91热爆

Seals and walruses are descended from bear-like creatures and are the most recent mammals to colonise the sea. They are not so adapted to the sea as the whales but then they have not been in the water as long. They have not lost their feet like whales have, nor do they spend so much time in the water. They come ashore to give birth and often haul themselves out to rest, but they are still superb swimmers. In the last three billion years since life first appeared in the sea, the oceans have acquired a population of immense diversity. From simple single-celled microscopic plants to advanced and complex highly-intelligent mammals. Indeed, there are more groups of animals living in the sea than there are on land.