Why Darwinism played an important role in shaping Nietzsche's Overman (Ubermensch)?

As Nietzsche states in Thus Spoke Zarathustra, mankind is placed in between a monkey and the Overman. Moreover, he states that the Overman is the final aim of men, and the process will go through 3 stages and 2 metamorphosis. From the Camel to the Lion and finally to a toddler who sits at the table, rolling dice with the Gods.