You already know about capitalizing Earth, right? (If you mean the planet rather than a synonym for dirt, capitalize it. Those Jovian gasbag critters don’t refer to their home as jupiter, lower-case, and there’s a difference between Mercury and mercury. 🙂 )
Radiation makes the surface of the Earth uninhabitable, leading to the creation of floating cities. Thousands of years later, a scientist discovers that a new breed of surface-dwelling humans has evolved.
*sigh* I know I keep saying I’m going to stick to the mechanics of writing and ignore such “trivia” as Bad Science, but there’s only so much I can take before I snap and… explain why something is scientifically unsound.
(Feel free to ignore the rest of this post…)
First, water can also become radioactive. If the entire land surface of the Earth is uninhabitable due to radiation, the surface of the ocean isn’t a safe place to set up, either. Even if the oceans were all right in the beginning, there’s this thing called fallout (if you’re young, you may have heard of this from your parents, who may have grown up under the threat of The Bomb and its attendant horrors), which doesn’t give a damn whether it’s falling on land or on sea. Radiation doesn’t stay put. Air currents, water currents… If you’ve got that much radiation, you’re screwed unless you can find a way to shield yourself from it.
Second, anything floating on the ocean is on the surface; it’s just not on the surface of the land. The sample does say these are floating cities, not domed cities protected from the radiation by being under many meters of water. (That would make too much sense, I suppose.)
Third, evolution cannot produce a new species of humans in a few thousand years. The humans who first colonized North America (more than 12 thousand years ago) were the same species as the humans who live here now, and it has been a very long time since we split off from our closest relatives on the evolutionary tree. Had that part said a new society or a new culture, no problem, but a new breed doesn’t work (and not just because it’s BS, either).
And… if the land is radioactive, what did they build those floating cities with? Did they somehow mine raw materials from the ocean floor? ‘Cause if the land is radioactive, you really don’t want to be using metal and whatnot you dig up there. How did they have time to build all these cities (either you have enough room for a viable population — I think the most common minimum estimate is around twenty thousand — or you’d end up with changes, one way or another, to that gene pool) if the cities weren’t built until after the radiation had made the land uninhabitable?