Earth in Twilight — Doris Piserchia

Laredo Space Base hadn't sent a ship to Earth for hundreds of years before the Project Deep Green survey craft was launched. Only one thing was known: the planet humankind had so long ago vacated was a wasteland with nothing on it but poisonous flora and small, murderous denizens.

That's what they taught astronaut Ferrer Burgoyne and as a result he was totally unprepared for the teeming jungle stretching farther than his eyes could see. He was even moer unprepared for the slightly green humanoids who greeted him. Obviously the scientists of Old Earth had done more in their labs than just mess around mixing human and plant cells. As sure as Ferrer Burgoyne was an astronaut the new men of Earth were the descendants of those hidden, forbidden experiments.

How then could Burgoyne continue his mission: to defoliate Earth with the deadly chemical Deep Green and prepare Earth for the return of his species?