|Public Domain, Courtesy of NASA|
Assuming we don't transform ourselves into silvery machines to survive the lethal amounts of radiation involved in any realistic voyage between stars, there's a whole spectrum of possible outcomes for humans interacting with the biosphere of another world. At one end is mass death - something along the lines of the Colombian Exchanges of biota between the Old and New Worlds here on Earth. Scenarios in this cluster involve pathogenic microorganisms (viruses, bacteria, fungi, etc...) running rampant in either the alien biosphere or the visiting humans, and killing, and killing, and killing.
No one is sure what the actual body count was in the Americas in the years that followed Columbus' arrival. Estimates often correlate with a scholar's political beliefs, but it does look like we can safely say that malaria, measles, and above all small pox killed 70% or more of the population during the first few generations of sustained contact between the hemispheres since the Siberian - Alaskan land bridge of Beringia closed at the end of the last major glacial period. And then it a happened again. Over and over, with subsequent generations of North and South Americans succumbing to new epidemics, some of which appear to have claimed up to 90% of afflicted populations. Even as late as 1800s, the movement of European, Canadian, and US missionaries or settlers into new areas sparked major epidemics among populations that had already experienced mass die offs during earlier Columbian pandemics.
By the time the United States appeared and began its westward expansion, the American Indian peoples were essentially living in post-post-post apocalyptic age after the centuries of repeated outbreaks. The full cultural dimensions of this tragedy will probably never be fully known since there are only sparse artifact records and no written primary source documents to tell us about societies whose first contact with Eurasian and African pathogens came at one or two removes from the European carriers.
There are hints though, about the magnitude of the subsequent social transformations that took place.
Enough early first contact accounts and physical evidence appear to show that the Great Plains were not probably not dominated by the nomadic tribes that populate popular imagination today. Rather villages or agricultural settlements that were inhabited for much of the year seem to have housed the bulk of the population, clustered heavily in riparian zones. The full-time nomadic lifestyle did not become the dominant mode of life until waves of foreign illnesses devastated the more vulnerable villagers, who lived in close quarters. This human population crash may have also enabled a massive spike in the population of Great Plains bison.
The arrival of one of the most prolific Eurasian invasive species, the horse, seems to have sealed the deal as far nomadic hunter-gather dominance. It did so by opening up a vast new realm of protein dense calories in the form of bison meat, while retaining a low population density lifestyle. As historian Elliot West describes the coming of the horse and its adoption by American Indians, it "...ended the bison's advantage against hunters afoot. In effect, man and animal fused into a single hunting creature, the ultimate bison nightmare: a fast, big-brained, grass-eating predator."
Biology Says No
The history above gives a glimpse of how the worst case scenarios might play out during planetary biosphere encounters when it comes to microbiological exchanges. Or at least how they might go without a lot of advanced biotechnology, evolved quarantine methodologies, tremendous professionalism, and above all, ethical restraint. With all the biological data, new flavors, potential pharamacological agents, genes, valuable imagery, and other resources at stake, it will always be much, much cheaper to take shortcuts and chances. Especially if the risk is primarily to the other biosphere and its inhabitants.
Devastating macro exchanges in the wild are another possibility when it comes to the meeting of biospheres. The enormous ecological damage wrought by European rabbits in Australia gives an idea of what could happen on the low end. Then there are scenarios more akin to the movie Aliens.
Or, on the far side of the spectrum, there might be few or no biological interactions at all. Chemistry, including the chirality of proteins, might render the lifeforms of two biosphere largely agnostic to one another.
Chirality? It breaks down like this. Bioactive molecules such as proteins come in two broad types, left- and right-handed, depending on which side a symmetry-breaking carbon atom is attached. Earth's biosphere is left handed. Why left handed? No one knows. Many chemists and biologists think that the choice was an arbitrary lock in. The earliest life, by sheer chance, synthesized slightly more left handed amino acids than right.
Dextro-based (right-handed) organisms cannot consume, incorporate, or interact well with levo proteins. Levos likewise find dextro proteins toxic. So once a preponderance on the left was reached, even very early on, right-handed chiral lifeforms would have found Earth's biosphere a lethal place when it came to most of the building blocks of life.
The hugely popular Mass Effect franchise worked this neat bit of chemistry into its mythos, with dextro and levo sapients interacting in a galactic society, but unable to eat each other's food.
In reality, there could still be some sharing of foodstuffs. Carbohydrates and sugars are not chiral. That said, you can also only go so long without proteins before malnutrition sets in. The fuel of life maybe largely non-discriminatory, but again, those more advanced, synthesized building blocks lean one way or the other.
Fusion or Segregation?
What does this mean for biosphere encounters?
Science fiction television series, movies, and most novels never address an interstellar version of the Columbian Exchange. Instead, beings from different worlds meet and speak, war with, or even mate with one another without a smallpox outbreak or plague epidemic to slow things up. Sapients even settle in each other's biospheres with nary an disease incident, 'cept for the occasional, ridiculously exotic illness. Typically a one-off throwaway sicknesses with a cheesy sounding name, designed to generate cheap plot tension. In other words, nothing close to the series of exchanges we can expect if humans ever visit another levo world.
When it comes to dextro worlds, however, those kinds of easy disease-free interactions might actually be possible. Right-handed chiral viruses would probably be completely incompatible with human DNA and cells, and dextro bacteria would probably find the environment of the human body uninhabitable. It might be different, though, with some fungi, which are notoriously adaptable when it comes in living in harsh or toxic conditions.
Of course it'd be difficult for us to live on a dextro world. So while we could probably interact with sapient life from these planets without too many issues, we probably wouldn't want to invade or settle alongside them on their home turf. Or at least not in our present form.
When it comes down to it, I hope we never colonize a world with its own biosphere. Or at least a biosphere with multi-cellular life. Doing so would make the Gaiasphere - the sum total of Earth's life - into a kind of virus. An infection obliterating the diversity of complex biological life in the galaxy. While there are no doubt those who'd like a carefully mediated fusion of biospheres, I'm hoping that we choose to terraform lifeless worlds and bring life to the inorganic, while allowing already existing forms to continue their own development.