Cell theory provides a very simple, succinct definition of life:
- All life forms are composed of one or more cells. - Cells only
arise from pre-existing cells. - The cell is the smallest unit
of life. The major contributors to cell theory were Matthias J.
Scheiden (often spelled Scheiden, Sheiden, Schlieden, etc, as
the Germans weren't too picky about how they spelled their last
names at the time) and Theodore Schwann, who coined the term.
Scheiden came to the conclusion that all animals are composed of
cells; Schwann took the next step and found that all plants were
as well. Of course, this is only considered a theory, and is
highly likely to change in the future. The question of "where
did the first cell come from?" (or DNA/RNA for that matter) is
little more than a "chicken / egg" kind of question, and forms
the basis of the scientific study of abiogenesis. It's obvious
that there must be some sort of exception to the rule that
"cells only arise from other cells," but this is how science
works. When we ob