Once upon a time there was a girl who lived in a tree. She had deep-set brown eyes and brown hair. She ate fruit – orange mangosteen and black juniper berries – crunched on nuts, sucked on sweet grasses and chewed juicy leaves, and dug up tubers and roots, knowing which ones were good, and which were hard or poisonous.
Sometimes, she followed the trails that crisscrossed through the grass, but much of the time she clambered through the broad crowns of the trees, reaching up for branches and feeling the texture of the bark against her hands, balancing against the trunks and springing along boughs. At night she tucked herself into the fork of several branches and curled up to sleep, watching stars like diamonds and branches against the sky.
One day, high up in the canopy, she slipped. She fell more than 40ft (12 metres) and hit the ground at more than 35mph (55km/h), feet first, then fell forwards on to her outstretched arms. The impact was too much, and her legs and pelvis shattered, as well as her arms and ribs. There was no medical help, and she died from her injuries in a couple of hours.
Three million years later in what is now Ethiopia, her fossilised bones were taken into an archaeological camp, where they were playing a Beatles song on repeat: Lucy in the Sky with Diamonds.
By 2016, Lucy One, as she was called, was an icon, and made a posthumous tour of American museums from her base in the museum in Addis Ababa. Scientists used an MRI machine to scan her bones, and reconstructed the breaks in them with the help of a trauma pathologist.
They found that Lucy had complex compression fractures in her larger bones, and the smaller ones had broken in the “greenstick” manner, a half break, half-split often recorded in pathologist’s reports on children who have fallen out of trees. They put together a picture of how she might have fallen, and thus a famous fossil came alive.
Some scientists have dismissed this as a just-so story, pointing to the fact that a 3.2m-year-old fossil will inevitably have breaks. So we can’t be certain about her death. But we can be pretty certain that she lived her life in trees. Lucy was famous because she was the first hominin known to have walked upright and straight-legged like a modern human.
Her species, called Australopithecus afarensis after the Afar area of Ethiopia where she was found, had adapted to walk upright for a good distance. Her kneecaps sat as they would on us, her pelvis was a similar shape to that of a modern woman. However, she also had hefty shoulders and strong arms for climbing and swinging, and she was almost certainly still sleeping in trees and using them to move around when she slipped.
Human history, the human brain, the human hand and the human leg, all start in the forest canopy. Our ability to bounce along branches standing upright, our desire to build nests and smell cedar wood – all the adaptations of primates – are adaptations to trees. Binocular vision, with both our eyes pointing forwards, allows us to judge distances and map a way through the canopy.
Standing upright you can reach a branch high above your head, and grip it with strong and flexible hands that have the perfect adaptation to smooth bark. Fingers backed by hard nails have fat and fluid-filled pads, allowing them to deform like a slightly deflated tyre to the shape of the branch and maximise surface area. Fingerprints, which we share via convergent evolution with the otherwise very distantly related koalas, channel away a film of water that might cover a branch, and interlock with rougher bark.
High in the treetops, it is likely that our pre-human ancestors learned to build nests – weaving together branches into the messy, leafy structures that our closest relatives, chimpanzees, still create.
Cosseted in these protective tree-nests, primates could do something that is rare in the animal world: long, deep sleep. Sperm whales, floating like megaliths, sleep for up to half an hour at a time. Birds sleep with one eye open, employing the opposite half of their brain to watch for any predators. Monkeys that sleep sitting on branches wake every five minutes or so lest they fall off. But in a nest, both non-rapid eye movement (NREM) and rapid eye movement (REM) sleep are possible for long periods. The result, broadly speaking, is brain development and dreams. Humans spend about 20% of the night in REM sleep, and it is during this time that we dream.
Dreams are an invaluable way for another world to bolster the waking world. Freud famously interpreted them as a window into the unconscious mind, but recent research suggests that dreams are something more tangible than this, a way of re-enforcing your kinaesthetic memory, the memory that makes you reach for the branch in the right way and swing for the correct distance. Current thinking suggests that the little twitches made by your body in REM allow your brain to map precisely where in the body and the brain that twitch is happening.
In other words, in REM we re-integrate our bodies, essential for an ape searching through a three-dimensional tree world for food, or bouncing along branches to flee a predator, and for an ape developing an idea of self-awareness. Asleep, we develop memories and maps, a shadow world that can map on to the real one, and at times expand it. Trees didn’t only shape our bodies; they enlarged and shaped our brains every single night we slept in them.
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It is easy to see tree-influence in the shape of human bodies. But trees are chemical manipulators above all, using sun and time and carbon to make incredibly complex compounds, which are finely tuned through evolution to domesticate people.
The cacao tree does so using theobromine, the active ingredient and distinctive taste in chocolate. Theobromine, like cocaine, caffeine and nicotine, has a mild but pleasurable effect on the nervous system, which makes it irresistible to primates. They get a psychoactive high, which allows them to swing through the canopy for miles.
This is an essential trick for a tree that can only grow in specific niches that are scattered about the forest, often far from another of the same species. Ruthlessly, the trees select for larger monkeys able to travel further distances by using the toxicity of theobromine: just as chocolate can kill dogs, so too can a large dose knock off a small monkey. The trees grow large, heavy pods that can develop without breaking branches on the parts of the tree that can support the weight of larger monkeys. These large monkeys can eat the pods and turn the very high fat content of cocoa butter into the physical energy required to travel long distances, and the theobromine hit into mental energy to spread the seeds.
Here, the tree uses extraordinary powers of chemical synthesis to manipulate animals it will use to travel. Chemically, physically and existentially, animals have been shaped by trees, and few have been more shaped by trees than humans. Whole societies have arranged themselves around trees.
The striking relationship between the cacao tree and monkeys appears to have been seamlessly transferred into humans, even though our genetic paths parted from them 5m years ago. As a result of cacao domesticating humans, cacao trees currently grow everywhere in the world where they can survive, from south-east Mexico to the Philippines, Ivory Coast, Angola and India, and we plant them, nurse them and consistently destroy their competition and pathogens.
How many more examples of trees shaping human behaviour, domesticating or taming us, are there? The soothing scent of a new book is a response to the smell of lignin in the paper, specifically a subunit, vanillin, that is also released into some whiskies aged in oak, and concentrated by the vanilla orchid, which grows on trees. Unlike most orchids, which have seeds dispersed by the wind, both bees and animals disperse vanilla – one reason for its extremely wide distribution. The smell of cupcakes and books can be traced back to the appeal of our woody nests.
Since we came down from the trees we have been learning how to manage without them – how to replace them in our lives. Sometimes it can feel as though we are now separated from trees; that, although they shaped us once, we can do without them in our bodies and our lives. But our lives remain intertwined. The links remain in unlikely places: it is too easy to think of trees as passive because we cannot see what they are doing. Victims, because we can take a chainsaw and clear a hillside of them. Still, crime reporters flocked to the trial of those who cut down the Sycamore Gap tree in Northumberland. Does our aesthetic appreciation of trees incline us to preserve them? Have trees in some way, indeed, cultivated that aesthetic sense?
Trees have profound agency over earth, wind, fire and water in ways we can only partly elucidate. Scientifically, trees are full of surprises. To try to apply physical laws to them is to invite disaster – there are so many species with their own peculiarities, and own laws of behaviour. Defining them chemically is equally risky – we will never catch up with the new compounds they invent while we sleep. Classifying them genetically is a slippery business. An occasional glimpse of root in a cave roof, the taste of an unusual fruit or a mirage-like cluster of palm trees in the desert, gives us a glimpse of the living unknown; a life form that shaped the world we grew into, that wove it around us into a peaceful form, but is still largely incomprehensible. And yet, when we reach out to grasp a branch, it fits into our hand.
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This is an edited extract from The Genius of Trees by Harriet Rix, published by Vintage (£25). Order your copy at guardianbookshop.com.