It isn't a premise - it's an inference based on the theory of common descent. If common descent is true then if you trace it back far enough then you eventually end up at a first lifeform. Various lines of evidence suggest this would be single celled.
inference based on the theory of common descent
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Makes no diffrence if the word premise or inference is used. In either case it is impossible to test and observe. Therefore Darwin's hallucinations cannot be accepted as a theory either.