<em>Posidonia oceanica</em> (Linnaeus) Delile is one of the most important endemic species of the Mediterranean Sea. The long phylogenetic pathway of this species deserves special attention because its evolutionary history allows us to think back over the debated issue of life evolution on Earth. Hence, this marine plant, by terrestrial origin, was born from an ancient ancestor named <em>Posidonia cretacea</em> Hosius & Von der Marck. This seagrass species, currently extinct, lived in the coastal waters of the Tethys Ocean about 100 million years ago, during the geological times of the late Cretaceous. In the following Cenozoic Era, during the Miocene period, in the time lag from sixty to forty million years ago,a long process of allopatric speciation by separate areas led to eight species of Posidonia genus developed in the northern and southern hemispheres. <em>Posidonia oceanica</em> is established in the boreal marine regions, so representing, within the long and winding road of life evolution on Earth, a typical example of paleo-Mediterranean relict. Nowadays, <em>Posidonia oceanica </em>meadows perform important ecological functions in the Mediterranean Sea, not only for biodiversity conservation but also to mitigate climate change. In this way, meadows play a leading role in carbon sequestration, reducing the concentration of carbon dioxide in coastal waters. Lastly, the meadows of <em>Posidonia oceanica</em> promote an effective removal process of plastic debris from Mediterranean sea waters.