Complex sentence: He conducted a number of expeditions into tropical areas in the 1930s, and later gained widespread fame for his popular writings on nature and travel, and later on topics such as cryptozoology, especially sightings of the Yeti, and reports of extraterrestrial visitors.
Split into simpler sentences: He conducted a number of expeditions into tropical areas in the 1930s and gained widespread fame for his popular writings on nature and travel. Later he became known for writings on topics such as cryptozoology, especially sightings of the Yeti, and reports of extraterrestrial visitors.