What rare materials are needed for life on earth ?
Steven Pinker , in his book " How the Mind Works " , cautions that the idea that evolution of life ( once it has reached a certain minimum complexity ) is bound to produce intelligent beings relies on the fallacy of the " ladder of evolution " : Since evolution does not strive for a goal but just happens , it uses the adaptation most useful for a given ecological niche , and the fact that , on Earth , this led to technological intelligence only once so far may suggest that this outcome of natural selection is rare and hence by no means a certain development of the evolution of a tree of life .