Why do scientist think whales evolved for acient mamals not fish?

Answer 1

Because they believe that mammals evolved from land-based creatures. When scientists studied the whales they noticed that whales share a lot of bodily functions with land living mammals despite their fishy apperance. The most obvious one is that they need to breath air, fish on the other side can breath water. Whales are warmblooded, fish are coldblooded. Whales get pregnant and have a womb where the fetus grow until it is born, fish on the other hand lay eggs which develop outside the mothers body. A whales skeleton also has several details in common with land based mammals (see http://en.wikipedia.org/wiki/Evolution_of_Cetaceans). In more recent years, scientists have compared the DNA of whales with land based mammals and found that they have a lot in common, and thus must be closely related.

Answer 2

The first thing to notice on this evogram is that hippos are the closest living relatives of whales, but they are not the ancestors of whales. In fact, none of the individual animals on the evogram is the direct ancestor of any other, as far as we know. That's why each of them gets its own branch on the family tree.