And it is almost guaranteed that these two bodies will have transfered material over the billions of years of the evolution of life. It would be likely that bacteria from the first planet to evolve life would infect the second planet, and so life on both would have a common origin. However, that would still mean that animal and plant life would have to evolve from prokaryotic life separeately on both planets. And given that we had about 3 billion years of unicellular life on earth before animals and plants evolved, it strikes me as likely that one planet might have animals and the other nothing but pond scum.
Of course, this reasoning comes from a sample size of 1 biosphere. Results may not be typical. Invest at your own risk.
I’ve also seen the hypothesis that DNA is the best molecule, by far, for passing down
information to future generations, and that DNA would be found in just about any alien lifeform
which evolved on a planet akin to Earth and its climate. A sort of cosmic convergent evolution
of sorts, tho I doubt any alien virus/bug would be capable of infecting native life forms.
Foreign viruses (which depend on hijacking the machinery of the cell for replication) certainly could not; acteria, perhaps. While DNA is a uniquely capable molecule in terrestrial life, it does not follow that it is the only possible configuration for storing replication data, transcribing said data into proteins, and catalyzing anabolic functions; it’s just the one that was overwhelmingly successful on planet Earth. In any case, even a genome based upon the same structure as DNA but evolving from an entirely seperate linage would not be compatible any more than you can run a Microsoft application natively on a Unix operating system. The interface specifications, to extend that analogy, would be completely different.
As for life spreading between the two Earth-like worlds, it’s not improbable that bacteria could be transferred from one to another by asteroid impacts, but it’s doubtful that beyond a very simple stage of development it would be particularly successful at thriving in relation to indigenous species. It’s possible it might be about to outcompete an existing species, but then it would itself be transformed into a terrestrial species largely distinct of its origin. Viruses, on the other hand, almost certainly couldn’t survive the trip–most viruses can’t stand more than a few seconds of UV–and would be ill-suited to exploit complex life on the other world. It’s possible that a virus that infests a bacteria and is suitably protected could survive, but it would be capable only of infecting that bacteria, either becoming completely synergetic or destroying both the host and parasite.
Stranger, I tend to agree with your description of the unlikelihood of the two development histories having the ability to take advantage of the other planet’s life. But simply because something is an unlikely threat doesn’t mean I don’t think that great care shouldn’t be taken, when the potential for disaster is so great.
Life can be screwy at times - Until the 1970’s, it was considered axiomatic that the only source of energy for life on Earth was ultimately solar energy. Until black smokers were found. It’s not, I don’t think, the same level of improbability we’re talking about with the Andromeda Strain scenario, of course. But it is something that until they were found was completely unsuspected. And still seems damned unlikely. Most critters we know would be cooked, or poisoned, in those waters, let alone thriving there.
Since I’m an idiot and had a few more thoughts on this, and I couldn’t get the down in the five minute limit, I’ve got to reply to myself. Oops.
I want to emphasize, I’m talking about reasonable, if stringent, precautions. Not the “banana in ear to keep elephants away” sort. If you can convince a panel of experts (which I’m not and frankly I’ve only minimal interest in getting that well versed in that level of biology) that precautions against casual cross-contamination are that unreasonable.
Of course I’d imagine that the remote probes we all seem to be assuming would precede any kind of manned mission would check for biological compatibility between the chemistries of the two ecospheres as a top and early priority.
You could launch a Saturn 5 arrangement like before except the return vehicle could not orbit the 2nd planet because the landing craft would not be able to accelerate into orbit. It would have to launch straight out of the atmosphere.
The landing craft would either meet up with a return vehicle or a fuel reserve for the return journey. Picture something like Rutan’s Spaceship One that is launched by a Saturn booster with 2 fuel reserves. One for the return trip and one to land autonomously on the 2nd planet.
Bob Shaw wrote the ‘Land and Overland’ trilogy (The Ragged Astronauts, The Wooden Spaceships & The Fugitive Worlds) which featured, initially, a pair of habitable planets.
They were so close, though, that they shared an atmosphere and I think the first expedition was made in a special dirigible or balloon which handled the ascent and descent portions of the journey, with some sort of propellors to drive it across the dodgy central part of the trip from one planet’s gravity well to the the other’s!