It is quite apparent that there is an aspect of Darwin’s discoveries which has never penetrated to the mind of the general public. It is the fact that once undirected variation and natural selection are introduced as the mechanism controlling the development of plants and animals, the evolution of every world in space becomes a series of unique historical events. The precise accidental duplication of a complex form of life is extremely unlikely to occur in even the same environment, let alone in the different background and atmosphere of a far-off world.
In the modern literature on space travel I have read about cabbage men and bird men; I have investigated the loves of the lizard men and the tree men, but in each case I have labored under no illusion. I have been reading about a man, Homo sapiens, that common earthling, clapped into an ill-fitting coat of feathers and retaining all his basic human attributes including an eye for the pretty girl who has just emerged from the space ship. His lechery and miscegenating proclivities have an oddly human ring, and if this is all we are going to find on other planets I, for one, am going to be content to stay at home. There is quite enough of that sort of thing down here, without encouraging it throughout the starry system.
- Loren Eiseley, The Immense Journey (1959)