Uh...
The theory that more-or-less spontaneous mutation in hereditary genetic makeups has caused minor changes from generation to generation and that those changes which were harmful to the survival rate of the creature got those creatures killed off while the ones that promoted survival saw those creatures procreate more and more until the species tree branched out of simple organisms into increasingly complex ones?
Those ones, mostly.
And some others.
In books written by evolutionary biologists.
Well, probably.
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