Piscanthropus profundus, is a deep sea-dwelling descendant of the original aquatic, from 5 million years (the 50020th Century), evolved to live life in abyssal zones of the oceans and is the only surviving tetrapod since the travelers of the stars came and left, from Man After Man: An Anthropology of the Future.
Throughout the oceans of the world there is a network of volcanic ridges. New crust is constantly being generated here, built up from hot material welling up from below Earth's surface. The newly-formed crust is continually moving away from the ridge axis as even newer molten rock constantly forces its way up. This is the mechanism that moves the continents and alters the geography of the planet.
The seawater along these ridges seeps into the newly-formed crust, is heated by the volcanic activity, dissolves many of the minerals that it finds there, and erupts once more as hot springs on the ridges. The instant cooling brings the minerals out of solution forming dense smoke-like clouds of suspended chemicals in the water around the vents.
The chemical energy present around these "smokers" is immense, and bacteria thrive on it. Traditionally, organisms have relied on the energy of the sun to produce their food energy. Plants have used their chlorophyll to harness the sun's energy and use it to make their own food from the gases of the air and the minerals of the land. Animals have also eaten the food produced by the plants, and other animals have eaten the herbivores. All animals die and decay into the gases of the air and the minerals of the land, which are turned back into food by the plants. The sun's energy turns this wheel.
Around the smokers, well away from any light of the sun, the chemical energy is used by the bacteria to produce their own food. Simple, unicellular organisms feed on the bacteria. More complex multicellular organisms feed on these, and so on. Gigantic worms and blind crabs thrive in these hot oases in the cold dark depths of the barren ocean - descended from remote ancestors that were once part of the sun-driven ecosystem far, far above them.
There is also another creature, only come to the smoky oases in the last few million years, and unnoticed by anything on the surface. This creature has a fishlike body that allows it to swim, and prehensile hands that help it to feel its way about and find its food. Its total blindness is no disadvantage in the solid blackness, but it has a sensitive organ on the top of the head, developed from an organ that its remote ancestors had forgotten, that is sensitive to heat and can give a temperature-based picture of the surroundings. More importantly, this creature has a brain that is complex enough to give it an intellect. This intellect tells it that something strange has taken place far above it.
Maybe someday it will be possible for its descendants to travel upwards, and even possible for them to live in conditions that are totally alien to it, if they can change enough.