- 20 different amino acids, in various sequences, define almost every protein on Earth.
- However, a large number of other amino acids exist in addition to those twenty, but occur relatively rarely in our biosphere.
- Theoretically, other amino acids might exist in the universe that have not been documented on Earth.
Given those latter facts, Alien life might be based on different amino acids than terrestrial life. Organisms based on a slightly different mix of amino acids might have tissues chemically dissimilar to our own, and have properties distinct from those of earth life. They may have increased or diminished tolerances to temperatures, chemicals, pressures, etc. These alternate versions of the amino acids don't particularly require any specific conditions on the other planet. Even an otherwise fairly earth-like planet could have life focused on a different subset of amino acids, and thus use very different proteins than those seen on earth.
Going one step further (and this is where it gets interesting) the Chirality of those amino acids could be reversed as well (or instead). Basically, this would mean a mirrored or inverted structure to how the amino acids are constructed, sort of a subatomic analogy to the difference between your left and right hand. The particular chirality arrangement common in amino acids here on earth is often called "left-handed" amino acids, and the arrangement common in sugars (cellular fuel) is "right-handed". With the inverse, "right handed" amino acids and "left-handed" sugars, you'd end up with life that couldn't interact biochemically with ours, and would be biologically incompatible with our own.
Among other effects, it would mean that such life couldn't derive nourishment from earth foods, and we can't digest foods native to their planet, either. Eating chiraly-mirrrored food would be like eating plastic, you could starve to death with a full stomach. This would also prevent crossbreeding or hybridization between the species, and even splicing genes via genetic engineering would likely be doomed to failure. The incompatibility could be far worse than this however, as laboratory tests have shown that some right-handed amino acids are actually toxic to plants and bacteria on earth. On the plus side this makes it unlikely for a virus or bacteria to be able to jump species and affect life from both worlds - though there are a few extremely simple Archaea here on earth that seem able to handle right-handed or mixed aminos, so that assumption is far from guaranteed.
Amino Acids have been found in meteorites and the tail of at least one comet, suggesting that the source and cause of life is likely to be found in abiogenesis, exogenesis or panspermia theories. However, the amino acids found in meteorites do not always have the same chirality as those here on earth. Currently, no one really knows why life on Earth chose for left-handed amino acids and right-handed sugars. One evidentiary link is water - meteorites with ice / water content tend to have mostly left-handed amino acids. Meteorites that appear to have never had any water content tend to have a mix of right- and left-handed versions. When both left-handed and right-handed amino acids are dissolved in solution, the right-handed ones tend to break down. Other proposed mechanisms for the dominance of left-handed amino acids on earth include the nature of the weak force, and the direction of the orbital spin of our planet and solar system.
Game and Story Use
- Life from a Mirror Universe or Counter Earth might be built upon right-handed amino acids, since it is essentially just a microscopic chemical reflection.
- This chirality flip could also be a side-effect, or root cause, of differences between our earth and a parallel universe.
- Life from a planet with very little water might be based on right-handed chirality, since water seems to be a major factor in the dominance of left-handed aminos on earth.
- Such a planet would also need a non-water biological solvent, such as ammonia, for life to utilize, and so you're more than likely talking about a very cold world, where ammonia would be a stable liquid.
- More exotically, such a planet could theoretically have two different biospheres, with some organisms based on left, and some on right. The two biospheres would be very limited in interaction, however, as it's unlikely that life could survive the chemical ups and downs of mixing chirality in the same organism.
- So, the planet may be tide locked to it's star, with one hemisphere using one type of amino acids, and the other, darker, hemisphere using the other chirality of aminos.
- Alternately, the biospheres may be seperated by elevation. In a Gas Giant, you could have a right-handed biosphere above or below a left-handed biosphere, with miles between them. Whenever and wherever wind currents end up mixing the two biospheres, it's toxic to both.
- A spacecraft crew ends up marooned, cast aways on an uncharted planet, but at least there's food here. When their crashed cargo hold runs out of food, they have no choice but to take a few risks with the native produce. At first, they're thrilled to find it doesn't kill them or make them sick. Eventually, however, they come to realize that their bodies can draw little or no nourishment from it, either, because the chirality is all wrong. When rescue finally comes, they are half-starved, with uncomfortable bellies full of undigestible fruit.
- Terraforming a planet with the wrong dominant chirality would be nearly impossible. The amino acids in the soil (left over from millions of years of alien cycle-of-life) would poison most crops. Sterilizing an entire planet is probably not worth the trouble, but I suppose one might engineer some sort of WMD to manage it. Amino acids are hardy, though, and there may be ongoing problems - only colonize such a world if you have no other choice.
- Two empires exist in space, one founded by humans, the other by creatures with opposed chirality. So like Jack Sprat and his wife, they divvy up the universe. Any planet where left-handed Chirality is natural goes to the humans. Planets where right-handed chirality dominates goes to the alien race.
- The bizarre amino acids in their tissues result in an alien race that cannot touch human flesh, and vice-versa. The reaction may range from mild allergy up to lethal toxicity, depending on the nature of the campaign and themes.
- For a tale of star-crossed lovers, it's like an allergy, ever-so-mild as long as they don't consummate their passion.
- For species that are sworn enemies, make it incurably fatal. There can never be peace between these races, for their very presence is lethal to each other.
- Of course, if the lethality is only one-way, then one side of the conflict has an advantage.