This would mean that free oxygen on Mars built up 1.5 billion years before it became a large component of our own...

This would mean that free oxygen on Mars built up 1.5 billion years before it became a large component of our own atmosphere. Atmospheric oxygen on Earth is largely the product of life - it was not until later in Earth's history that living organisms began using the oxygen. It's possible that oxygen on Mars formed from the same processes, but it's also possible that it was the result of chemical reactions in the atmosphere.

More info: http://www.guardian.co.uk/science/2013/jun/19/mars-oxygen-rich-atmosphere