Earlier quoted context omitted.
Motion sickness is body's way to protect itself against eating hallucinogens by mistake. Large mismatch between what your inner ear is feeling about your motion, and what your eyes perceive as your motion, imply possible hallucinogens, so your body is trying to make you throw them up. Humans haven't evolved riding in vehicles. Interior of car is not moving much relative to you, but your inner ear feels movement.
This seems very odd as by the time you start feeling the effects of an hallucinogen, your body has already absorbed them past the point of expelling them being effective.
Perhaps our distant ancestors also tended to eat a small bite first, then proceed with other foods, before consuming the remainder? For that matter the drug wouldn't be absorbed all at once. If it is toxic better to eject whatever hasn't been digested than continue absorbing it. There's a bonus psychological side-effect: vomiting is not pleasant and the individual (if they survive) will be more likely to remember what they ate and how sick it made them to avoid it in the future.