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Didn't for a moment think you meant us, Raouf.

If you think porpoises are something, read about the Orca. They are the only other species on this planet considered to have a culture. They live in tribes, they speak to each other and they pass learned behaviors from generation to generation. Even more important, they learned a very long time ago not to eat humans. They knew they couldn't win a contest with us. In fact, that single behavior underlines their intelligence more than anything else.

Just to put the Orca in perspective, they rule the seas the way we rule everywhere else. When a fisherman asked what Orca eat, he answered, "anything they want." Their diet, for example, includes the largest sharks: the great whites and the hammerheads. My personal opinion is that the Orca did in the Megatooth Sharks, the sixty foot super great whites that ruled the seas for 20 million years. Call it a clash of titans. A sixty foot Megatooth battling a tribe (pod) of Orca. The Megatooth would put up one hellava fight and then then the Orca dine on its enormous liver.

Why is all this important? Mainly because Gliese 581 d (nicknamed Water World) probably has organic life. In fact, all we await is confirmation of that fact to say conclusively that we have found a living planet 20 light years away. And, since Water World is three billion years older than Earth, we can assume it has sentient life. Whether that life has evolved to become interstellar is another matter, entirely.

Still, remember Michio Kaku's caution: “Anything that is not proven to be impossible, is mandatory.”
Michio Kaku PHYSICS OF THE IMPOSSIBLE

Since it is 3 billion years older than Earth , might it not be equally plausible to assume that life on " Water World " evolved to the point where it polluted or warred itself out of existence several billion years ago ?

They were all abducted by aliens. All that was left was their pet Orcas. This is verified by the Beatles song, "Yellow Submarine". And aside from sharks and salt-water crocodiles, Orcas like peanut butter and jelly sandwiches.