I’ve been wondering lately about how exactly whales and dolphins evolved as mammals in an aquatic environment. As I understand it, the evolutionary tree goes amphibians from bony fish, reptiles from amphibians, and mammals and birds split off separately from there. So how exactly did whales and dolphins make it back to the oceans, evolving from (I assume) land roaming reptiles? Were there salt water reptiles for them to evolve from?
First, there was a bit of a discussion about the nature of the tree and who branched off of whom in this thread (starting about halfway down).
Second, aquatic mammals evolved from full-fledged mammals, not from any reptilian (or reptile-like) ancestors.
Third, in addiiton to Mjollnir’s link, you can read more about whale evolution here, here, a little blurb here (near the bottom), and here.
As an interesting side-note, I read somewhere (it may have been a Cecil column, but I’m really not sure) that despite the highly evolved brains of many aquatic mammals, they can’t really evolve much further (on, not in the build-cities, make cars, design high-tech porography aperatures sense) because of their inability to create fire. The obvious impact of a fireless society is that you can’t create an energy source for anything mechanical, and…well, it’s been a long time since I read the column, and now that I think about it I may have read it in an old Encyclopedia Britannica. But, the moral of the story is: No Fire = No McDonald’s on every corner. Or maybe that’s DEvolution.
True Phocid seals (Family Phocidae swim with the legs and have no external ears) are believed by some to be related to mustelids (Weasel family), while Otariid seals (Familiy Otariidae, swim with the forelimbs, have external “ears”) evolved from bear-like carnivores. Other scientistd disagree, considering all “seals”, or Pinnipeds to be evolved from a common land mammal carnivore.
Walruses, Steller’s Sea Cows (extinct), Manatees and dugongs (Sirenia) are believed to be evolved from the same protoungulate mammal stock as elephants.
Whales, Dolphins, Porpoises and Beaked Whales (Cetaceans) are believed to be evolved from an even toed ungulate (hoofed mammal) that survived the Cretacean extinction, but there are different explanations for when the toothed and baleen whales diverged.
The skeleton of Ambulocetus “walking whale” has been hailed as an evolutionary missing link between land and marine mammals.
Boy, you know me better than I do!
For a bit of further proof, check out the nails at the ends of my flippers.
Stan Doubt: I feel a bit left out.
That’s what happens when you go extinct, Desmostylus: everybody forgets about you. These guys still remember you, though.
It may be helpful to consider the ecological niches currently occupied by otters, seals and the like - the transition from fully terrestrial to fully marine could be quite a smooth one.
I had a long discussion with some door-to-door evangelists (my wife hates it when I do that), a few weeks ago. When the discussion got around to evolution, they asked for just one example - I gave them the Ambulocetus, Pakicetus …whale story. You could almost see their heads fixin’ to explode.
I agreed to read their literature if they would read "The Demon Haunted World" by Sagan. Fat chance of that, though.Understanding of the ancestry of cetacea has been expanded by a number of great transitional fossils in the last 20 years or so. This Science News article gives recent conclusions on whale origins and the paleontological reasons for them. This more technical paper still holds to the mesonychid theory of cetacean origins.
There’s an excellent essay on the proto-whales by Stephen Jay Gould in one of his last collections, IIRC Bully for Brontosaurus. (In passing, I found the recreations of Ambulocetus and Basilosaurus on the recent Discovery channel special, supposedly about Ice Age mammals but actually covering the entire Cenozoic, to be quite well done.)
Poly: your Science News link doesn’t work. I did a quick search at Sciencenews.org, but only found this article discussing the controversial Himalayacetus, which doesn’t quite sound like what you are referring to.
Huh. I’m fairly sure I pasted the URL in, but that parsed it as “http colon doublebackslash this.science.news.article”!! :o
Anyway, this Science News article: http://www.sciencenews.org/20010922/fob1.asp
The gist of this article was that the rather famous and diagnostic ankle structure of the artiodactyls (even-toed ungulates) was what the first pre-whales with intact ankle bones had – implying strongly that whales’ and dolphins’ closest living relatives are pigs and hippos.
There was a big article on the subject in National Geographic in (I think) the December 2001 issue. Maybe your local library has a collection.