Humans probably shouldn’t be capitalized in the first sentence. If you prefer to capitalize the common name of your own species (the way we generally don’t capitalize the common, non-scientific names of any other species), be consistent about it, and capitalize it at all times.
Earth-bred needs to be hyphenated. On the other hand, maybe I ought to be grateful it was spelled correctly; I’ve seen bread used in place of bred far too many times, as in “My dog is a pure-bread poodle.” *shakes head* Kudos, too, for correct use of a colon in the first sentence and for correctly capitalizing Earth.
Earth is actually an animal sanctuary to preserve a critically endangered species: humans. Today the first reintroduction of Earth-bred humans back into the wild will take place.
That second sentence is a bit awkward, isn’t it? How about Today will be the first reintroduction of Earth-bred humans into the wild, or something like that?