"Galaxie peinture" by Pat Rawlins / Nasa - 
Panspermia (from the Greek pan, meaning “all” and sperma, meaning “seed”) is the hypothesis that life exists and is distributed abundantly throughout the universe. It spreads in a dormant form such as spores that develop in the right environment. Panspermia is not meant to address how life began, only its method of distribution in the universe.
I was amazed to learn, as a youngster, that all life on earth, from primitive archaeobacteria to the most complex multi-cellular creatures share the same DNA. I wondered: Did this mean life on earth arose only once? Or did several forms emerge and compete until one destroyed all the others? Is there life on other worlds or are we it?
Most scientists now suspect that life is ubiquitous throughout the cosmos. It has not (so far) been discovered even on other bodies in the solar system. Yet it has adapted to bizarrely different ecologies here on earth. It seems reasonable to think it could adapt to strange ones we can’t even imagine. When I researched my first novel I found that bacteria could live anywhere a little moisture could be found. Later I learned that some genera lived deeply in perfectly dry rock and live solely on electrical energy. Some live sublimely in lakes of poisonous heavy metals like arsenic.
Microbes can survive as spores for millions of years. If they end up in a propitious neighborhood they return to life and carry on as if they had never been dormant. It’s easy to see how spores travel from one world to another. We get debris from space on a regular basis: bits of comets, asteroids, meteoroids, planetoids and even spacecraft unintendedly contaminated by microbes. Bits of other planets are usually ejecta from volcanoes or asteroid or comet impacts. The last pieces of Mars that came to Earth that I’m aware of landed in Morocco in July, 2011. These can be easily identified now that rocks have been studied on their native planet. Research done in outer space has shown that microbes are tough enough to withstand most of the rigors of space travel, including reentry into the atmosphere.
Halley 1986 by ESO
No one knows how far Earth’s countless other invaders have traveled. Despite what appears to us the vast nothingness of space, gravity warps objects’ random paths to land them on all sorts of heavenly bodies. Some end up sizzling in stars’ deadly furnaces, others alight on frigid worlds where they await patiently frozen until some perturbation moves them elsewhere. (Or doesn’t move them and they end up perishing in the great heat death of the collapsing universe. Or remain frozen forever if the universe isn’t collapsing.) Those which reach a planet in the Goldilocks environment (not too hot, not too cold … etc.) surrounding the right sun awaken and evolve.
So maybe the current life-forms based on our DNA didn’t originate on earth or anywhere else in the solar system. Spores could have emigrated here from some distant star, say Epsilon Eridani. They then could have gobbled up and replaced whatever life was struggling to establish a beachhead here. Or maybe spores from Mars, when rivers ran there, infected both earth and Epsilon Eridani.
Once the argument over whether there were many worlds or only this one was intense. Contrarians believed there were many. Metrodoros the Epicurean said, “To consider the Earth the only populated world in infinite space is as absurd as to assert that an entire field sown with millet will produce only one grain of millet.” Metrodoros lived 2,300 years ago. The Kepler Mission, which has resulted in the discovery of hundreds of planets revolving around many suns, has proved him right. With so many planets whizzing around all those suns, why not abundant life?
Of course a planet doesn’t have to rely on spores from outer space; it has the right to allow evolution of its own life. But I must agree with scientists and contrarians that life is likely to be rampant in the universe and to travel about in it. Perhaps thinking creatures on a planet encircling Epsilon Eridani who share our DNA are discussing similar thoughts at this very moment.
Or perhaps not.
We’re unlikely to ever know.
Though some day we might find life somewhere else in our own solar system.