There is no such a thing for any theory in science as "foundational evidence."
Various ideas are suggested to explain observable phenomenon, then evidence is gathered to support the hypothesis.
I think you must mean what is the foundational observations which the idea of evolving seem a possible explanation.
Along those lines, I would say that finding sea shells in rock layers of earth, high up in the mountains convinced Leonardo Da Vinci that the earth was way older than people thought.
Then, the layers of rocks that contained fossils not present in layers below and above suggested that life arose, disappeared, and started over again many times.
These were the foundational observations that gradually lead to hypothesizing evolution of complex life from more simple.
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