How come a reptile evolved into something like mammal that have a really different charateristic from reptile?

Dear Evan,
I think you have the idea that one organism evolves directly into another. Our understanding of the process of evolution has moved on from the idea of direct ancestor-descendant relationships to one in which we understand the relationships between organisms in terms of shared, new characteristics.

In the case of mammals and reptiles, they both shared a common ancestor (imagine a fork in a tree) that had some features we can still discern in both groups. The reptile fork went along and evolved some new features that defined them as a dsitinctive new group. Mammals did the same. So rather than imagining a vertical line running from reptiles to mammals, you need to picture a v-shape with one side of the v leading to mammals and another leading to the reptiles and at the base of the v an organism that shared some of the features of both groups.

If you then added a group like sharks, you would be able to grasp that mammals and reptiles had a more recent common ancestor, as they share more shared novel features, than either does with sharks. So our view of evolution is much more about 'who is more closely related to who' than 'who evolved from who'.

Richard Dawkin's explains this view of evolution in detail in his book 'The Ancestor's Tale'
http://en.wikipedia.org/wiki/The_Ancestor%27s_Tale

"Hope is a duty from which palaeontologists are exempt."
David Quammen