It's 250,000 years from now and people are moving back to the sea.
The land is crowded with a human population of two trillion. Humans have reached the Malthusian limits of what the land can support. Enough real estate for a bed is too precious for many, and the homeless are in the billions.
Living outdoors from one generation to the next, breeding among themselves on the streets, the homeless have begun to diverge physically. They are becoming a new species.
And nowhere more clearly than among the homeless living on the shore. To compete with commercial fishing and one another, a fringe spends its time literally in the water, as close as possible to edible fish and plants.
Competing for food means getting there first, and getting there first means swimming right up to your catch. Webbed toes help, especially for those too poor to buy fins. The ability to hold your breath longer follows quickly. Larger eyes to see in dark water. New digestive enzymes to process seaweed and kelp. Faster reflexes to catch fish with your hands. It's a new race.
But not the only one. The same thing is happening in muddy estuaries, in sulfur baths, in the jungle canopy, underground. In the world after the mass extinction caused by humans, and the followup population bubble, there's an explosion of species stand upright and have opposable thumbs, as post-humans evolve to fill the ecological niches that their ancestors emptied.