Natural selection is the process by which an organism uses its traits to survive and reproduce. In this mechanism of evolution, organisms have favorable traits for survival and reproduction. The more adapted an organism is to its environment, the more likely it is to have heritable traits that are passed down to new coming offsprings, and really, this also applies for the likeliness of the organism to survive and reproduce in general. The more these heritable traits are passed down and used with offspring, the more common the favorable traits are in the community.
I believe the answer you are looking for has to do with cells, they are the basic building block of life and are the smallest life that can support itself.
