Marine mammals are a K-selected relict megafauna that escaped the mass extinctions that affected their terrestrial counterparts at the end of the Pleistocene. Over the last 500 years they have been severely impacted as humans have mastered the marine environment. Some stocks and species went to extinction, and extinction of others was narrowly averted through international efforts regulating commercial exploitation. In the late 20th century new threats arose through contamination of the marine environment and these will be exacerbated and added to as human numbers double and economies grow over the next century.