the viridian twilight and watched a troop of chimpanzees make their beds. Standing in a tangle of underbrush, I listened to the sounds of rustling as the dark, distant bodies deftly wove their nests, rapidly bending and tucking the springy branches—a flurry of haphazard-looking movement that resulted, within minutes, in something startlingly intricate, much as a magician’s frantic gestures give birth (et voilà!) to a balloon animal.

I was visiting the research camp, known as the Greater Mahale Ecosystem Research and Conservation area (GMERC), which was run by a primatologist named Fiona Stewart. Among her colleagues, Stewart was revered for an audacious series of experiments she once conducted in Fongoli, Senegal. Curious to know why chimpanzees prefer to sleep in nests, she spent five full nights sleeping in a chimpanzee nest. Then she spent five more nights sleeping on the ground, without any sleeping bag or tent. Throughout the night, she took her temperature, counted her insect bites, and marked down how many times her sleep was disturbed. In effect, she turned her body into a scientific instrument.

What Stewart discovered is that in the nest, she was woken up far less often than on the ground, where roaming animals frequently startled her. She also discovered that she was bitten by fewer insects in the nest, even though mosquitoes are perfectly capable of flying that high; she theorized that the broken branches emitted a scent that acted as a natural insect repellant. A nest, in other words, is an intricately engineered sleep machine. Indeed, since our arboreal ancestors also built nests, the aerial sleeping platform may well be the very earliest form of human technology—a technology older than humanity itself. 

(Read Jane Goodall's iconic 1963 article on the chimpanzees of Gombe Stream Game Reserve)

Watching the chimps bunk down for the evening, I felt a sudden urge to join them. I was curious if, like Fiona, sleeping in a nest might teach me something about human evolution that my mind, in reading stacks of research and conjecture, had missed. Plus, I was just curious to see what it felt like.

I ran this unusual proposition by Fiona.

“I don’t see why not,” she said. After all, chimpanzees abandon their nests after only one night. (Making a new nest each evening, Fiona informed me, is only a little bit more onerous to them than making the bed each morning is for us. They prefer fresh nests, which, being made of green branches, are stronger, safer and, presumably, more comfortable to sleep in.) She assured me I wouldn’t be bothering them, only inconveniencing myself.

On the origin of human sleep

One of the great mysteries of human evolution resides in a dark, amnesiac realm: the enigma of sleep. Studies show that getting a better night’s sleep improves our memory, creativity, immune response, metabolism, and emotional regulation. And yet, spending eight hours or so each night in a state of bodily paralysis and mental hallucination leaves us open to attack, both from predators and from other humans. And the deeper we sleep, the deeper the risk. One of the peculiarities of our species is that we sleep for fewer hours overall than any other primate, but we spend more time in the phase of deep sleep known as REM, when the body is immobilized. Evolutionarily speaking, this is somewhat baffling. ‘Sleeping like a log,’ as my grandpa used to say, would have left our early ground-sleeping ancestors profoundly vulnerable to attack. A nest lifts the sleeper above any ground-dwelling predators and provides her with a kind of natural alarm system; any large predator that attempts to sneak up on her will jostle the branch, startling her awake. On solid ground, this alarm system is disabled. So how, evolutionary biologists wonder, did humans come to feel safe enough to sleep so deeply on the ground, surrounded by predators?

(Meet Tatu and Loulis—the last of the 'talking' chimpanzees)

The anthropologist David Samson has dedicated much of his life to solving this riddle.  He began his career by studying the sleeping habits of chimpanzees; his fieldwork included climbing into 72 nests in order to study their construction. He then moved onto orangutans, and then human hunter gatherers, namely the Hadza people of Tanzania. In his papers, he highlights the fact that building nests was a major step in the evolution of apes (compared to, say, monkeys, who sleep on bare branches, and who, perhaps not coincidentally, exhibit the skittery irritability of the perpetually sleep-deprived). But early humans, by abandoning nests and learning to sleep on solid ground, made an even greater breakthrough.

The most common explanation for the origins of human ground-sleeping is that by that point in our evolutionary history we had already mastered fire, which would have provided heat and deterred predators. But this theory faces a major challenge: wielding fire undoubtedly allowed us to become a smarter species, but how did humans become smart enough to master fire? Anthropologists sometimes refer to this Catch-22 as the “gray ceiling,” a reference to the brain’s gray matter. Samson suggests an elegant solution: it could be that mastering the art of sleeping on the ground, over the course of many generations, gave us the increased cognition necessary to master fire—among a thousand other innovations unique to our species of ape. 

Still, the central question remains: without fire, how did the earliest humans manage to leave the nest?

Samson has a theory about that, too.  

A fitful night in the branches

The following evening, I packed my gear and hiked with my husband, Remi, down to a nearby river, about 30 minutes outside of camp, where there was a Brachystegia tree with a big, fresh, comfortable-looking nest that an adult male chimp had built the night before. With relatively little effort, I managed to anchor a climbing rope to a branch just above the nest, which meant that all I needed to do was to climb up the rope, using mechanical ascenders, and then lower myself into it. In theory at least, it was dead easy. 

With us that day was Pascal Gagneux, a lean, leathery, alert-eyed, voluminously talkative evolutionary biologist. During one phase of his research he had climbed into more than 300 chimp nests to collect samples of their DNA. Pascal gave me three warnings: First, he warned that, as I ascended, I should take care not to jostle the nest too hard, or else it could “explode” in my face. Second, he pointed out that chimpanzees frequently soil their nests in the morning, so I should be sure to check it for droppings. Third, he said that while I was up there, I should be ready to come down at a moment’s notice. It was not likely, but it was possible that a chimp might not be happy to see me up in their domain. He said that one time while he was climbing into a nest, he had encountered a female chimpanzee carrying the corpse of her recently deceased baby. The mother, in a fit of grief and rage, screamed, threw the baby at him, and then fled.

(These slumbering fish may offer clues to the origins of sleep)

While I prepared my climbing gear, Remi began unpacking the rest of our stuff. He was acting as my support crew for the night, and was armed with a walkie talkie should anything go wrong. Pascal warned Remi that the area was frequented by lions, leopards, and hyenas. Before leaving, he told us a story about a local man, the grandfather of a staff member at the research camp, who had been attacked by a hyena and had half of his face ripped off. “So all his life, to eat, he had to hold one hand against the missing half, to keep the food in,” he said.

Never in all of our years traveling together have I seen Remi set up our tent so quickly.  

As the daylight began to dim, I attached my ascenders and inch-wormed up the rope. The nest sat about 35 feet off the ground. I paused for a moment just below it to admire its construction. It resembled a small green cloud. The branches had been woven into an ovoid shape, then lined with an extra layer of fresh leaves. I was somewhat humbled to realize that, if asked to, I, with all of my human intelligence, would have no idea how to replicate it.

Once I was above the nest, I switched over to my rappel device and lowered myself down into it. It sank beneath my weight, with a sickening softness. For a moment, I worried I would fall right through the bottom. But then the layers of branches compressed into a kind of net, gained tension, and held me. I slacked out my rope completely, so that all of my weight was resting on the nest. It was comfortable, in a tenuous kind of way. The only problem was that my legs—those wonderful, quintessentially human appendages—are much longer than a chimp’s legs. When chimps sleep, they tend to lie on their backs and fold their short legs up, frog-like, with their heels near their crotch. I was too tall and too inflexible to do that, so my legs dangled in the open air. The blood began to pool in my feet. After only a few minutes, it was clear that this would be a very uncomfortable night.

Down below, in the tent, I could hear Remi inventorying all of his cozy accouterments.

“Okay, I’ve got my pillow, I’ve got my sleeping mat, I’ve got my sleeping bag…” he muttered to himself. A few minutes later, I heard him sighing contentedly as he settled into his soft, crinkly, synthetic little nest. Then he went quiet.

(Exploring the mesmerizing physics of animal locomotion)

Night spread; the sky turned lilac, then indigo, and a single fiery orange star appeared through the branches.

Down below, I heard Remi gasp.

“Oh my god,” he said. “The ground is alive.” He’d unzipped the door to his tent for a moment, to adjust something, and within that brief window of time, dozens of bugs had gotten in; I could hear him down there, hunting down the infiltrators. “They’re everywhere!” he yelled, slapping the sides of the tent.

From her studies, Fiona concluded that the relative absence of creepy-crawlies was the biggest reason chimpanzees build nests. When she slept on the bare ground, she found that just after sunset it became a single crawling, slithering, hopping carpet of insects, which continually interrupted her sleep. “It was unbearable,” she said.

Fifteen minutes later, Remi, having evicted the last remaining insects, was softly snoring. For the next few hours, I lay there in the dark, growing increasingly bored and miserable. My dangling legs caused my lower back to arch, which caused it to cramp up, so I tried propping them up on a branch, but each time I drifted off to sleep, they would slip off and I would jolt awake with that terrible plummeting sensation familiar to anyone who has tried to sleep on an airplane. This phenomenon, known as a “hypnic jerk,” is surprisingly common; it is estimated that 70 percent of people experience it at some point. It might well be an ancient holdover from our arboreal past, a time when the act of sleeping and a fear of falling were deeply intertwined.

Hours passed, a small eternity. I fished out my phone to check the time, thinking it must be only a few hours from dawn, at which point I could climb down and stretch out on the ground. 

The clock read 11:04 p.m.

By this point, I had been aloft for five hours. My lower back was already so badly cramped that I was afraid I would soon become trapped in the nest. What’s more, every time I shifted my weight or repositioned my legs in an attempt to find a more comfortable position, I could feel the twigs below me sproinging loose, which meant that my sleeping platform was gradually disintegrating; I recalled Pascal’s warning that with too much jostling, the nest could simply explode.

Utterly defeated, I heaved myself upright, crawled out, rappelled down to the ground, and, glancing about warily for hyenas, slipped into the tent beside Remi. My camp-bed—nothing more than a stinky old sleeping bag and a mouse-chewed foam pad—suddenly felt like the pinnacle of human innovation.

(Inside the hunt for the 'other humans')

To sleep, perchance to dream

Fire, it turns out, is not as essential to sleeping comfortably outdoors as it might seem. Studies of the Hadza, a group of modern hunter-gatherers living in the Central Rift Valley, show that what seems to most reliably ward off lions are large numbers of humans in a tight formation, ideally in a hut or fenced-in area, with at least one individual serving as a sentinel in shifts throughout the night. Samson believes our early ancestors learned to sleep in precisely this manner around 1.8 million years ago, which is what allowed us to safely leave the nest. If true, this would make literal the maxim, put forward by Martin Nowak and Roger Highfield, that life is not just a “struggle for existence”; it was also a “snuggle for existence.”

This theory meshes nicely with other recent findings in evolutionary science, which emphasize the necessity of cooperation among even the earliest human ancestors. In 2021, the paleoanthropologist Jeremy DeSilva published a book entitled First Steps. While studying various hominin fossils around the world, he had noticed that they often showed signs of injury. (Lucy, for example, likely had both an injured back and an infected femur.) This was not surprising; sprinting away from predators on two spindly legs is inherently risky. What surprised DeSilva was how often those same injured bones—what in many cases should have been life-ending injuries—had healed. In order to survive, DeSilva surmises, our early ancestors would have needed friends and family members to help care for them when injured. We could not afford to be lone wolves or petty tyrants. “Bipedalism in an overly aggressive ape with purely selfish tendencies and a low tolerance for other group members would have been a recipe for extinction,” he writes.

Cooperation functions as a kind of evolutionary skeleton key, unlocking many of the constraints on the growth of human intelligence. Social sleeping may have given us that key. It would have created a feedback loop between increased group cohesion, increased emotional regulation, and increased cognitive capacity: in other words, we got calmer, kinder, and smarter, the one feeding into the next in a virtuous circle. It also would have created a negative selective pressure against psychopathically dominant individuals, who couldn’t be trusted to watch one another’s backs. Nestled together like a pack of dogs, dreaming alongside one another, early humans would have quickly become tightly bonded in ways no other apes are.

Once individuals or families cohere into tribes, their collective abilities mushroom. The invention of cooperative parenting, for example—where extended family and community members occasionally pitch in to help with the burden of child-rearing, a practice human communities almost universally share, but chimpanzees altogether lack—allowed mothers to participate in more aspects of social life and be more productive while still nursing their infants. It also allowed those infants to explore, play, and learn from various adults, before finally setting about the hard work of gathering food for themselves.

Indeed one of my favorite theories of the origin of language is that it emerged not among adults, but among children, who had both open minds and free time, allowing them to experiment with sounds until they began to formulate humanity’s first words. Once language was invented—once our thoughts and dreams could be shared to create entire conceptual worlds—humanity acquired a power to coordinate our actions and achieve complex tasks no other animal could even imagine. We became, in the words of the anthropologist Loren Eiseley, “a dream animal.”