I have read something about a proof of Mar's pre-existing life forms a few months ago. If I remember correctly, the proof is from this meteorite from Mars. It is a current study though, because it was found in a glacier located in Antarctica, and was formed on the red planet about a billion years ago and was altered because of the water of the planet's wet past before it wafted to Earth.
You can check the internet about this, I'm sure the article is still up. Good luck!