This paper provides evidences of the evolutionary pathway followed by one of the main groups of marine angiosperms, the Hydrocharitaceae. Current molecular data has confirmed the aquatic origin of these plants. The Hydrocharitaceae group has a cosmopolitan distribution and is well represented in the fossil record in Europe and North America. Morphological and phylogenetic data has shown dramatic differences between the Hydrocharitaceae and the other marine angiosperms. Furthermore, it supports the hypothesis that aquatic monocot ancestors were able to adapt to a continuously changing environment caused by widespread continental flooding in the Cretaceous Period when seagrasses first occur, to a gradual regression of inland seas during the Eocene leading to subsequent adaptation to a completely submerged marine environment within the subfamily Hydriloideae.