God decides mankind is so utterly corrupt that it's time to hit the reset switch and just flood the planet. Similar stories come up in folklore all over the world, from the ancient Greeks to the Babylonians, always with a huge flood that kills almost everyone, and often with mankind having to recover its population. For instance, in China, it's a goddess named Nuwa who stops the flood and creates humans out of clay.
Some are more clay-like than others.
In the Bible's version, God tells Noah that he is less of a dick than everyone else on Earth, and instructs Noah to build a really big boat. Really, really big. So big that it could hold at least two of every single animal on the entire planet. It rained for 40 days, flooding the world and killing off all life except that which was on Noah's boat. When the flood ended, all of the animals got off the boat and immediately started boning for their lives, because two individuals needed to repopulate their entire species.
At some point a duck wandered into the wrong tent and POW: Platypuses.