Fun fact : this was the (slightly hidden) premise for Splatoon.
Those happy, colourful descendants of squids and other marine animals are playing paintball over the ruins of our civilization, long after human extinction.
They worship an old fax machine they found, too, for some reason.
Yeah. It was revealed mostly through a couple of "scrolls", rewards in the single player mode.
There was a human fossil (somehow petrified while playing Wii U) dated 12,000 years ago, and documents from scientists warning about global warming and oceans rising.
Last scroll was a message from a scientist, "the professor", in the middle of the big extinction event 10,000 years before Splatoon. At that point all land life would disappear very soon. The professor did the logical thing and saved his cat.