-- IF I evolved on that planet, then my body is perfectly matched to the conditions and the environment over there, and I am approximately just as happy and care-free la de da over there as I really am on Earth, and I can't imagine living anywhere else.
-- IF I evolved on Earth, and I move this body to a planet with less surface gravity, then there are all kinds of complications.
. . . Even if the atmosphere over there has the same gases in the same ratios as Earth's atmosphere, the lower gravity means that the atmosphere has lower weight. In turn, that means that the surface pressure is lower, and I have to work harder to take in enough oxygen to keep my Earth body functioning. Breathing alone could be a full-time job.
. . . The acceleration of gravity is less than 9.8 m/s² over there. This means that when I drop something, it falls slower than I'm used to, and I can usually grab it before it hits the floor. When I lift something with the normal force of my Earth arm, it jumps up faster than I expect. Until I get used to things, I'll probably overshoot, lift things too high and too fast, maybe spill the coffee etc.
. . . With my Earth muscles, almost everything takes less force. It's a lot easier to walk around. I bob up and down more than usual, and my steps are longer. When I run, it's really funny. Every time I take a step, I sail several feet into the air, and come down farther than I ever did, but it all happens in slow motion.
All of this is fun while it lasts, but it doesn't last long. My Earth body gets adjusted to the new planet. Before long, I actually lose muscle mass AND bone, so that if I were to go back to Earth, I would be so weak that I'd have to be carried around for a while.
We know this from what happens to the astronauts who spend 6 months or a year in zero G on the International Space Station. Even when they do resistance exercises for an hour a day, they come back with less muscle strength and less bone mass.
