Complex sentence: Cooper left cryonics activism in 1969, and was lost at sea in 1983, however, his activities with LES provided the basis for the formation of the first Cryonics Societies.
Split into simpler sentences: Cooper left cryonics activism in 1969, and was lost at sea in 1983. But his activities with LES provided the basis for the formation of the first Cryonics Societies.