system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some people get seasick and some don't?
gpt
Motion sickness is caused by conflicting signals from 

a: your eyes

b: your sense of balance

When your eyes see the deck of a ship as stationary, and your balance tells you "shit's moving, yo!" you get confused.

When we were cavemen, such conflicting signals primarily appeared whenever we'd eaten something toxic/poisonous, and hence our body responds by throwing up - getting rid of the poison. Those who did so, had a statistically better chance at surviving and thus having more offspring and passing on the genetic predisposition of motion sickness.

Apart from that, it can be mediated to some extent by not having an empty stomach or by dulling the balance system (which is what most motion sickness drugs do).