McCoy changes history (Star Trek)

KIRK: Strangely compelling, isn't it? To step through there and lose oneself in another world. SPOCK: I am a fool. My tricorder is capable of recording even at this speed. I've missed taping centuries of living history which no man before has ever (But McCoy has woken up, and dashes towards the Guardian.) SCOTT: Doctor McCoy! KIRK: Bones, no! (McCoy leaps through the doughnut and the pictures stop. He's gone.) KIRK: Where is he? GUARDIAN: He has passed into... what was. UHURA: Captain, I've lost contact with the ship. I was talking to them. Suddenly, it went dead. No static, just nothing. KIRK: Kirk to Enterprise. Scotty. SCOTT: Nothing wrong with the communicator, sir. GUARDIAN: Your vessel, your beginning, all that you knew is gone. KIRK: McCoy has somehow changed history. SCOTT: You mean we're stranded down here? SPOCK: With no past, no future. UHURA: Captain, I'm frightened. KIRK: Earth's not there. At least, not the Earth we know. We're totally alone.