oldest rocks on Earth, 3 million year old human tools, and future directions for astronomy. So, ignore all of that and bring focus to bear on a persistent idea. The notion of panspermia - the transferral of viable organisms between planets, and even between star systems and further.
There is no doubt that planetary surface material is continually being shipped around between rocky planets and moons in our solar system. Ejected by asteroidal or cometary impacts, chunks of stuff follow a range of orbital trajectories that result in both eventual return to their origins or transferral to the surfaces of other worlds. Increasing evidence suggests that a variety of (typically microbial) organisms could be carried along, surviving both the extremes of pressure and acceleration, as well as exposure to thousands to millions of years of interplanetary space. There is a real possibility for life to both cross-infect, and even to be 'seeded' from planet or moon to planet or moon.
Enthusiasts for panspermia go further, and have been known to invoke this as a mechanism for galaxy-wide dispersal of life - taking one rare occurrence of life and spreading it. There is however a factor that to my knowledge is rarely considered, and that is natural selection. You or I, fluffy bunnies, and daffodils are all unlikely candidates for interplanetary or interstellar transferral. The sequence of events involved in panspermia will weed out all but the toughest or most suited organisms. So, let's suppose that galactic panspermia has been going on for the past ten billion years or so - what do we end up with?
Although it's a complex problem, it seems likely that life driven by cosmic dispersal will end up being completely dominated by the super-hardy, spore-forming, radiation resistant, rock-eating (endolithic) type of critters. There will be no advantage to a particularly diverse gene pool. Billions of years of galactic transferral will have whittled it down to only the most indelicate and non-fussy microbes - super efficient, super persistent, and ubiquitous - the galactic top dogs.
Now, we might argue that there are many organisms on Earth that could fit the bill, and could be the links to these ancestral interlopers. The problem, and the potential paradox, is that if galactic panspermia is real then the type of life it will evolve should be everywhere. There would be stuff on the Moon, Mars, Europa, Ganymede, Titan, Enceladus. Every nook and cranny in our solar system would be a veritable paradise for these ultra-tough lifeforms. It may be too early to rule this out, but if life is sparse in our neighborhood then it would seem to argue strongly against the possibility of Galactic panspermia.