McCoy has somehow changed history (The City on the Edge of Forever)

GUARDIAN: (The doughnut pulses bright in time with the words) A question. Since before your sun burned hot in space and before your race was born, I have awaited a question. 
KIRK: What are you? 
GUARDIAN: I am the Guardian of Forever. 
KIRK: Are you machine or being? 
GUARDIAN: I am both and neither. I am my own beginning, my own ending. 
SPOCK: I see no reason for answers to be couched in riddles. 
GUARDIAN: I answer as simply as your level of understanding makes possible. 
SPOCK: A time portal, Captain. A gateway to other times and dimensions, if I'm correct. 
GUARDIAN: As correct as possible for you. Your science knowledge is obviously primitive. 
SPOCK: Really. 
KIRK: Annoyed, Spock? 
GUARDIAN: Behold. A gateway to your own past, if you wish. 
(A camel train passing a pyramid, ancient Rome - images appear inside the doughnut.) 
MCCOY: Killers! Killers! I won't let you get me! I'll kill you first! I won't let you get me! Assassins! Murderers! Killers! 
(Scott and a guard manage to grab McCoy, and Spock neck-pinches him. An army of lancers charge across the doughnut.) 
KIRK: Spock! If that is a doorway back through time, could we somehow take Bones back a day in time, then 
SPOCK: Relive the accident. This time be certain that the hypo accident is avoided. Look at the speed with which the centuries are passing, Captain. To step through on precisely the day we wish 
KIRK: Guardian. Can you change the speed at which yesterday passes? 
GUARDIAN: I was made to offer the past in this manner. I cannot change. 
(The US cavalry charges.) 
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.
Captain's log, no stardate. For us, time does not exist. McCoy, back somewhere in the past, has effected a change in the course of time. All Earth history has been changed. There is no starship Enterprise.