Life on Earth is a lot stranger than commonly supposed. Science fiction aliens are often a lot more like humans than many of our real-life fellow Earthlings. Even alien life-forms suggested by serious scientists may be limited in conception, compared with Earth's more exotic flora and fauna. In their book Evolving the Alien, Jack Cohen and Ian Stewart comment on this gap between 'the biology known to most astronomers', and genuine Earth biology.
A simple way of getting in touch with a known sample of biological possibility is to stop thinking of Earth as a planetary 'land', but to consider it as a mostly water-based world.
Consider, for example, marine creatures with a manageably exotic kind of alien-ness, such as the octopus, with its three hearts, eight arms, hundreds of suckers and millions of tastebuds. This would make a good 'alien', for a start, especially if encountered as a giant life-form somewhere in outer space (rather than a snack-sized corpse on a sushi conveyor). Or consider the squid, with its jet-propulsion chassis, or 5000 species of sponge - with no mouths, nervous systems or internal organs, but skeletons of lime or silica.
Go deeper, and we get yet more exotic fauna. Consider the bioluminescent creatures living in the pitch black depths of the abyssal plain. These have evolved to emit their own light to see by - without forgetting to evolve the eyes to see with in the first place.
Even more alien are the bacteria that live around deep sea vents: they do not require light or oxygen, but live off the heat and hydrogen sulphides, by a process - chemosynthesis - that Bill Bryson has described as 'an arrangement that biologists would have dismissed as preposterous had anyone been imaginative enough to suggest it.'
The creatures living here must surely be among the most incredibly alien of our fellow Earthlings, having evolved in a rather un-Earthlike environment with temperatures of 400 degrees and pressures of 200 atmospheres. (If we were searching for life in outer space, we could be forgiven for by-passing a planet like this).
Such an alien physical environment, harbouring an alien biology, may also suggest an alien cosmology, as viewed by any sentient inhabitant.
Imagine the totally dark 'universe' of the abyssal plain, in which the darkness just goes on and on - as far as any eye could see. Even the combination of eye and telescope might not be much use for a would-be 'astronomer' of the ocean depths. Aquatic cosmology might necessarily be rather theoretical, involving much speculation on the existence of what might lie beyond the hydrosphere.
Indeed, there might not be any reason to suppose there were anything out beyond the watery firmament in the first place. An ocean bottom-dweller would have even less reason to believe their world was surrounded by air than a land-dweller should believe 'The World' to be a globe floating in space, since the ocean floor has no 'starry sky' revealed at night to hint at an outer beyond, and no equivalents of moons or wandering planets to hint at possible formats for celestial bodies. At least land-creature science can speculate, by analogy, that if a landmass is an island surrounded by water, the whole Earth might be an 'island' surrounded in a yet wider spatial void.
Imagine the scepticism directed at a sea-floor cosmologist speculating that, instead of the abyssal universe extending forever in darkness, you could journey upwards to be touched by light, and become increasingly bathed in it, until bursting out of the water into sunshine - a sudden phase transition from an 'atmosphere' of liquid to one of gas.
Traditional sea-floor cosmologists assuming the 'common sense' position of a homogeneously dark, aqueous universe - regarded with the same nothing-there nonchalance as we consider the invisible air or space around us - might be suspicious of this fantastic break from the normal scheme of things.
An even greater leap of faith might be needed to conceive of a cosmological fabric in which the very terra firma of the ocean floor could ruck up in some places, to project out of the watery firmament entirely, and directly touch the face of the (hardly less hypothetical) gaseous heavens.
And on top of that, who would believe that this surely barren, almost topologically-inconceivable solid-air interface might be the habitat of some kind of alien creatures creeping around - or flitting above - the bed of a volatile nitrogen-oxygen ocean...?
That could be enough to tax the imagination of the brightest abysmal scientist.
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