little green men... and women

In Brian Aldiss' 1962 masterpiece Hothouse the Sun is in its final phase before going nova. On earth, increased heat means that plants have taken over the earth (hence the title). New, deadly plants have evolved, plants that fly, vines that slash at moving objects, all kinds of variations of the venus flytrap. All this takes place in a seemingly bottomless forest, which has only one kind of tree: a successful tree species has destroyed competitors and taken over the planet as one huge, knotted, labyrinthine organism. It is, effectively a single tree.
High in the branches of this forst/tree live the descendants of modern humans. No longer masters of the world, they are tiny, green, and spend their short lives hiding from predatory plants.
The story centers Gren, a young man who inadvertantly gets a parasite lodged in his brain. This parasite talks to him about his heritage, and sends him on a quest around the world to recover man's forgotten greatness.
The future of mankind described by Aldiss may seem to be pessimistic. After all, it is rather depressing to think that the great works of mankind amount to nothing in the long run. At least, unlike Wells, or some evolutionary theorists, there is no beautiful master race.
Aldiss' story, however, far from depressing, is in fact enlightening and reinforces the value of the lives that we have today, rather than the chimeras of tomorrow. In the course of his journey, Gren learns what is really important: love, and an appreciation of the here and now, rather than the pursuit of ambitious dreams. Such dreams are mirages, receding forever out of reach. In Hothouse, those who chase their dreams of future greatness are represented by a group of people who are trying to escape the world before the sun goes out.
Gren rejects their lofty aspirations, choosing instead to settle down with a mate and live out his life on earth.