Complex sentence: The history of the Agency began during the time of growing concern on the dangers posed to the environment through careless human activities which prompted the United Nations to convene a conference in Stockholm on the human environment in June 1972.
Split into simpler sentences: The history of the Agency began during the time of growing concern on the dangers posed to the environment through careless human activities. These prompted the United Nations to convene a conference in Stockholm on the human environment in June 1972.