Marine life refers to plants, animals and other organisms that live in the sea or in the saltwater areas of coastal estuaries. Marine life can produce oxygen and sequester carbon. Much of the land life on Earth evolved from Marine life. Vertebrates, for example, first lived only in water, and later some of them evolved into amphibians. Other fish evolved into mammals and came to live on land, but some returned to the sea, such as seals, dolphins and whales.