Complex sentence: He was a member of Maurice Herzog's 1950 expedition to Annapurna, at which time he was 27, and he made the first ascent of Makalu with Lionel Terray on May 15 1955.
Split into simpler sentences: He was a member of Maurice Herzog's 1950 expedition to Annapurna, at which time he was 27. Couzy made the first ascent of Makalu with Lionel Terray on May 15 1955.