Well many millions of years ago there was no Antarctica. Perhaps a civilization came about when there was only one continent, which has happened a few times throughout the earths history.

Why would a civilization even be terrestrial, wouldn't an ocean living civilization be even more likely?

Why does a civilization need to invent agriculture. Perhaps ever evolving hunting technics or even just a sudden massive change in diet could free up enough time for a mass population of a species to start concentrating their efforts on cross societal constructions.

I'm not saying that there was an ancient aquatic civilization of squids that mutated plankton to swim to their mouths. But if we want to ask such an extraordinary question, we must not limit our self in such a way that it makes it easier to deny it.