Spontaneous generation
The first step was taken by the Italian Francesco Redi. In 1668, Redi proved that no maggots appeared in meat when flies were prevented from laying eggs.  From the 17th century onwards it was gradually shown that, at least in the case of all the higher and readily visible organisms, the previous sentiment regarding spontaneous generation was false. The alternative seemed to be omne vivum ex ovo: that every living thing came from a pre-existing living thing (literally, everything from an egg).