Good question! Most people have heard the old story about Isaac Newton coming up with the theory of gravitation when an apple fell on his head while he was sitting under a tree some time in the 1860s. The apple is said to have inspired him to figure out that there was a force called gravity that pulled the apple down from the tree. Scientists were starting to figure out some things about gravity almost 100 years before that, though. For centuries people believed that heavier objects fall faster than lighter objects. Galileo Galilei climbed to the top of the leaning Tower of Pisa in 1585 and performed an experiment to prove everyone wrong. He dropped two objects, one heavy and one light, from the top of the tower and, to everyone's shock, they reached the ground at the same time! This was the first proof that all falling objects accelerate at the same rate (a rate that experiments later showed is 9.8m/s²) no matter how heavy they are. So, while Newton usually gets all the credit, he owes a lot to Galileo's work from 100 years earlier.

That's often the way science works out: ground-breaking research building on all of the great research that has come before it.