When did biologists confirm that living things are composed of cells?

Biologists confirmed that living things are composed of cells in the mid-19th century, particularly through the work of scientists such as Matthias Schleiden and Theodor Schwann, who formulated the Cell Theory in 1839. This theory established that all living organisms are made up of one or more cells, which are the basic units of life. Their findings built upon earlier discoveries by Robert Hooke in 1665, who first coined the term "cell" while observing cork under a microscope.