This piece from Listverse got my mind going.
In the search for extraterrestrial intelligence, some have been accused of harboring a sense of “carbon chauvinism,” expecting other life-forms in the universe to be made of the same biochemical building blocks as we are and tailoring our searches accordingly. Here are 10 examples of biological and nonbiological systems that stretch the definition of “life.”
I've long heard of the hypothesis that there were multiple geneses on Earth. The reason why we don't see evidence of alternate forms of DNA (i.e. XNA)? DNA-based life simply outproduced XNA-based life on the surface.
It's possible that Earth's situation means that DNA-life was the best method of life evolving. We have a cool, temperate climate with lots of salty water, just enough solar radiation, and a close and very large tidally-locked satellite. For DNA-life, that (or a variant of it) is the best possible circumstance. Thus, DNA-life molds the planet to more totally allow it to survive over the course of billions of years. Sure, paramecia aren't going to construct moons, but they can release gases that alter the very chemistry of the planet. Cyanobacteria did this billions of years ago, turning our sterile and inhospitable world into the oxygenated zoo it is now.
But XNA may require something much different. If XNA took root on Earth, it'd inevitably become something more like DNA because our environment favors DNA-based life. If it can't adapt, it all goes extinct. Move XNA to Titan, however, and everything changes. XNA actually thrives on Titan.
Why might XNA not have evolved into something intelligent on Titan?
1) We don't know it hasn't. We've only gone there once, and we've only gone to a single part of Titan. It would be as if an alien probe landed in the Atacama desert. It's possible that, underground, a very alien form of intelligent life has evolved, something similar to a fusion of animalia and fungi.
2) Energy. The reason why evolution happens is because life seeks more energy efficient means of reproduction. If you're not receiving enough energy in the first place, you won't have enough energy to reproduce. Titan receives very little solar energy. XNA would need to be ultra-highly efficient to survive, unless it utilized a way to combust methane on a microscopic scale in order to reproduce. Which is unlikely, because chemistry doesn't work that way. And that's on a universal level (hence why we have Laws of Physics but no Laws of Life).
In order for intelligence to rise, you need some way of creating neural networks. Even fungi that have developed a sort of intelligence do so through creating a form of fungal neural network. If the material you're working with can't act as a neural network for some reason (energy isn't flowing well, for example), then you're not developing life.
So let's keep discussing this, "alternative lifeforms."