Is there any proof of life having developed in more than one instance on Earth?

You may have seen the announcement today of the discovery of a bacterium that thrives on the deadly poison, arsenic. The bacterium was discovered in California’s Mono Lake, and it uses a new form of DNA where the phosphorus is replaced by arsenic. It is exciting because it, coupled with life thriving in superheated acidic water around volcanic fumarole on the ocean floor, means that our search for life elsewhere in the Universe may need to be reevaluated. We had assumed that life would only be found where conditions closely matched Earth, with temperatures and chemistry similar to familiar life forms here. That assumption is clearly flawed.

That brings up the question of whether the DNA we find here in life forms can all be traced back to a single source, or may have had multiple beginnings. Is there any evidence one way or the other? If so, can you point us to it?