What were the main discoveries in Biology?
Here are a few things coming to my mind


Discovery of cells by Robert Hook
Discovery of bacteria by Antonie Philips van Leeuwenhoek
Discovery of antibiotics by Alexander Flemming 
The basis of heredity by Gregor Mendel
Evolution (On the origin of species by C. Darwin)
Watson and Crick discovery of DNA structure
Vaccination
The concept of ecosystem