First Frontier (Star Trek, Book 75)

While testing a new shielding device, the "U.S.S. Enterprise" TM is caught in the middle of a Klingon/Romulan battle. The "Enterprise" crew rescues a lifepod, and they are confronted by a Klingon who claims to know nothing of human existence. Convinced the Klingon is telling the truth, Captain Kirk hurries to Starfleet Headquarters in search of answers. But upon arriving on Earth, the "Starship Enterprise" crew finds that Earth is a vast jungle-like paradise where large, reptillian animals rule, with no signs of human life anywhere. Kirk must travel to the past in search of the key to the mystery, or face the destruction of the human race.