Imagine if nature had evolved its own version of the modern blimp, or the old fashioned zeppelin. Picture a creature built like a huge hydrogen balloon, a thin tissue of living flesh and tenuous skeleton supporting vast empty sacs filled with lifting gas. A lighter-than-air animal much, much larger than the biggest whale or dinosaur that has ever lived in our world. An enormous living island, large enough to support a tenuous topsoil and bedecked in hanging vines and sprouting trees from its back.
This island drifts on the winds up in the sea of clouds, living for thousands of years without ever once touching the ground far far below. The plants and small animals that thrive on its back have lived there for countless generations. When the winds chance to bring two of these huge creatures together, animals clamber from one to the other, and birds fly the huge distances between the living islands. At first they were unwitting passengers, marooned in the sky when the islands first rose up to the clouds, or else temporary visitors, flying birds stopping to rest on long migrations. Over time however they have all evolved to fit this curious niche, forming a new ecosystem high above.
Beneath the island trail long, gossamer thin roots and tendrils, drinking water from the clouds to quench the thirst of it and its many passengers. It survives by living off the produce of the plants on its back, or perhaps a more direct symbiotic relationship; siphoning nutrients from their roots in exchange for water. All is not entirely peaceful however, and it keeps a wary eye on the skies and clouds for its larger carnivorous cousins, menacing shadows beneath the cloud tops.
How did these living islands evolve? What would drive them to the skies? How plausible is this ecosystem in the sky? Is the air thicker here and so floating easier perhaps? Could they survive off the plants on their own backs? Or something else?
Note that there are other questions about flying this or flying that and how might it work. This question is different, in that the lifetform can be anything and the focus is on the evolution and how such a thing came to exist.