[ Alex looked confused at the question for a second, then shook his head, remembering. ]

Right. Sorry, 'course you wouldn't know.

Mars ain't the only colony in the solar system. Just one of the biggest ones, and the only planet-side one. Out in the Belt - the asteroid belt between Mars and Jupiter - there's millions of people that eke out a livin' clingin' to rocks. Ceres, Eros - big enough asteroids that ages ago they could build minin' stations on them. Only thing is, it's been a few generations since they were first built. Most of the people who lived out there grew up there, too. And they ain't even got as much gravity as Mars does.

On Mars, we all have to train at one gee - build our muscles up to get used to it, just in case. [ In case of a Martian invasion of Earth, he doesn't say. ] But in the belt, or out beyond 'er, they live at minimum gravity all the time. Their bodies don't quite grow up right. On Mars it just means we tend to get taller and leaner than your average Earther - but that effect is multiplied in the Belt.

Thing is, means no one on the Belt is ever gonna set foot on Earth. The gravity would literally crush 'em. So Earth loves to treat 'em like they ain't even human, anymore. Or at least a hell of a lot less than one.

[So did Mars, but he really didn't like thinking about that.]

So when they wanted to experiment with the protomolecule on people... [He trailed off to let Frank finish that sentence on his own.]