Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: Eventually plants evolved. Plants produce oxygen as a product of photosynthesis. Oxygen spread around the planet about 2.5 billion years ago. Many organisms died off because they could not handle the oxygen. But this development was extremely important for other life. Animals need oxygen to breathe. If photosynthesis had not evolved there would be no animals. Answer: photosynthesis
A:
Because it produces oxygen, what plant process was necessary for the evolution of animals?