I believe I understand the theory as much as possible for a beginner. Meaning that the theory is that there is one or more starts of life from something that happened on the earth, and these things replicated and eventually evolved to become plants and animals.
As a description of evolution 101, this description is missing the most important aspects. I also see a common misunderstanding in your description.
Evolution theory does not in fact say "things evolved". In fact, that things evolved (ie: species share ancestors and life changes over time) are part of the set of
facts that evolution theory explains.
Evolution theory (today at least) is not in fact the assertion that species share ancestors. That species share ancestors is a
genetic fact. A fact that requires an explanation.
Evolution theory is a model that addresses
the mechanism of evolution.
And that mechanism in a simplistic nutshell is:
- reproduce with inheritable variation (mutation)
- survive (natural selection)
- repeat
If tomorrow, this
mechanism is shown to be false and / or inadequate... the
facts of genetics still exist. They would still require an explanation. Species sharing ancestors would still be a genetic fact.