Manatees and dugongs are marine mammals of the order Sirenia. They originated in the middle Eocene period around 45 to 50 million years ago. They are gentle, plant-eating herbivores sometimes nicknamed "sea cows". The name manatee comes from the Haitian word manati. Today, their closest living relatives are elephants [Proboscideans], hyraxes [Hyracoidea] and aardvarks [Tubulidentata].