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I’m designing planets for a hard (ish) science fiction RPG setting. So I want to find out if there are any clues or theories as to what proportion of planets with life will have the basic biochemistry compatible with Earth’s. (I’m aware the higher level stuff like will be far less likely to have compatibility).

Earth has right-handed sugars and (mostly) left-handed amino acids. But there are another 3 possible combinations:

Chirality grid

So… are all these four outcomes equally likely to happen? Is there any energetic reason which would favour right-handed over left-handed forms once proto-biology comes on the scene? E.g. if grabbing or manufacturing one form rather than the other is more energy efficient, natural selection should favour it. Is there any basis in real world chemistry or biochemistry for this?

For context, I want planets to fall into the following categories, and am interested in some reality-based proportions of each type. It is all about ‘free’ ecosystem services versus humans having to build their ecology from the ground up.

  1. Earth. Breathable atmosphere. Oxygen makes combustion engines and the like possible. People and domestic animals can eat a vast array of flora and fauna without worrying about digestibility or poisoning. Local microbes happily recycle everything from that cherry tree you chopped down, to sewage from the local town, to the corpse of faithful old Rover buried in the garden.
  2. ‘Near twin’ of Earth. Life has made a breathable atmosphere with high oxygen and low CO2. Same chirality of amino acids, proteins, sugars. Local microbes happy to eat Earth trees/sewage/corpses. There will be local quirks, unique to each planet, e.g. local meat has to be processed to remove the weird hormones which cause heart murmurs in humans, or life hasn’t emerged onto land yet, so the interior of the continents are barren wastelands.
  3. Incompatible biochemistry. Life has made a breathable atmosphere, but its sugars/proteins are the wrong chirality so eating it (or it eating you) is a no-no. You have to terraform the soil to provide the microbes and humus grow Earth crops, etc.
  4. Lifeless rocks with unbreathable atmosphere. There are a LOT of these planets.
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    $\begingroup$ One thing to consider: Even creatures with incompatible biochemistry can still break you down for the tasty materials in your body. Calcium is calcium regardless of provenance... $\endgroup$ – Joe Bloggs Dec 22 '16 at 12:09
  • $\begingroup$ @JoeBloggs Good point. I'm now having evil thoughts about Planet of the Bone Crunching Predators! :-) I have no idea if getting the calcium out of all that pesky cartilage is easy/difficult if you can't digest the proteins. $\endgroup$ – DrBob Dec 22 '16 at 12:23
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    $\begingroup$ It's not just calcium. To blatantly steal from The Expanse: from the point of view of an alien bacterium you're just a nice, exploitable pile of resources. It's not so much eating as mining. $\endgroup$ – Joe Bloggs Dec 22 '16 at 12:36
  • $\begingroup$ @JoeBloggs. I'm fond of the Lipovores of Ashen Stars RPG myself - they are 'vampires' who after your body's fats, including the phospholipids of your central nervous system. $\endgroup$ – DrBob Dec 22 '16 at 12:44
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There is nothing particularly special about the amino-acids used by terrestrial life-forms. Chances are that elsewhere life will use at least some different amino-acids.

I'm not sure about amino-acids, but for sugars the left- and right-handed forms are produced in equal proportions by many chemical reactions. The preference of life-forms for just one of the enantiomers is an accident, and on other worlds other accidents could have indeed resulted in a preference for the other enantiomer, or, at least in the case of sugars, in complete or partial indifference.

Even on Earth, the preference of life-forms for, for example, D-glucose, is not absolute; there are bacteria such as Burkholderia caryophylli which are able to process L-glucose. This ability is not widespread simply because there is very little L-glucose available. Should terrestrial bacteria find an abundant source of L-glucose they would surely and quicky evolve the ability to process it -- after all, strains of Flavobacterium evolved the ability to eat nylon in no more than 40 years (nylon was invented in 1935 and the first nylon-eating bacteria were discovered in 1975).

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  • $\begingroup$ That nylon-eating fact is awesome! $\endgroup$ – DrBob Dec 22 '16 at 12:36
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I believe that raw statistical mechanics/thermodynamics favors neither left nor right handed molecules of proteins or sugars, respectively. However, biochemical enzymes are not generally chirality (handedness) blind. Self-reproducing molecules in life have evolved to expend energy/resources to reproduce their own specific versions, including the handedness of sugars and proteins.

Searching for unequal concentrations of chiral isomers has been proposed as a mechanism for searching for life on mars. See http://www.gillevin.com/Mars/GVL_SPIE_2010.pdf

If you're interested in this topic, I suggest you read Zelazny's novel Doorways in the Sand Which includes speculation about what sense-reversed foods would taste like. (Not an experiment I'm eager to try personally.)

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