... That explains it.

[ Turning to face him... ] I have no idea how far in the future it happens for you, but at one point humanity went extinct due to rising sea levels and total warfare. Thousands of years later, sea creatures like jellyfish and cephalopods emerged from the oceans and evolved to take humanity's place.

[ All of this said matter-of-factly, like they're summarizing ancient history for a youtube video. ]