The following is an excerpt from the new book Beyond Words: What Animals Think and Feel by Carl Safina (Picador, 2016):

How might we discern an elephant’s or a mouse’s sense of the world? Elephants and mice might not tell us what they’re thinking. But their brains can. Brain scans show that core emotions of sadness, happiness, rage, or fear, and motivational feelings of hunger and thirst, are generated in “deep and very ancient circuits of the brain,” says the noted neurologist Jaak Panksepp.

Researchers in labs can now trigger many emotional responses by direct electrical stimulation of the brain systems of animals. Rage, for example, gets produced in the same parts of the brains of a cat and a human. Further evidence of shared experience: Rats can become addicted to the same euphoria-producing drugs that humans get addicted to. Dogs with compulsive behaviors show the same brain abnormalities as humans with obsessive-compulsive disorder; they respond to the same medications. It’s the same disease. Under stress, other animals’ blood carries the same hormones that the blood of stressed-out humans does. Crayfish hid for extended periods after getting mild electrical shocks and showed elevated levels of serotonin—evidence of clinical anxiety. When researchers gave the same crayfish a drug commonly used to treat humans suffering from anxiety—chlordiazepoxide—they resumed normal crayfish activities and explorations. The researchers wrote, “Our results demonstrate that crayfish exhibit a form of anxiety similar to that described in vertebrates.” Having subjected crabs and lobsters to worse treatment than mild electric shocks, I find this discomfiting. Perhaps try the pasta. Anxiety in many species apparently shares ancient chemical systems largely unchanged during evolution. Makes sense; being afraid of venturing out while danger is lurking has obvious survival value for all kinds of animals.

Complex animals have inherited very ancient emotional systems. The genes that direct our own bodies to create the mood-making brain hormones oxytocin and vasopressin, for instance, date back at least seven hundred million years. They “likely arose when animals became mobile and started to make experience-based decisions,” wrote researchers. “When a worm is suddenly illuminated,” Darwin wrote, it “dashes like a rabbit into its burrow.” But if you keep scaring it, the worm stops withdrawing. Such apparent learning suggested to Darwin “the presence of a mind of some kind.” Watching as worms evaluated the suitability of objects for plugging their burrows, Darwin offered the idea that worms “deserve to be called intelligent, for they then act in nearly the same manner as a man under similar circumstances.” Ridiculous? Consider this: “The same neural mechanisms are at work in worms and humans,” wrote S. W. Emmons in a 2012 article with the intriguing title “The Mood of a Worm.” He’s referring to the tiny one-millimeter-long C. elegans—the elegant nematode. Here’s the thing: the worm has nearly the same suite of genes that underlie the nervous systems of humans, giving the worms “connectivity patterns also found in the human brain.” C. elegans has just 302 nerve cells. (Humans have roughly 100 billion.) Yet C. elegans produces a motivating chemical similar to oxytocin, called nematocin, and its function is familiar. It makes the worms seek sex. Mutant males lacking it spend less time looking for a mate, take longer to recognize one, then initiate copulation more slowly “and execute poorly.” Poor worm! Emmons, who is a professor at the Albert Einstein College of Medicine, leaves us with this insight: “Just as today’s major roads and highways may once have been ancient trails, biological systems can retain essential features derived from their origins.” He cautions, “It is a mistake to consider small invertebrates as primitive.” Oxytocin drives bonding, and it makes elephants and many other species act social or sexual. Block the hormone; many mammals and birds lose interest in socializing, pairing, nesting, and contact. Oxytocin and opioid hormones create sensations of pleasure and social comfort in many species, including humans. Given a sniff of oxytocin, human fathers get more playful with their babies, increase eye-to-eye gazing, and show greater interest in the child. This is the chemistry of bonding.

When we do something that we know is a bad idea, it’s often because hormone-flooded ancient parts of our own brains disable our intellectual override switch. Hormones can unlock—for instance—the cages that contain deep sexual feelings, setting loose behaviors we become powerless to resist, leaving rationality bound and gagged while emotions hijack our minds. Sex is often so risky and costly that we might never reproduce if our brains weren’t chemically sparking urges for obtaining our next fix. It sounds quite animal, doesn’t it? And it feels that way—because it is. It so deliciously, so frighteningly is. In 1883, George John Romanes recognized that “with nerve-tissue in a jelly-fish, an oyster, an insect, a bird, or a man, we have no difficulty in recognizing its structural units as everywhere more or less similar.” Sigmund Freud observed that the nerve cells in a crayfish were basically the same as the nerves in human beings. Freud grasped that the nerve cell is the signaling unit of the animal nervous system. As Oliver Sacks explains, neurons “are essentially the same from the most primitive animal life to the most advanced. It is their number and organization that differ.” Uncertainty, anxiety, worry, pain, fear, terror, defiance, defensiveness, protectiveness, anger, disdain, rage, hatred, distrust, disappointment, reassurance, patience, persistence, interest, affection, surprise, happiness, delight, joy, exuberance, sadness, depression, remorse, guilt, shame, grief, awe, wonder, curiosity, humor, playfulness, tenderness, lust, longing, love, jealousy, loyalty, compassion, altruism, pride, vanity, shyness, calm, relief, disgust, gratitude, abhorrence, hope, modesty, sorrow, frustration, fairness—Is it possible that humans alone feel all these things, that elephants and other animals feel none of them? I don’t think so. If we deny the possibility that they have feelings, and they actually do have feelings, we’d have been wrong. I think we’ve been wrong in exactly that way. I’m not suggesting that humans and elephants have all the same emotions. Self-loathing seems uniquely human.