How Falling Down (Maybe) Made Us Human

To be human is to trip, to stumble, to stub your toe—it really feels that other animals do this far less than we do.

The enormous benefits we gained by becoming upright and walking easily on two legs, as no other mammal does, are balanced by some costs. Our backs hurt, our knees hurt, we’re unsteady in childhood and in our old age. One serious cost—humans’ relatively great difficulty giving birth because of the bipedal pelvis shape—goes to show how very beneficial that pelvis shape must have been in other ways (having our hands free to carry things, for example).

It’s not easy to learn, this top-heavy way of locomoting with only two points of ground contact, and it requires a finely tuned orchestra of joints and balance. Anyone who has watched the lurch of a toddler knows the challenges even on flat ground—and kids must of course learn to adapt for uneven surfaces: to lean back a bit when going downhill, and lean forward a bit when going up.

Why, when, and over what time course we became bipedal is a compelling question, and likely has to do with many things including climate change, predators, food, community size, and much more. Something about our bodies and our environment must have combined to create conditions under which this (at least initially) inefficient way of moving provided benefit at the time.

But a mismatch between bodies and the environment, that required adjustment as you moved rather than chugging forward on autopilot, might have had wider-reaching consequences. What if having to gauge your body in space offered some novel twinkle of insight that allowed for much more? The need to gauge as you go might have arisen in a few different ways, and the thought experiment is fascinating.

Risky Maneuvers

Here is an interesting idea, and the authors’ inspiration for it comes from a wonderful animal that likely resembles our evolutionary ancestor at the moment of that “mismatch”: the orangutan. As a very large (200-pound) almost entirely tree-dwelling individual, the orangutan is in the unique position of thinking daily—moment to moment, really—about his own body weight.

The consequences of misjudgment are grave: a plummet to the forest floor from a hundred feet in the air. He must over and over put a tentative foot out and test things—will this hold me? Yes? No, I think not. Will this? Hmm, yes. Am I heavy enough to use this sapling as a pendulum to get over this gap? Am I too heavy for that? Neither the squirrel (very light in the trees) nor the cow (very heavy on the ground) needs to think this way.

The famous child developmentalist Jean Piaget considered it a key moment in self-awareness when children can explicitly reflect on their body as having actual weight. When they get off a blanket they’re standing on, if they want to move the blanket. They do this at about 18 months of age, and not before. Very few other animals do this, and very few other animals recognize themselves in mirrors—in human toddlerhood, when both skills are being developed, the very kids who step off the blanket are the ones who recognize themselves in mirrors.

The Conditions for “Self”?

Once we could reflect on our body-as-object, maybe, the tiny seed of being aware of the self was planted, being able to explicitly reflect on our bodies as existing in the world, to think of ourselves as distinct from others and from what surrounds us. This would gain steam—evolutionarily speaking—and give rise, over the generations, to increasingly more sophisticated conceptions of self.

As with all matters of evolution, nothing is static; everything depends on the way that chance and environment interact to produce more offspring, at any given time. Might our closest evolutionary relatives, the chimpanzees, offer some clues to this chain of events?

A good deal of interesting work has been done about chimps’ abilities to recognize their own bodies in mirrors. Much less has investigated the question of their explicit representation of their bodies as objects, in that old Piagetian sense of stepping off the blanket.

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One strategy is to test this in a laboratory, which has the benefit of careful experimental design. Another is to observe them in their natural environment, which presents chimps with challenges that likely resemble those under which our common ancestor evolved.

Modern chimpanzees spend about half their time on the ground and half their time up in trees, where they do most of their eating and sleeping. They are not as big as orangutans, but still have a lot of weight to navigate through a high, delicate canopy—about 110 pounds for an average adult male. Females are also carrying their babies, an extra and precious burden.

The Real-World Problem

Most of the trees a chimp eats from have enormous trunks, far too smooth and wide around for her to climb. So she doesn’t move straight up the one she wants—she scales a smaller “satellite” tree and uses it to access the canopy of the larger one. Sometimes she needs two or three satellite trees, in succession, to solve the problem.

If she has a tiny infant, it hangs onto her belly, so no worries. But if she has a preschooler, it will insist on its own slow path, with adventure and mishap along the way; it is a long day of patience for her, to which many of us can relate. If she’s not in the mood, she hauls him onto her back, and then he rides up like a little prince-jockey while she navigates the route.

She uses her weight to carefully bend the smaller, satellite trees in slow arcs of connection. Each new one might actually bend in a different direction than she’s headed, or she might need to build momentum by swinging back and forth, so that progress is sometimes indirect.

Adults use their own weight to go just far enough to get a tree to bend, but not too far. Young ones are still learning: if they don’t climb high enough before trying to sway-arc, the tree is unyielding, and they remain motionless. If they go too high the bend is deep, and their own weight takes them back down—thud—to the ground.

Little ones who are learning their own body practice with saplings over their mother as she tries to sleep. If they do it just right, their teeny dangling feet barely brush her, paddling as they hang one-armed, open-mouthed with breathy laughter; she spends naptime gently swatting them away, eyes half-closed.

A Mistake and a Lesson

One day I witness a rare mistake by an adult. Carter arrives at the base of a huge fig tree and decides too quickly on a satellite route. The top of his slender, chosen tree doesn’t reach the fig’s canopy. He tries swaying back and forth but no luck; he scratches his chest and surveys the scene.

Basie then arrives through the underbrush, chooses a different route—a complex four-tree maneuver—and within minutes is stuffing figs into his mouth. Carter quickly slides down his own tree, trots across to Basie’s choice, and journeys upward to success.

As they eat, Jackson arrives on foot. He makes the same initial mistake as Carter, quickly realizes the problem, and turns to descend, but the tiny branches at the top can’t support the sudden weight shift of his larger body. The whole tree collapses with a ringing snap, and he plummets a crashing 50 feet straight down.

There is no movement at all, total silence for almost a minute. Then Jackson sits up, sucking slowly on his hand. He moves gingerly off along the forest floor, and does not rejoin the group for two hours. Maybe he’d gotten the wind knocked out or was unconscious for a moment, but he was lucky. His body was extremely tough—chimps are, in general—and he’d not been carrying a child. He learned. Not just to avoid that tree, but to recognize what it feels like to be too heavy in the moment before catastrophe.

The Seeds for Self?

If Jackson were a fast learner of this concept he’d likely live longer to have more offspring who, like him, were fast learners. If he were a slow learner, he’d be around for less time, and have fewer kids.

To make use of that lesson he’ll need to recognize the signals his body sends him, about its weight and its proportions, and the relevance of these to what happens in the world. He might be on his way to thinking of himself as something distinct, as an individual.

How many animals are en route to the same?