Preview:
Given the current state of origin-of-life research that is starting to support tying early biochemistry to geochemistry rather than 'primordial soup' type situations and the apparent speed of life's origin on Earth, it's easy to imagine life arising multiple times in multiple places in our solar system. Can't say that it did or didn't though, we'd need way more data.
Given the differences between Earth and other places in our solar system, there's definitely not going to be any other big biospheres with huge amounts of biomass and complexity that utterly remake the geochemistry of a body; instead any other biospheres would necessarily be both tiny and very hard to see due to hanging onto small matter and energy fluxes in protected regions of their homes.
Given the sheer difficulty of exploring other bodies for these small things in protected niches, it's entirely possible that we could've missed several of these small biospheres (past or present) so far. As such I would not be terribly surprised at future evidence of these things. Mars is great because you've got an exposed surface that may have been very different in the past that you can look for traces on plus inklings that there could still be productive niches in places its not inconceivable you could look at, it's 'close' (all things considered), and the interesting spots aren't all under kilometers of ice. It's also still an entire planet that has barely been looked at up close at all so far and where it costs an instrument's weight in gold to send it there.
This is the first in a series of posts I am putting together on a personal blog I just started two days ago as a collection of my musings on astrobiology ("The Great A'Tuin" - sorry, I couldn't help it), and will be reposting here. Much has been written here about the Fermi paradox and the 'great filter'. It seems to me that going back to a somewhat more basic level of astronomy and astrobiology is extremely informative to these questions, and so this is what I will be doing. The bloggery is intended for a slightly more general audience than this site (hence much of the content of the introduction) but I think it will be of interest. Many of the points I will be making are ones I have touched on in previous comments here, but hope to explore in more detail.
This post references my first two posts - an introduction, and a discussion of our apparent position in space and time in the universe. The blog posts may be found at:
http://thegreatatuin.blogspot.com/2015/07/whats-all-this-about.html
http://thegreatatuin.blogspot.com/2015/07/space-and-time.htm