Check out the work of Ilya Prigogine - he won a Nobel prize in the 70's for his work on self-organizing complexity. There is a selection force in the universe for increasing complexity, being driven by the dissipation of energy. When you have systems in non-equilibrium, there is a strong pressure to explore the possibility space to find ever more efficient ways to dissipate energy.
A bacteria the size of a grain of sand dissipates much, much more energy than the grain of sand, and so there is a very strong "preference" for bacteria in this regard.
A bacteria the size of a grain of sand dissipates much, much more energy than the grain of sand, and so there is a very strong "preference" for bacteria in this regard.