Complex sentence: In the course of their investigations, they mounted expeditions to Mexico to study the religious use of mushrooms by the native population, and became the first westerners to participate in a Mazatec sacred mushroom ritual.
Split into simpler sentences: In the course of their investigations, they mounted expeditions to Mexico to study the religious use of mushrooms by the native population. They became the first westerners to participate in a Mazatec sacred mushroom ritual.