Conventional science says whales and other ocean mammals are descended from a wolf-like animal who lived about 50 million years ago. This is, in fact, one of the key "icons" of evolution promoted in practically every biology textbook in the country as "proof" for Darwinism.
I'm not going to get into the strengths and weaknesses of this argument because, quite frankly, I'm not qualified to comment on it. I'll leave that to others.
I just wanted to bring it to your attention that there was a news announcement this week that the final "missing link" connecting the wolf-like creature to whales has now supposedly been found. Here is a link to the popular news article on the BBC web site.
"Whale 'missing link' discovered"
Now, I'm taking this news rather cautiously because experience tells me the media usually reports between 50 to 85% of the content in news stories incorrectly. Also, from what I can tell, there is no peer-reviewed paper connected with this discovery so I'm not sure how other scientists are viewing this specimen.
UPDATE (12/20/2007): Dr. Fuz Rana discusses this discovery in a short podcast, explaining the evidence within an old-earth creation framework.
Whales May Have Come From Deer-Like Animal