The invention that was an early step in the discovery of cells was the microscope.

Here's why:

* Before the microscope: People couldn't see things that were too small for the naked eye.

* The microscope's impact: The invention of the microscope (around the 16th century) allowed scientists to see tiny objects, like cells, for the first time.

* Early observations: Robert Hooke, using an early microscope, observed the structure of cork and named the tiny compartments he saw "cells" because they reminded him of the rooms in a monastery.

While the microscope wasn't the only factor in discovering cells, it was a crucial tool that enabled scientists to see the building blocks of living things.