Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Scientists think that protists are the oldest eukaryotes. If so, they must have evolved from prokaryotic cells. How did this happen? The endosymbiotic theory provides the most widely-accepted explanation. That's because it is well supported by evidence. Answer: protists
What do many scientists think are the oldest eukaryotes?