Can non-intelligent life forms evolve to leave their home planet and travel in interplanetary space, for instance, grow on atmosphereless icy moons and transfere spores over interplanetary space?
Yes, via panspermia.
Panspermia is the idea that extremophiles "hitch a lift", as it were, on ejecta from collisions between celestial bodies. There are a few obstacles, because the microbes would have to survive all three phases of travel:
- Launch (as well as the impact event)
- Travel in the harsh environment of space
- Atmospheric entry and landing, with high temperatures
Extremophiles are really the only types of organisms that could survive such a journey. More complicated organisms (with more needs) would surely die en route.
On Earth, here have been some discoveries of materials related to organic matter that might be evidence of panspermia; see Bell et al. (2015) for one example.
Image courtesy of Wikipedia user Beao under the Creative Commons Attribution-Share Alike 3.0 Unported license.
Yes it can be done but I would list a few caveats.
Odds are the natural selection process is not going to select for surviving in space...why would it. Natural selection selects the individuals most fit in their respective environments. This simply means that if an organism makes it to space accidentally survival will also be random/accidental.
The more complicated the creature the less likely they are to pull this off. The more biologically complicated you become the more you are tied to your environment, mainly because you need more quantity and variety of resources.
The most likely scenario in my mind is a single celled organism. The organism would be launched into space via natural phenomenon, to be trendy lets go with the ejection of water from a nice little moon called Enceladus. If there are single celled organisms in the theoretical oceans on the moon, the plumes of water vapor could contain little critters.
Over time some could randomly have the ability to survive in space by essentially being dormant while they travel. Say they land on an asteroid or another moon that has the simple resources they need to survive...a particular mineral maybe and viola, you have your space faring critters.
If there was a low gravity moon with an atmosphere but in which its tallest mountains were so high that their tips were to stick out above the atmosphere then that could potentially lead to some life evolving to survive in the vacuum of space. Because the transition from atmosphere to vacuum on the mountains could be very gradual individuals that don't travel very far from where they started life might live with basically the same amount of atmospheric pressure as their parents.
Over many generations populations could migrate however from regions with normal pressure on the ground to the peaks of the mountains that would be in a vacuum. As populations migrate up the mountains their skin and the openings to their bodily holes could become more and more pressurized to hold in their bodily fluids. Plants could also evolve to become more and more and more pressurized as they migrate up the mountains. Animals that fly would at first evolve more and more powerful wings as they migrate up the mountains but would eventually evolve to have smaller and smaller wings as wings become useless in a vacuum. Animals and plants living at higher and higher elevations might start to evolve to extract more and more energy from chemicals in the ground as it would be inefficient to extract energy from the air.
Animals and plants on the peaks of the mountains would tend to evolve skin that protects their insides from deadly radiation considering that they would have no atmosphere to protect them. If there was ice at the peaks of the mountains life on the peaks of the mountains could use that as a source of hydration considering that water could not exist as a liquid in a vacuum.
If some animals that live on the peaks of mountains use chemical defenses that involve chemical reactions that could end up getting modified into a means of locomotion as they could eject fuel out of their backsides in order to move. Animals that evolve this means of locomotion could diversify the fastest out of the animals living in a vacuum as they would tend to be able to travel the furthest the fastest. If the planet had multiple mountains that stick out above the atmosphere then animals that can move by ejecting fuel out of their back sides that evolve on the peak of one mountain could migrate to the peak of another mountain.
If some of the animals were able to move fast enough using chemical propulsion they could escape their moons gravity and travel to other moons including ones that have no atmosphere as they would have evolved to live without atmosphere. Some of them could carry the seeds of plants as well as having smaller animals that can't use chemical propulsion hitch a ride on them so that they spread other animals and plants with them meaning that they moon hop in the same way that some animals and plants island hop on Earth. These types of animals would be the most diverse as they would be able to spread to moons that have no atmosphere where life could never have started.
Some could perhaps evolve to eat parts of their gas giants rings so that they could get nutrients on journeys between moons. Some species could perhaps even evolve to subsist entirely on space junk and they could perhaps live entirely in space between moons and eventually between planets. Species that live between moons and planets could get very large as they wouldn't have to fight against gravity in the same way as animals on the surface of a planet and being very big could allow them to go longer without eating. Being very large would also allow for larger eyes that would be better at detecting food from large distances as in distances potentially larger than the radius of a moon.
There is no known reason to believe there is any limitation on what can evolve, simply because we do not understand the process to assign limits, so the answer almost has to be a default "yes."
Escaping a gravity well is very tricky, requiring a lot of energy, but if there was a reason, there might be a species that figures out how to use things like winds, solar energy, and possibly a small packet of chemical energy to try to escape. I don't think it's likely (as in it'd be a miracle if it happened), but there's nothing in the laws of physics that forbids it.
Waterbears are known to be able to survive the rigors of a deep vacuum, so we know its possible for organics to function in extremely inhospitable worlds.
That all being said, when it comes to world building, we must always remember Sanderson's First Rule of Magic:
An author's ability to resolve conflict using magic is directly proportional to the reader's ability to understand it.
This rule goes not only for high-fantasy magic, but science fiction magics as well. This species is highly "exotic," as in a real scientist would scoff at the possibility (but "can" is such a great open-ended word in a question!). You would have to make sure you don't resolve to much conflict with it unless you fleshed out why such an astonishingly unique species came into being.
There are three main routes to natural evolution into space-going life:
Panspermia is covered in other answers. I think that this is cheating though, as panspermia is planet-based life that travels through space in an inert form and then resumes life on a new planet. I don't consider it to be life in space, as it's not really alive in space.
Life could develop on planets and then evolve into something that can live in space. Note that this works best if there is a lot of life somewhere that is becoming progressively more like space. It needs to be a lot of life because space kills life optimized for planets. We need enough base material to leave room for natural selection. It needs to happen over a long period of time so that new generations can build life for the new circumstances. Note that panspermia may be a stage in this.
It is conceivable that life might find moments on comets or in a sufficiently soupy region of a nebula to develop in space. Maybe when we explore enough we'll find some. Note that there is Earth-based life in extreme conditions like volcanoes. Perhaps an environment like Saturn's rings could hold life. It seems like it would take a really long time to develop though. Realize that we think that what we now call one-celled life developed from even smaller life (e.g. we think that mitochondria might have started outside cells). Given how vast space is, it might be difficult for the life to encounter enough additional life to even get as far as one-celled life. It might still be stuck at the prokaryote stage or even the protocell stage. Realize that it took something like 3.5 billion years for the first animal life to develop on Earth. This route may be slower than that.
I'm assuming that what you want is to eventually get to a form of life that can reproduce in space. Perhaps you even want a form of life that can travel in space willfully. So a space plant that floats through a nebula collecting materials may not count.
It seems at least potentially possible. We really haven't traveled far enough to disprove the existence of such life. Is there life on Mercury? Or the sun? We don't know. We haven't found life on Mars, but that merely suggests that it is difficult.
I think that a form of life that travels via a solar sail, collecting material as it goes would be possible. I think that an intelligent race could create some. Since the universe is a big place, it should be possible for randomness to duplicate that feat -- somewhere.
some bacteria survives in space: http://www.panspermia.org/bacteria.htm
They could be launched into space when their planet is smashed into fragments by an astroid.
You can't get much more unintelligent than that.
Well, you said 'non-intelligent' but if we loosen the definition a bit I see an interesting option.
How about custom made 'Pets' modified to live in space by an intelligent species. Think something like flying cats or dogs with large solar energy gathering 'wings', GMed to thrive in vacuum and naturally extract organics from rocks and space debris. 'Built' to aid in asteroid mining but then abandoned or escaped and gone wild. Perhaps very long lived and with 'natural' solar sails or other propulsion systems. Migrating back and forth from comets and small moons.
Perhaps with enough time, they might evolve into a complex biome with multiple competing species.