Contrary to what some religious types would have us believe, evolution has an extraordinary effect on the plants and animals that live on earth. There are some very simple, very inescapable facts associated with this planet and life on it. The earth has been here for billions of years, and for much of that time, there was life on it. The earth was not always the way it is now; it has undergone the most profound changes throughout the millenia of its existence. Life, however it came to exist, has undergone the most profound changes as well. As the earth evolved and changed, conditions that supported life changed. The life that was here was forced to change with it, with those changes in the conditions on the earth. And if it couldn't, it died. Life changed, it evolved. Nothing could be more simple.
Evolution as it is presented in modern evolutionary synthesis is a fact. It isn't a theory any more. Be clear that evolution does not say how life came to be, but speaks to the changes it has undergone. There is something else to take note of. The facts do not support the theory of evolution. It's the other way around. The theory of evolution explains the facts. There are a zillion facts, and evolution ties them together. That's why modern evolutionary synthesis is considered fact and is taught as such in science classes.
Wikipedia has some information about modern evolutionary systhesis posted under the article of the same name. Need a link? Got one for ya.