The cell theory is 150 years old, sort of—it's not really a theory, more a set of ideas initially promulgated back in 1839, a culmination of observations by many scientists using the relatively new tool back then, the microscope.
At any rate, here's a textbook version of it:
- All organisms are made of one or more cells.
- The cell is the basic building block of all living things.
- All cells come from pre-existing cells. Omnis cellula e cellula.
The first two parts are credited to Theodor Schwann and Matthias Schleiden, apparently inspired by an after-dinner coffee chat. Schwann published it (and neglected to mention Schleiden) in 1838 with a third part, something about “free cell formation,” akin to spontaneous generation. Rudolph Virchow proclaimed omnis cellula e cellula in 1858, now usually listed as the third tenet of the cell theory.
Still, each time I bring it up in class, I hope a student will challenge it. I rattle on about the Earth's finite age, about the vastly different planet it was a few billion years ago, then how, according to the cell theory, all cells come from pre-existing cells.
The obvious question—and school has a way of discouraging true questions, even the obvious ones:
A little less obvious, but still powerful question:
If a cell formed from a certain set of conditions once, how come it didn't happen hundred or millions or billions of times other times?
Maybe Scwhann's free cell formation idea was not so crazy after all.
Finally, and now colleagues may wander away as I don my tinfoil hat:
Science is not knowing the answers, science is observing and questioning and putting together some framework to explain what we observe. Science is, after all, telling stories.
I'd rather hang with the skeptics.