Assume a crewed spaceship lands on a planet that matches earth in terms of gravity, temperature, atmospheric composition and pressure, and the presence of water. Life has evolved independently, and while it’s based on Carbon, it has many but not all of the components are the same than for earth life. The amino acids differ, many metabolic pathways are different, the genetic code differs, and so forth. There is no intelligent life and the megafauna is not directly dangerous to humans.
Would those two biologies be compatible? We can only speculate, and this is exactly what I’m going to do.
On the day the humans first steps on the planet and takes off their helmets, the only sign there’s a problem is a skin rash one of the crew members develops. It’s an allergic reaction caused by air-borne alien microorganisms. Once he has returned to the spaceship and cleaned the area, the rash disappears within a few hours.
Within a week, after repeat exposure to the alien molecular biology, the humans’ immune system has been sensitized. Several but not all crew members experience severe immune reactions and have to be hospitalized in the ship’s sick bay. Going forward, they must be fully isolated from the alien planet’s biosphere. Even so, one of them eventually dies from severe anaphylactic shock. The other crew members also experience immunological symptoms, but they can be managed by not exposing the skin and breathing through a mask when outside. Some of the byproducts of alien biochemistry are toxic to humans, but again this can be managed by being careful.
Even though the human scientists and doctors nervously run test after test, there is no sign of anyone getting infected with alien microorganisms. The aliens’ biology is to different from ours. There is also no evidence of humans incorporating alien molecules into their bodies. While the amino acids have the same chirality than ours, there is no molecular mechanism for our bodies to incorporate them into our proteins.
What about the planet’s biosphere? When the humans took off their helmets, they immediately shed some of their microorganisms. Most of them die on contact with the alien soil, but some microbes and molds establish locally. Since the humans are careful not to introduce and of their plants or animals, there are no invasive multicellular species taking hold. After a few years, there is evidence of a few Earth microorganisms having spread to the rest of the planet, but their impact on the established ecosystem is minimal.
This changes once terrestrial photosynthesizers take hold. Earth photosynthesis turns out to be more efficient than the alien equivalent, and within a few decades large regions of the alien planet have been ecologically transformed, with many native species displaced.
One hundred years after the first humans landing, their descendants, partly habituated to the alien antigens and partly knowing how to manage them, have established a growing colony. The planet’s ecosystem has changed. While most life is still based on the alien molecular biology, many species have been displaced, resulting in widespread ecological breakdown. There are still no reports of hybrids making use of both biologies.
Again, this is just one scenario. The range of possible outcomes goes from the crew dying within minutes from exposure to an alien atmospheric toxin and terrestrial microbes completely destroying the alien ecosystem, to everything being fine and the two ecosystems coexisting peacefully.
Nehaveigur 