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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.

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

  • Half-Human Hybrid - the most blatant offense against conventional biological wisdom. See that entry for examples.
  • 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 - Humans or aliens affected by alien pathogens or parasites that should not have had time to adapt to a different species' physiology. Even on Earth, most viruses, bacteria, and parasites only affect a single species or a few related ones, and so-called "zoonoses" have generally only been a big deal amongst livestock, which we've intentionally cultivated in close proximity for ages.
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  • 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.

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Similarly, some people claim that most life-forms in the universe would be fundamentally compatible with each other because they would be formed from the same basic chemicals. Considering that life on other planets wouldn't even necessarily be carbon-based,note  this is a very naive position. Also, arsenic is one of those "same basic chemicals" as well.

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.


Examples:


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