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But if you are attempting to write a work of [[MohsScaleOfScienceFictionHardness hard science fiction]], your aliens have to be ''realistic''. The sophisticated reader, who is knowledgeable in physics, chemistry, and biology, must believe that these otherworldly creatures could ''actually evolve'' on real planets in the real universe.

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But if you are attempting to write a work of [[MohsScaleOfScienceFictionHardness [[SlidingScale/MohsScaleOfScienceFictionHardness hard science fiction]], your aliens have to be ''realistic''. The sophisticated reader, who is knowledgeable in physics, chemistry, and biology, must believe that these otherworldly creatures could ''actually evolve'' on real planets in the real universe.


Much is made about the "first animals to leave the oceans", with most folks usually pointing to either lizard-like amphibians that used their arms for brachiation under water, or lungfish that could pull themselves across land for short distances. The people that think these were the first animals to leave the oceans are ''hopelessly'' vertebrate-centric in their thinking. The first land animals weren't fish. They weren't amphibians. They were '''insects.''' Insects evolved the ability to breathe air and survive on land 40 million years before vertebrates did. With no vertebrate predators to threaten them, some of these insects grew to nearly 3 feet long. They couldn't get any bigger than this, though; without an endoskeleton, all the squishy guts inside their bodies have to be anchored to the inside of their chitinous exoskeletons. The bigger the insect, the more those guts will inevitably "sag" toward the bottom of their body cavity, and the thicker (relative to the overall length of the insect) the exoskeleton needs to be. Compoounding this problem was the need to extract oxygen from the air. Insects have neither lungs nor oxygen-carrying blood; they have to draw air in from ''spiracle'' valves on their skin, and carry it directly to the tissues through networks of tiny tubes. This means no point inside an insect's body can be more than a couple of centimeters from its exterior.[[note]]The earth's atmosphere also had a higher partial pressure of oxygen in the Paleozoic than it does today. A three-foot-long insect would probably asphyxiate in today's air.[[/note]] These twin problems form the insect version of the SquareCubeLaw, and it piles up a lot quicker than the square-cube law does for us bones-on-the-inside vertebrates.

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Much is made about the "first animals to leave the oceans", with most folks usually pointing to either lizard-like amphibians that used their arms for brachiation under water, or lungfish that could pull themselves across land for short distances. The people that think these were the first animals to leave the oceans are ''hopelessly'' vertebrate-centric in their thinking. The first land animals weren't fish. They weren't amphibians. They were '''insects.''' Insects evolved the ability to breathe air and survive on land 40 million years before vertebrates did. With no vertebrate predators to threaten them, some of these insects grew to nearly 3 feet long. They couldn't get any bigger than this, though; without an endoskeleton, all the squishy guts inside their bodies have to be anchored to the inside of their chitinous exoskeletons. The bigger the insect, the more those guts will inevitably "sag" toward the bottom of their body cavity, and the thicker (relative to the overall length of the insect) the exoskeleton needs to be. Compoounding Compounding this problem was the need to extract oxygen from the air. Insects have neither lungs nor oxygen-carrying blood; they have to draw air in from ''spiracle'' valves on their skin, and carry it directly to the tissues through networks of tiny tubes. This means no point inside an insect's body can be more than a couple of centimeters from its exterior.[[note]]The earth's atmosphere also had a higher partial pressure of oxygen in the Paleozoic than it does today. A three-foot-long insect would probably asphyxiate in today's air.[[/note]] These twin problems form the insect version of the SquareCubeLaw, and it piles up a lot quicker than the square-cube law does for us bones-on-the-inside vertebrates.


(This article still needs WikiMagic.)



Eventually, reptiles emerged, who produced hard-shelled eggs that could be safely stowed away on land. They out-competed the amphibians in much the same way that the amphibians had out-competed the insects. But like the insects, the fish, and the amphibians -- in fact, like every other organism living on Earth at this point in history -- reptiles were cold-blooded. Their body temperature depended entirely on the temperature of their surroundings. Since a lot of biological processes depend on chemical reactions that only happen within a certain narrow range of temperatures, they had to evolve all sorts of tricks to keep warm. Some got large enough that they wouldn't lose heat very quickly in cold weather. Some evolved enormous sails on their backs which they could turn toward the sun for gathering heat, and then fold up at night to prevent heat loss.

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Eventually, reptiles emerged, who produced hard-shelled eggs that could be safely stowed away on land. They out-competed the amphibians in much the same way that the amphibians had out-competed the insects. But like the insects, the fish, and the amphibians -- in fact, like every other organism living on Earth at this point in history -- reptiles were cold-blooded. Their body temperature depended entirely on the temperature of their surroundings. Since a lot of biological processes depend on chemical reactions that only happen within a certain narrow range of temperatures, they had to evolve all sorts of tricks to keep warm. Some got large enough that they wouldn't lose heat very quickly in cold weather.weather, a trick known as ''gigantothermy''. Some evolved enormous sails on their backs which they could turn toward the sun for gathering heat, and then fold up at night to prevent heat loss.


The simplest animals on Earth are the sponges. They're basically filter-feeders; they just sit there and let water pass through their bodies, and feed on any plankton that happen to come drifting through. ("Plankton" is a broad term that covers all tiny water-bound organisms that can't swim against the current; some plankton are bacteria, some are algae, some are plants, some are even small animals.) They have no central nervous system, and only the most primitive of means for distributing nutrients throughout their tissues (i.e. no blood, and certainly no circulatory system). A central nervous system, or something like it, probably first evolved with the anemonies; touch one part of an anemony, and other parts of the organism will contract in response. The development of a nervous system marked the beginnings of ''behavior'', rules governing how a large multicellular organism responds to stimuli.

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The simplest animals on Earth are the sponges. They're basically filter-feeders; they just sit there and let water pass through their bodies, and feed on any plankton that happen to come drifting through. ("Plankton" is a broad term that covers all tiny water-bound organisms that can't swim against the current; some plankton are bacteria, some are algae, some are plants, some are even small animals.) They Sponges have no central nervous system, and only the most primitive of means for distributing nutrients throughout their tissues (i.e. no blood, and certainly no circulatory system). A central nervous system, or something like it, probably first evolved with the anemonies; touch one part of an anemony, and other parts of the organism will contract in response. The development of a nervous system marked the beginnings of ''behavior'', rules governing how a large multicellular organism responds to stimuli.



Warm bloodedness came at a price, however. To produce all that chemical heat, you needed much more glucose than you'd use otherwise. Over 75% of the glucose consumed by a human's metabolism goes solely into producing heat, for example. That means needing a ''lot'' more food. A warm blooded creature has to eat, and eat, and eat, nearly all the time. The 3 meals a day we humans take for granted as normal is in stark contrast to the one meal every week, or every month, that a reptile needs. As a consequence, a much smaller percentage of warm blooded creates could afford to be carnivores. While 1 in 5 cold-blooded fish or reptiles is carnivorous, only about 1 in 100 warm-blooded creatures are carnivorous, because a warm-blooded carnivore has to kill and eat a lot more prey than a cold-blooded one does. (Kinda backwards from the notion of the "cold-blooded killer," isn't it?)

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Warm bloodedness came at a price, however. To produce all that chemical heat, you needed much more glucose than you'd use otherwise. Over 75% of the glucose consumed by a human's metabolism goes solely into producing heat, for example. That means needing a ''lot'' more food. A warm blooded creature has to eat, and eat, and eat, nearly all the time. The 3 meals a day we humans take for granted as normal is in stark contrast to the one meal every week, or every month, that a reptile needs. As a consequence, a much smaller percentage of warm blooded creates creatures could afford to be carnivores. While 1 in 5 cold-blooded fish or reptiles is carnivorous, only about 1 in 100 warm-blooded creatures are carnivorous, because a warm-blooded carnivore has to kill and eat a lot more prey than a cold-blooded one does. (Kinda backwards from the notion of the "cold-blooded killer," isn't it?)


Ribozymes, like modern protein enzymes, sometimes need "cofactor" molecules to function properly. Some of these cofactors are individual amino acid molecules, or very short molecular chains (2-3 long) of amino acids. Amino acids occur naturally in some comets and asteroids, and were doubtlessly present on the ancient Earth. This was likely how proteins first started becoming bound up in living organisms: Those Ribozymes that had the necessary chemistry to capture free-floating amino acids had access to more cofactors than those ribozymes that didn't, and could thus out-compete them. The ability for two amino-acid-carrying ribozymes to joing their amino acids together in a chain would also have been useful; indeed, such behavior can be (and has been) "evolved" in a laboratory. Eventually, one specific short RNA strand can become associated with one specific amino acid throughout a given protocell, forming what we in the modern world would call Transfer RNA. From there it's a short step to associating a specific string of RNA nucleotides with a specific piece of tRNA, and thus with a specific amino acid. This is the origin of the genetic code. [[https://www.youtube.com/watch?v=rtmbcfb_rdc This video]] explains this process in greater detail.

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Ribozymes, like modern protein enzymes, sometimes need "cofactor" molecules to function properly. Some of these cofactors are individual amino acid molecules, or very short molecular chains (2-3 long) of amino acids. Amino acids occur naturally in some comets and asteroids, and were doubtlessly present on the ancient Earth. This was likely how proteins first started becoming bound up in living organisms: Those Ribozymes that had the necessary chemistry to capture free-floating amino acids had access to more cofactors than those ribozymes that didn't, and could thus out-compete them. The ability for two amino-acid-carrying ribozymes to joing join their amino acids together in a chain would also have been useful; indeed, such behavior can be (and has been) "evolved" in a laboratory. Eventually, one specific short RNA strand can become associated with one specific amino acid throughout a given protocell, forming what we in the modern world would call Transfer RNA. From there it's a short step to associating a specific string of RNA nucleotides with a specific piece of tRNA, and thus with a specific amino acid. This is the origin of the genetic code. [[https://www.youtube.com/watch?v=rtmbcfb_rdc This video]] explains this process in greater detail.


Thus far, we know of only one planet where life actually arose. But just in our one biosphere alone, we've seen an amazing diversity, an astonishing spectrum of bizarre and wondrous organisms that all fall under the umbrella of "life." Much has been made of how alien the humble [[StarfishAliens starfish]] is when compared with a human being, but even a starfish still has cell nuclei and mitochondria and shares the same genetic code with us. There are life forms on, and in, the Earth whose very ''chemistry'' is different from ours, and that show us unequivocably that the route from microbes to man didn't have to play out anything like it actually did.

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Thus far, we know of only one planet where life actually arose. But just in our one biosphere alone, we've seen an amazing diversity, an astonishing spectrum of bizarre and wondrous organisms that all fall under the umbrella of "life." Much has been made of how alien the humble [[StarfishAliens starfish]] {{starfish|Aliens}} is when compared with a human being, but even a starfish still has cell nuclei and mitochondria and shares the same genetic code with us. There are life forms on, and in, the Earth whose very ''chemistry'' is different from ours, and that show us unequivocably that the route from microbes to man didn't have to play out anything like it actually did.



Of course, you can't respond to a stimulus unless you can ''detect'' that stimulus, hence the evolution of senses. Of great benefit was the sense of sight. Vision evolved independently no less than ''sixty-four different times'' within different animal classes. The first "eyes" were nothing more than a light-sensitive patch of skin, attached to a nerve that sent different signals depending on how much light was received. Just the ability to detect the difference between being in light and darkness had a huge survival advantage; if the lights suddenly went out, it likely meant that a predator was casting its shadow on you, and by running in a random direction you might be able to escape. Wiki/TheOtherWiki has an article on [[http://en.wikipedia.org/wiki/Evolution_of_the_eye the evolution of the eye]]. What's important to note is the ''convergence'' of eye evolution in different species. Human and octopus eyes, for example, evolved totally independently of one another, yet their similarity in structure is striking -- in some ways, an octopus eye is ''better'' than a human eye, in that there are no blood vessels cluttering the area in front of the octopus's retina. Many insects have "compound eyes", which are basically a retina turned inside-out -- each facet on a fly's eye can see only one "pixel" of the world around it, and it assembles a vague, low-resolution picture of its surroundings from these pixels.

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Of course, you can't respond to a stimulus unless you can ''detect'' that stimulus, hence the evolution of senses. Of great benefit was the sense of sight. Vision evolved independently no less than ''sixty-four different times'' within different animal classes. The first "eyes" were nothing more than a light-sensitive patch of skin, attached to a nerve that sent different signals depending on how much light was received. Just the ability to detect the difference between being in light and darkness had a huge survival advantage; if the lights suddenly went out, it likely meant that a predator was casting its shadow on you, and by running in a random direction you might be able to escape. Wiki/TheOtherWiki has an article on [[http://en.wikipedia.org/wiki/Evolution_of_the_eye the evolution of the eye]]. What's important to note is the ''convergence'' of eye evolution in different species. Human and octopus eyes, for example, evolved totally independently of one another, yet their similarity in structure is striking -- in some ways, an octopus eye is ''better'' than a human eye, in that there are no blood vessels cluttering the area in front of the octopus's octopus' retina. Many insects have "compound eyes", which are basically a retina turned inside-out -- each facet on a fly's eye can see only one "pixel" of the world around it, and it assembles a vague, low-resolution picture of its surroundings from these pixels.



!!Living in air:

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!!Living in air:
air



!!Size:

To build spacecraft, the aliens will need to be intelligent. This means that whatever they have that passes for a "brain" will need to house billions of neuron-like switching elements for abstract thought. There is probably a certain minimum size that any biological neuron-analog will need to be, so the aliens' brain itself will need to be at least, oh, several cubic centimers in size. This puts a lower limit to how small the aliens can be. ''[[LittleGreenMen Little]]'' [[LittleGreenMen green men]] may be feasible, but ''microscopic'' green men are impossible so long as we limit ourselves to life based around organic molecules.

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!!Size:

!!Size

To build spacecraft, the aliens will need to be intelligent. This means that whatever they have that passes for a "brain" will need to house billions of neuron-like switching elements for abstract thought. There is probably a certain minimum size that any biological neuron-analog will need to be, so the aliens' brain itself will need to be at least, oh, several cubic centimers in size. This puts a lower limit to how small the aliens can be. ''[[LittleGreenMen Little]]'' ''{{Little|GreenMen}}'' [[LittleGreenMen green men]] may be feasible, but ''microscopic'' green men are impossible so long as we limit ourselves to life based around organic molecules.



!!Manipulatory ability:

You can't make tools without something you can use for "hands". An elephant's trunk, an octopus's tentacles, a monkey's prehensile tail, a dog's mouth, or even a bird's beak can be used to pick things up, but performing fine work requires either fingers or tools that you can shape to use like fingers.

!!Communication:

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!!Manipulatory ability:

ability

You can't make tools without something you can use for "hands". An elephant's trunk, an octopus's octopus' tentacles, a monkey's prehensile tail, a dog's mouth, or even a bird's beak can be used to pick things up, but performing fine work requires either fingers or tools that you can shape to use like fingers.

!!Communication:
!!Communication



!!Motivation:

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!!Motivation:
!!Motivation



Once you've got the basics of your species' psychology figured out, consider checking out SoYouWantTo/DesignAnAlienMind to flesh out the rest of the thought processes of your new aliens.

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Once you've got the basics of your species' psychology figured out, consider checking out SoYouWantTo/DesignAnAlienMind to flesh out the rest of the thought processes of your new aliens.aliens.
----


Normally, when a single-celled organism (or its zygote) divides, the two daughter cells separate and go on their merry way. But in some circumstances, the daughter cells can stay clumped together -- and sometimes, banding together with your siblings gives you advantages over all the loner organisms out there. Eventually, the group sticking together could evolve so that it ''always'' sticks together, and then the members of that cellular colony have a golden opportunity: They can ''specialize'', so that (for example) a few cells become gametes while others stay "normal" for their entire lifetime. In such a case, where most cells never become gametes, the colony is similar to a beehive: A few reproducing organisms (queens/drones) supported by a non-reproductive mass of organisms with the same genome (workers). Ever-greater cell specialization is possible from this point on, so long as the colony sticks together. The colony can even move and act as a unit, assumining intercellular communication is possible. At that point, the colony has become a ''multicellular organism.'' More than a dozen times in the history of life on Earth, in both eukaryotes and prokaryotes, multicallularity has evolved; even fossilized cyanobacteria shows some evidence of multicellular organization. [[http://www.youtube.com/watch?v=JVqxyYBuI_U This video]] discusses probable ways that it might have evolved.

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Normally, when a single-celled organism (or its zygote) divides, the two daughter cells separate and go on their merry way. But in some circumstances, the daughter cells can stay clumped together -- and sometimes, banding together with your siblings gives you advantages over all the loner organisms out there. Eventually, the group sticking together could evolve so that it ''always'' sticks together, and then the members of that cellular colony have a golden opportunity: They can ''specialize'', so that (for example) a few cells become gametes while others stay "normal" for their entire lifetime. In such a case, where most cells never become gametes, the colony is similar to a beehive: A few reproducing organisms (queens/drones) supported by a non-reproductive mass of organisms with the same genome (workers). Ever-greater cell specialization is possible from this point on, so long as the colony sticks together. The colony can even move and act as a unit, assumining intercellular communication is possible. At that point, the colony has become a ''multicellular organism.'' More than a dozen times in the history of life on Earth, in both eukaryotes and prokaryotes, multicallularity multicellularity has evolved; even fossilized cyanobacteria shows some evidence of multicellular organization. [[http://www.youtube.com/watch?v=JVqxyYBuI_U This video]] discusses probable ways that it might have evolved.

Added DiffLines:

Or rather, several disasters all struck at the same time. Land masses slowly drifted together to form the supercontinent of Pangaea, squeezing out oceans that used to harbor life between them. Volcanic activity caused devastating global warming, and exposed underground coal beds which caught fire and caused more global warming; and all this warming melted the frozen methane hydrate under the oceans, which caused even ''more'' global warming. Oxygen levels fell, and hydrogen sulfide levels soared. A giant asteroid may have struck the earth too. With all these catastrophes adding together, 96% of marine species and 70% of land vertebrates vanished. Over half the Earth's taxonomic ''families'' went extinct. This event, known as the Permian-Triassic Mass Extinction, is the worst mass extinction that still survives in the fossil record.

As a result of the Permian mass extinction, the modern-mammal-like domination of the therapsids evaporated. But what if the Permian mass extinction hadn't happened, or had been milder? We still to this day might be living in a world dominated not by furry mammals, but by scaly-skinned warm-blooded lizardlike therapsids.


You can't make tools without something you can use for "hands". An elephant's trunk, an octopus's tentacles, a monkey's prehensile tail, a dogs's mouth, or even a bird's beak can be used to pick things up, but performing fine work requires either fingers or tools that you can shape to use like fingers.

to:

You can't make tools without something you can use for "hands". An elephant's trunk, an octopus's tentacles, a monkey's prehensile tail, a dogs's dog's mouth, or even a bird's beak can be used to pick things up, but performing fine work requires either fingers or tools that you can shape to use like fingers.


Perhaps a species of herbivores, such as Creator/LarryNiven's Puppeteers, would be motivated to see what's over the horizon by a simple desire to ensure the safety of the herd -- if they discovered a leopard, they could prepare for it and thus decrease their odds of getting eaten. Or perhaps the grass that the Puppeteers graze on (or whatever ground-covering organism passes for grass on their planet) only grows in random patches that last a few weeks, so they have to find the next grass patch or starve to death.

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Perhaps a species of herbivores, such as Creator/LarryNiven's Puppeteers, [[Literature/KnownSpace Puppeteers]], would be motivated to see what's over the horizon by a simple desire to ensure the safety of the herd -- if they discovered a leopard, they could prepare for it and thus decrease their odds of getting eaten. Or perhaps the grass that the Puppeteers graze on (or whatever ground-covering organism passes for grass on their planet) only grows in random patches that last a few weeks, so they have to find the next grass patch or starve to death.


Of course, you can't respond to a stimulus unless you can ''detect'' that stimulus, hence the evolution of senses. Of great benefit was the sense of sight. Vision evolved independently no less than ''sixty-four different times'' within different animal classes. The first "eyes" were nothing more than a light-sentitive patch of skin, attached to a nerve that sent different signals depending on how much light was received. Just the ability to detect the difference between being in light and darkness had a huge survival advantage; if the lights suddenly went out, it likely meant that a predator was casting its shadow on you, and by running in a random direction you might be able to escape. TheOtherWiki has an article on [[http://en.wikipedia.org/wiki/Evolution_of_the_eye the evolution of the eye]]. What's important to note is the ''convergence'' of eye evolution in different species. Human and octopus eyes, for example, evolved totally independently of one another, yet their similarity in structure is striking -- in some ways, an octopus eye is ''better'' than a human eye, in that there are no blood vessels cluttering the area in front of the octopus's retina. Many insects have "compound eyes", which are basically a retina turned inside-out -- each facet on a fly's eye can see only one "pixel" of the world around it, and it assembles a vague, low-resolution picture of its surroundings from these pixels.

to:

Of course, you can't respond to a stimulus unless you can ''detect'' that stimulus, hence the evolution of senses. Of great benefit was the sense of sight. Vision evolved independently no less than ''sixty-four different times'' within different animal classes. The first "eyes" were nothing more than a light-sentitive light-sensitive patch of skin, attached to a nerve that sent different signals depending on how much light was received. Just the ability to detect the difference between being in light and darkness had a huge survival advantage; if the lights suddenly went out, it likely meant that a predator was casting its shadow on you, and by running in a random direction you might be able to escape. TheOtherWiki Wiki/TheOtherWiki has an article on [[http://en.wikipedia.org/wiki/Evolution_of_the_eye the evolution of the eye]]. What's important to note is the ''convergence'' of eye evolution in different species. Human and octopus eyes, for example, evolved totally independently of one another, yet their similarity in structure is striking -- in some ways, an octopus eye is ''better'' than a human eye, in that there are no blood vessels cluttering the area in front of the octopus's retina. Many insects have "compound eyes", which are basically a retina turned inside-out -- each facet on a fly's eye can see only one "pixel" of the world around it, and it assembles a vague, low-resolution picture of its surroundings from these pixels.


This is not an easy task. GeorgeRRMartin said as much in his 1976 essay "First, Sew On a Tentacle (Recipes for Believable Aliens)". Your alien species will come from a world with its own evolutionary history, its own flora and fauna -- if distinctions like "flora" and "fauna" even make sense in that world's biosphere -- and must occupy some evolutionary niche on that world, or by all rights it shouldn't exist at all.


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This is not an easy task. GeorgeRRMartin Creator/GeorgeRRMartin said as much in his 1976 essay "First, Sew On a Tentacle (Recipes for Believable Aliens)". Your alien species will come from a world with its own evolutionary history, its own flora and fauna -- if distinctions like "flora" and "fauna" even make sense in that world's biosphere -- and must occupy some evolutionary niche on that world, or by all rights it shouldn't exist at all.



Of course, distinctions like "warm blooded" or "cold blooded", and "herbivore" or "carnivore", are ''terrestrial'' ones. The alien biosphere might not have the sharp dichotomy between plants and animals that exists on Earth -- it may have mobile creatures with central nervous systems like animals, who subsist on photosynthesis (and a bit of decaying dead organic matter) like plants. But however they live, there must be ''something'' in their basic survival psychology that pushes them to explore, or they're never going to build space ships in the first place.

to:

Of course, distinctions like "warm blooded" or "cold blooded", and "herbivore" or "carnivore", are ''terrestrial'' ones. The alien biosphere might not have the sharp dichotomy between plants and animals that exists on Earth -- it may have mobile creatures with central nervous systems like animals, who subsist on photosynthesis (and a bit of decaying dead organic matter) like plants. But however they live, there must be ''something'' in their basic survival psychology that pushes them to explore, or they're never going to build space ships in the first place.place.

Once you've got the basics of your species' psychology figured out, consider checking out SoYouWantTo/DesignAnAlienMind to flesh out the rest of the thought processes of your new aliens.


But the genetic code we have today -- a mapping of 64 different nucleotide combinations to 20 different amino acids -- is not universal to all life forms on Earth. The mitochondria inside your cells, for example, have their own DNA and replicate themselves according to their own drummer, but their genetic code is slightly different from the genetic code in your cell nuclei. It's ''mostly'' the same, but not ''entirely'' the same. An RNA/DNA using organism that evolved on another planet could --and, indeed, almost certainly ''would'' -- have an entirely different genetic code. Maybe they only make use of 16 amino acids, not 20, and get away with having codons that are only 2 nucleotides long instead of 3. Maybe they don't use amino acids to build their bodies but something else, and that something else has 10,000 variants instead of 20; if they use 4-nucleodite RNA/DNA like we do, each codon would have to be at least 14 nucleotides long.

to:

But the genetic code we have today -- a mapping of 64 different nucleotide combinations to 20 different amino acids -- is not universal to all life forms on Earth. The mitochondria inside your cells, for example, have their own DNA and replicate themselves according to their own drummer, but their genetic code is slightly different from the genetic code in your cell nuclei. It's ''mostly'' the same, but not ''entirely'' the same. An RNA/DNA using organism that evolved on another planet could --and, indeed, almost certainly ''would'' -- have an entirely different genetic code. Maybe they only make use of 16 amino acids, not 20, and get away with having codons that are only 2 nucleotides long instead of 3. Maybe they don't use amino acids to build their bodies but something else, and that something else has 10,000 variants instead of 20; if they use 4-nucleodite 4-nucleotide RNA/DNA like we do, each codon would have to be at least 14 nucleotides long.



Of course, distinctions like "warm blooded" or "cold blooded", and "herbivore" or "carnivore", are ''terrestrial'' ones. The alien biosphere might not have the sharp dichotomy between plants and animals that exists on Earth -- it may have mobile creatures with central nervous systems like animals, who subsist on photosynthesis (and a bit of decaying dead organic matter) like plants. But however they live, there must be ''something'' in their basic survival psychology that pushes them to explore, or they're never going to build space ships in the first place.

to:

Of course, distinctions like "warm blooded" or "cold blooded", and "herbivore" or "carnivore", are ''terrestrial'' ones. The alien biosphere might not have the sharp dichotomy between plants and animals that exists on Earth -- it may have mobile creatures with central nervous systems like animals, who subsist on photosynthesis (and a bit of decaying dead organic matter) like plants. But however they live, there must be ''something'' in their basic survival psychology that pushes them to explore, or they're never going to build space ships in the first place.

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