Compatible biochemistry, or not?
I showed a draft of a short story I wrote to some friends and got a very derisive comment from the chemist in the group.
According to him the chemistry just isn't possible.
I'd like a second opinion.
The short of it:
It's the year 2970 and humanity has finally cracked the FTL drive and starts to colonize worlds far away.
They find a world with perfectly Earth-like conditions. It even has complex live (non-sentient) similar to Earth. The alien life is even carbon-oxygen based with a bio-chemistry that is very similar to our own. There are thr equivalent of plants, herbivores that eat those plants and carnivores that eat the herbivores.
Unfortunately for the colonists the bio-chemistry of the alien life-forms isn't compatible with Earth lifeforms. Earth live-stock can't eat the local plant-life. Meat from alien animals/alien plant-material is at best indigestible to Earth life-forms (including humans), at worst poisonous.
Luckily Earth plants can deal with the alien soil, so the colonists can setup an Earth food-chain starting with plants.
The opposite isn't true: The alien life-forms can and will eat Earth bio-mass. Alien herbivores find Earth plants tasty. The alien carnivores consider Earth live-stock (and especially humans) a nice snack.
My story revolves about the fight for survival of the colonists against the alien biosphere.
Now the chemist (an-organic chemistry, he is not a bio-chemistry expert) claims that this won't work.
According to him, if the alien bio-mass is indigestible (or worse) to Earth life-forms, the opposite has to be true as well. It works both ways.
Obviously that would kill my story altogether.
I'm not so sure he is right. I would presume that it heavily depends on the exact mechanics of the digestive system of the alien life-forms. If that first breaks down the Earth-chemistry bio-mass into small chemically simple compounds that are easily digestible this could possible work in my opinion.
Who has the right of the matter. Can this work or made to work?
This post was sourced from https://worldbuilding.stackexchange.com/q/12493. It is licensed under CC BY-SA 3.0.
1 answer
Well, I'm not a biologist or biochemist, but I think your scenario could be made to work (although still very unlikely) as follows:
Our genetic code is based on four nucleotides which make up two pairs of complementary bases. Now there's no reason to assume that the genetic code of the aliens is the same; indeed, there's no reason it should be restricted tot he same base set. However, in principle, it could be that it uses those four nucleotides we also use and then two more, so it is based on three different base pairs. Since the genetic code is that fundamental, every single of those alien life forms would have those extra nucleotides. Now if those extra nucleotides are poisonous for us, we cannot eat anything from their life. However we don't contain anything that's poisonous to them (all our DNA bases are used by them, too), so they can eat earth food quite fine.
Alternatively, their DNA could be made up of the same bases, but they could use a superset of the amino acids used by earth life forms to build proteins. Again, if one of those extra amino acids is poisonous for earth life, then you get what you want.
Now there's one open question: How can it be that it is poisonous for all life forms on earth? I think that would be easiest explained for the extra nucleotides: Since our genetic machinery (like the ribosomes to interpret the genetic code, or the DNA duplication/repair mechanism) is not prepared to those extra nucleotides, their presence might cause those processes, which are at the very base of everything going on in our cells, to fail. On the other hand, those alien life forms would have no problems with earth material since the nucleotides are the same.
A similar mechanism could be at work with the amino acid suggestion; our ribosomes may erroneously build those alien amino acids into our proteins instead of the correct ones, because they are not prepared to distinguish them; this leads to dysfunctional enzymes and proteins, and thus to death.
Note however that while this would be a possible solution to the problem, it would still be a highly unlikely one: It is far more likely that the alien life forms would have evolved to have a completely different biochemistry, which means we would be as useless as food for them as they would be as food for us.
Well, unless one assumes that evolution on at least one of the planets (earth or that alien planet) didn't start naturally, but as experiment by some intelligent species; then that intelligent species could have intentionally made that similarity plus specific difference, probably in order to study what effect that difference has on evolution.
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