How would the world be different if there was no photosynthesis?

Plants would require a different way to gain food, and some would go extinct or wilt many times before gaining strength back.

The most valid hypothesis for explaining the natural selection of bacteria is that of Dominique.

Antibiotics are, initially, molecules naturally synthesized by microorganisms to fight against bacteria competing with their environment. This created a pressure of selection on the bacterial populations, leading to the appearance of resistant strains. In fact, when an antibiotic is used, only bacteria with defense systems against this molecule survive (and reproduce).