Q: Given a scientific passage and an answer, generate a question for the given answer.
Passage: Prokaryotes were the first living things to evolve on Earth, probably around 3.8 billion years ago. They were the only living things until the first eukaryotic cells evolved about 2 billion years ago. Prokaryotes are still the most numerous organisms on Earth. Answer: prokaryotes
A:
What were the first living things to evolve on earth?