After mixing the album "Here at Last... Bee Gees"... Live, the Bee Gees began recording songs for what was to be their next studio album in 1976: "Children of the worl"d. But a call from Robert Stigwood asked them for songs for a film he was producing. The Bee Gees agreed to his request and gave him five songs, one of which was "How Deep Is Your Love" (source Wikipedia).