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No Biochemical Barriers

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I love the smell of methane in the morning.

R2: Obviously the air is highly caustic and poisonous.
Obi-Wan: Apparently not.

In Speculative Fiction, it is far too often the case that writers do not take into account the fact that the differences between terrestrial species and their alien counterparts would run deeper than appearance alone. Given that alien species would have evolved in environments vastly different to terrestrial ones, contending with different atmospheric conditions, different levels of background and cosmic radiation, different soil conditions, different pathogens and parasites, and so on, it is patently unlikely that any aspect of their biology would be directly compatible with their terrestrial counterparts. Aliens in speculative fiction are usually depicted less as properly alien creatures and more like humans from another country, and alien animals are merely exotic species from a different climate.

This, then, means that you will often find one or more of the following in speculative fiction:

  • Half-Human Hybrid — Humans and aliens are able to produce offspring together; the most blatant offense against conventional biological wisdom. See that entry for examples.
    • Hot Skitty-on-Wailord Action — The same as above, but with the partners being significantly different (e.g. a giant alien being able to reproduce with a human).
  • Alien Food Is Edible — Humans and aliens are able to share food and drink. As an extension of this, humans or aliens shown as being able to derive sustenance from utterly alien animal or plant life. (Aliens eating humans also falls into this category.)
  • Cross-Species Disease — Pathogens or parasites that should not have had time to adapt to a different species' physiology easily infecting other species, and affecting all organisms in the same way. Even on Earth, most viruses, bacteria, and parasites are only adapted to survive in certain hosts, sometimes as narrow as a single species. "First Contact" compatibility is extraordinarily rare- when a disease can infect multiple disparate species, these are usually species that interact frequently like humans and domestic animals or species that share a water source, giving opportunity for evolution to reward versatility. On the other side of the coin, when diseases do make the jump between species by fluke or mutation, their effects can be radically different in the new host- what causes a mild cold in the original species may be lethal in another (or vice-versa) as the immune system is faced with an Outside-Context Problem and the pathogen flounders trying to interact with anatomy and metabolism that doesn't exist in the new host.
  • All Atmospheres Are Equal — While species that breathe something other than oxygen are sometimes seen wearing protective gear when moving in oxygen-based atmospheres, species that are capable of breathing an oxygen-rich atmosphere are never shown to be hampered or even affected at all by the other gases (nitrogen, for examplenote ) that constitute the air familiar to humans. Nor do they suffer any problems related to the air pressure — presumably, All Planets Are Earthlike.

Some of these are more plausible than others, although not by much. While there is reason to believe that Earth's atmosphere carries life because it is particularly suited to life, this is basically a confirmation bias — we consider it suited to life because it is suited to our life. This position is called the anthropic principle.

Similarly, some people claim that most lifeforms in the universe would be fundamentally compatible with each other because they would be formed from the same basic chemicals. Science is currently undetermined on who's right — on one hand, carbon's properties mean it's more likely for carbon atoms to combine with other elements and form more complex life forms; on the other hand, with how many other elements exist there are way more chances for those elements to combine over carbon. Regardless even if the aliens we encounter are carbon-based that doesn't mean they'll be compatible: even here on Earth most species cannot reproduce with each other (and even when they can, the offspring are usually sterile) and can't eat the same food (chocolate, as an example, is poisonous to many species).

This is one of the problems to watch out for in How to Invade an Alien Planet. A technological counterpart is Plug 'n' Play Technology. If different species (including, for example, humans and aliens) are shown reacting to drugs in exactly the same way despite biochemical barriers, see One Dose Fits All.