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Background:

Chiral molecules or ions are particles which cannot be made congruent to one another via flipping one or rotating one. For example, the same property applies to your hands, assuming that you were born without a genetic abnormality; if both are facing down, the thumb will be on the right side of the left hand and the left side of the right hand, as opposed to both being on the same side of their respective hand.

Mirror life refers to hypothetical life-forms whose component building blocks - proteins, amino acids, et cetera - have a different chirality than those of Earth-based life. In this case, that means right-handed amino acids and left-handed sugars, rather than Earth's left-handed amino acids and right-handed sugars.

Let's say that you have an entire ecosystem of mirror life, completely identical to Earth's in every way except for that their amino acids are right-handed and their sugars are left-handed. Let's also say that this ecosystem evolved alongside Earth's more conventional one since the beginning of life on the planet, meaning that there are plenty of right-handed amino acids and left-handed sugars to go around so that these hypothetical organisms aren't deprived of their building blocks.

How this happened is irrelevant to the scope of this question. Just assume that, somehow, it did.

How much biological overlap would there between this mirror life ecosystem and a normal Earthly one?

For the sake of this argument, assume that this is a 50-50 split; that is, one out of every two organisms has one type of chirality, and the other one out of two has the other type of chirality.

Also, please note that all organisms in this question use either right-handed amino acids and left-handed sugars (anything that's part of the mirror life ecosystem) or left-handed amino acids and right-handed sugars (anything that's part of the Earth-standard ecosystem). There are no right-handed-amino-acid-and-right-handed-sugar organisms, nor any left-handed-amino-acid-and-left-handed-sugar organisms.

Good answers will list a way that an ecosystem of mirror life (see above for reference) is capable of interacting with Earth's opposite-handed-acids-and-sugars-based biology; for instance, interconnections between their food webs, competition for nutrients/energy, territorial conflicts, etc.

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L-organisms (particularly the ones higher up the food chain, who mostly consume their amino acids rather than synthesizing them) would not be able to satisfy all their nutritional needs by eating R-organisms. However, you would get some nutrition_ from eating species of the opposite handedness, so the two ecosystems would interact more or less as normal. That is, they wouldn't be completely walled off, as you might expect with human colonists on a silicon-based planet or whatever.

This is one of those questions that is hard to answer "realistically" while also stipulating the premise – if these two ecosystems coexisted, they would simply have evolved to be able to use each other's molecules, so you would end up with a single ecosystem anyway (perhaps one in which all organisms use both enantiomers of everything). We already know that the favorable evolutionary pathway is to standardise on a single enantiomer, because that's what happened on Earth at the very earliest stages of life.

But perhaps a wizard created this situation a few centuries ago. Some possible consequences:

  • if an L-organism eats an R-organism, their feces may contain a lot of undigested high-level R-nutrients. Generally it's not economical to eat other organisms' poop unless they are much higher up the food chain than yourself, but this could open up new ecological niches.

  • Since DNA is chiral (among other reasons), R-humans would be categorically unable to breed with L-humans, which would raise interesting issues relating to sex, racism, social geography etc.

  • For the same reason, viral plagues would affect only one or the other population

  • Many scent molecules are chiral, so predators and prey would likely be able to recognise each other by chirality. That would have consequences for things like predators' territorial behavior; e.g. L-cats might not feel compelled to drive away R-cats, and there could be new strategies for cooperation and competition

  • Competition among plants would be less affected by chirality (because they depend on mostly achiral nutrients). However, they are threatened by different populations of insects and herbivores, so again there would be new strategies. For instance, a grass that is too successful at crowding out its neighbors will just create a rich target for goats of the corresponding chirality, so L-grasses might learn to team up with R-grasses to create meadows that are only 50% delicious to any given goat.

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Life heavily relies on enzymes for successfully complete the chemical reactions needed for its sustenance. Molecules with the wrong chirality are simply not good for use, so feeding on them is pointless. If you are a R-chiral man, an L-chiral pizza is as nutrient as a stone. To put in mathematical terms, the two sets are disjoint.

A mix of R and L chiral organisms would make for a harsh environment to live in, because on average each organism would have access only to half of the potentially available biomass.

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there are 3(4) macro nutrients protein, sugar, fat (, and fibers). proteines of the wrong chirality are useless to harmfull. sugars would be just useless. but the fat can be shared. there is one chiral center in fat. i am not sure if that one is a problem. but the fatty acids themselves have no chirality. strong acids make the hydrolysis happen that breaks fats apart into fatty acids and glycol. i suspect that stomach acid is concentrated enough to break it apart. (fibers are already not metabolised and are just there to help move the other stuff along.)

the 2 groups of micro-nutrients are vitamins and minerals. minerals is easy none of them are chiral so no problems. vitamins on the other hand are numerous and complicated to asses. plenty are chiral. others aren't. and from some you need or can use both. as an example the taste of orange and the taste of lemon are partly from the same molecule limonene. in one fruit it is left-handed and in the other right.

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The resemblance would depend on how your mirrored world has been created,

Option 1: mirrored, you tweaked a Star Trek transporter: they live on happily everafter

Imagine you'd take planet Earth and a huge StarTrek transporter device, put the planet into the transporter, scan it in zero time, then swap a few settings in the molecular geometry.. at the other end, your mirrored Earth would pop out.

I think in that case, your new world would proceed its way, as it was before. Reason: the mirroring only affects biochemical processes. When all molecules involved have been mirrored in zero time, there would not be any effect, the world would proceed turned 180 degrees as if nothing happened. The change would hardly be noticed. When you visit your new Earth however, make sure you take food (see L.Dutch) and also.. wear an air tight space suit. This mirrored Earth will have some substances which are very toxic for you.. and quite common, like e.g. CH4.

Read https://academic.oup.com/toxsci/article/110/1/4/1668162

What we just can't know

(I've put this part in thanks to critical feedback of a friend)

Dream on, assuming a zero change copy, that's easy for a 45 min episode of Star Trek, but in reality, this could be complex ! Uncertainties: how about interaction with non-organic, non-chiral molecules ? Suppose a certain left-turned configuration is able to capture that molecule, and the right-turned is not, you'd get deprived of the nutrition.

.. and how about folding.. is it geometrically sound, to assume the mirroring will affect all biochemical transformations in the same way ? Do we have to take other transformations into account as well..

Option 2: mixed, or a mess

Suppose you'd use this for terraformation. Turn a planet around. Some animals won't notice. Ciliates (single-celled protozoans) vital organs come in right and left turned versions, per individual. Apparently, both are viable.

But I'd expect mass extinction actually.. it could be deadly for an ecosystem, if chirality would get messed up. Suddenly, certain body parts will cease to function, because - for example - it cannot digest certain nutrition (spore elements) anymore, that maintain the kidneys, or the liver. Also, the reserve fat a human has accumulated becomes a hostile substance. The burning of fat cannot be controlled anymore, or the fat would start to penetrate the veins, because the artery wands are not able anymore to prevent the substance to penetrate.

Read https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5104503/

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