Life, itself, evolved in Earth's primordial oceans about 3.6 billion years ago; these initial unicellular forms of life were likely chemoautotrophic bacteria, that arose near deep ocean vents, and cyanobacteria, that evolved in warm, shallow seas. Eukaryotic cells are thought to have developed by 2 billion years ago and, by the end of the Precambrian Era, 600 million years ago (MYA), complex, soft-bodied organisms (the Ediacaran fauna) and the first shelled marine life had evolved.
Based on fossil evidence, the diversity of shelled marine organisms exploded during the Cambrian and Ordovician Periods (550-440 MYA) and jawless fish had appeared by the end of the Ordovician; the first jawed vertebrates, including spiny fish and placoderms, appeared during the Silurian (440-400 MYA), when marine arthropods first crawled from the sea. Primitive sharks, amphibians, lungfish and boney fish arose in the oceans of the Devonian Period (400-350 MYA), having since diversified into a vast array of species; some evolved to breed in freshwater streams while others, stranded in shallow, inland seas, became freshwater species.
Reptiles evolved from terrestrial amphibians late in the Paleozoic Era (some 300-250 MYA) and turtles appeared in the Triassic (225-190 MYA); some turtles, of course, have since returned to the sea. In like manner, snakes evolved from lizards during the Cretaceous Period (about 70 MYA) and some became marine species. Primitive cetaceans (whales and dolphins) split from terrestrial ancestors and returned to the sea during the Eocene Period (about 50 MYA), as did sirenians (dugongs and manatees), the only marine mammalian herbivores. Finally, pinnipeds (seals, sea lions and walruses) diverged from the bear lineage during the Miocene (25 MYA) and returned to the sea by the Pliocene (5 MYA).
