I have always wondered how mammals, our live-bearing creatures with recently-discovered impressive brains, got back into the sea.
Mammals, from all accounts, including Darwinians, were descendants of sea creatures who, through fin and gill transformations, evolved into something resembling walking and breathing creatures that then stepped up onto the land and preferred berries and heron eggs to seaweed. Made sense to me.
So my question, is why did some of these mammals return to the sea? I guessed that a million years ago, they actually became even smarter and saw the rise of land mammals, including man-like creatures and Japanese and Chinese fish mongers, who would became the human-equivalent of meteor attacks. So they puffed up their lung-like organs and did cannonballs back into the oceans. Extinctions in the sea are rare. Exceptions are of course mermaids, who once existed until they got tired of sailor abuse, and turned themselves into manatees, who are now safe from extinction because ... well, nobody bothers with them (ever try to hug one).
Most recently, in 2016, scientists made a distressing announcement: There were fewer than 30 vaquitas still alive from the thousands living in the Gulf of California only a decade ago. Vaquitas are small porpoises found only in these Mexican waters. According to World Wildlife Fund Mexico, they are disappearing at an alarming rate, a situation that can be traced to something really mundane: soup made with fish swim bladders. I think we can all relate to how scrumptious that is. The Chinese and the Japanese love it. The swim bladder, also known as maw, is a gas-filled organ fish use to stay buoyant underwater. Leave it to the Chinese to eat SCUBA gear.