I used to go scuba diving, but I routinely got seasick on the boat. Since I had nothing better to do while leaning over the water, I wondered why I had to go through this wretched experience. I understood the origins of physical pain– an animal that didn’t realize it had sprained an ankle would likely hurt itself even more rather than waiting for it to heal. But why should I feel nauseous when on a boat? I wasn’t being hurt by the waves, so this incapacitating condition wouldn’t have provided any advantage to my ancestors and therefore shouldn’t have been favored by natural selection.

I’ve heard some proposed explanations for this mystery. A woman once told me that she thought Motion Sickness was a relic of our tree-dwelling ancestors. Young monkeys are probably similar to young humans in the sense that they’re full of energy and devoid of caution. Motion sickness acted as negative reinforcement that discouraged them from swinging through the trees too wildly and losing their grip on the branches.

Another explanation comes from Alastair Reynolds’s novel Redemption Ark. He notes that evolution shaped our genes for many generations before boats and other moving vehicles were invented. Being inside a moving vehicle isn’t a situation evolution ever had to deal with before, so we haven’t been optimized for riding in them. Normally, our sense of balance is maintained by the vestibular system in our inner ears combined with our vision. You can test this by standing on one foot with your eyes open, then close your eyes and find out how much harder it is to keep your balance.

It’s usually impossible for your visual cues to contradict the signals from your inner ears– unless you’re in a moving vehicle. Then the deck of the boat appears stationary to your eyes, but your inner ear insists that it’s moving. It’s not obvious to me why this conflict results in the specific symptoms of motion sickness, though. For instance, why does seasickness manifest as nausea rather than a headache?

Reynolds suggests that motion sickness arose as a defense against hallucinogenic plants. An animal that accidentally eats peyote will experience hallucinations that cause the inner ear to conflict with visual cues just like being inside a moving vehicle. This state of intoxication is potentially deadly to a prey animal who wouldn’t be alert enough to notice a stalking predator. Because the psychoactive chemical is absorbed through the stomach, throwing up is the quickest way to end these hallucinations.

So whenever I get seasick, I’m experiencing the side effects of an evolutionary war between plants and animals. Plants evolved thorns and disorienting chemicals to deter animals from eating them. Animals, in turn, evolved a physiological response to these chemicals which is accidentally triggered by riding on a moving vehicle.

Interesting idea, but how can it be tested? The only falsifications I can think of are that hallucinogenic plants had to have evolved before motion sickness, and species whose ancestors never encountered these plants wouldn’t get seasick. Those seem nearly impossible to verify, but some people seem to be immune to motion sickness, so their genomes could be compared to people like me to identify the genes responsible. I’ve also briefly looked into the role of the area postrema, the region of the brain that controls vomiting. Reynolds suggests this part of the brain can be activated by a conflict between the inner ear and visual cues, but this page notes that it lies outside the blood-brain barrier which suggests that it induces vomiting based on detecting toxins in the blood rather than as the result of sensory conflicts.