I stumbled across this article from the Boston Globe about what it would mean if life was discovered on Mars. The author argues that because we have no objective evidence that there is other life in the Universe, that there is a Great Filter that prevents the rise of civilisations technologically advanced enough to colonise the stars.
Based on the theory of this Great Filter, he goes on to argue that it either lies in our past or our future. His argument that if it is in our past, the probabilities are that we are the only self-aware civilisation in the Universe.
The opposite argument that this Great Filter lies in the future, and that at some point in our future the human race will become extinct before it develops the technology to colonise space.
Moving on to life on Mars, the author then argues that if we do find life on the red planet, this means that it is less likely that the Filter lies in our past and that it then lies in our future. If so, all we have to look forward to is the extinction of the human race.
I think the author makes one very naive assumption here. If life was found on Mars, no matter how biologically advanced it once was, that would suggest, to me anyway, that this Great Filter occurred at that point in the evolution of life on Mars – and on Mars only.
If you take the theory of a Great Filter as being true, you are still left with the fact that you are basing your results on the evolution of life on one planet, i.e. Mars. In fact, as long as any life found on Mars was less complex than that found on Earth, we have an example of a data set that contradicts the theory – we have our evolution on Earth. (Unless of course there is a more technologically advanced civilisation hiding deep under the Martian surface).
Based on this theory, we can then argue that if all live on Mars died out when life was less biologically complex than it is currently on Earth, that it is more likely that the Great Filter lies in our past, because we managed to pass that level of complexity a long time ago. (Though we do have to allow for the fact that I am also basing that result on a data set of 1.)
As for the fact that we have yet to find evidence of a technologically advanced civilisation outside of our solar system, all I can say is space is a big place. A very big place. While scientists have found exo-planets, the methods used, and the equipment they have at their disposal at this point in time, means that these planets are usually very large and very close to their stars, which is not where you’d expect to find recognisable life. (I say recognisable, because no-one is entirely sure what other forms of life are possible.)
No matter how much I think about, I can’t see that the finding life on Mars in any form, or complexity, means that we should be pessimistic about the future survival of the human race. In fact, it should be the opposite: life on Mars would show us that life can begin in more inhospitable places than Earth. If it can happen on a much smaller, colder planet than Earth, the chances of sentient life elsewhere in the galaxy will go up. If complex life did develop on Mars, then those odds go way up.