Flies are very different than humans, but they are built using the same building blocks and processes.
It is not belief it is observation: humans are composed of cells that contain chromosomes. Genetic data is mixed with errors during reproduction (both with flies and humans) resulting in different characteristics in the individuals of the next generation (observable with flies and humans)
Sexual attactiveness of individuals will depend on their genes and their environment (also based on observation), which will impact their number of offspring, effectively selecting some genes and discarding others.
All of this is based on simple observation and you sée that belief has no place in this line of reasoning.
Of course there is more to flies and humans than evolution, yet evolution is such a simple process that it applies to both! Nature is truly amazing