Personally, I don't know if evolution is true or not, no one can really "prove it" beyond a reasonable doubt, yet.
Lets remember what DNA is for though. DNA is transcribed into mRNA which is then translated into proteins. There are also other uses as well for RNA, such as tRNA and rRNA. So the main reason why there are so many similarities is that most organisms need to code for the same proteins.
The Classicist of the why am I taking a microbiology course when I'm in Arts? class