Which makes you wonder of the possibilities. Maybe life is not that different elsewhere in the universe. Maybe something fell down and was either destroyed, or assimilated into local Earth life (some viruses do leave their genetic mark behind on their host organisms after they’ve run their course). Maybe nothing’s fallen yet, and we’re just really lucky or unlucky because of it.

And that’s just the stuff off the top of my head. After all, there’s got to be billions and billions of possibilities out there

Still, if such a killer critter existed, it should have attacked and destroyed all earth life by now.
We see no hint of any life on earth not descended from the same primordial organism.

A critter without DNA would definitely be a stranger. If microbes could survive mega-years in deep space then Cretaceous Event bugs could be all over the Galaxy by now. One Cosmic Year (a complete solar orbit of the Milky Way) is 200 million years. Most of the stars in the Solar neighborhood now were not there 200Myr ago.

I’m no biologist, but I’d guess that any evolved DNA found out there as a result of that collision would be tracable to Earth of that time if that’s where it came from.

Something completely different would be evidence of independent evolution.

And as the article says at the end, maybe we’re the copy.

But although interstellar panspermia is certainly possible, it may not be necessary; the circumstantial evidence is that life has arisen idependently in many places in the past, and will continue to do so in the future. We’ll never know for sure until we start sampling biological material from outside the solar system. If it all arose in one place, it will be chemically very similar.

Frankly, I prefer a universe where life arises naturally over and over again to one where it has an origin in one place only. I prefer the principle of mediocrity, I find it much more reassuring than DNA exceptionalism.