Those would be reasonable objections if we were talking about life with a common origin. But life on a different planet would not only have experienced utterly different selection forces, but the mechanism for responding to those forces would be utterly different.
Even if AAs on Earth have been selected as “especially useful building blocks” as the article suggests, there is no reason at all to believe they would be especially useful to alien life. Alien life wouldn’t code for proteins via DNA triplets, it wouldn’t construct them via ribosomes and so forth. The properties that make an AA especially useful to Earth life would likely be meaningless or actually detrimental to alien life. To pick a random example from your article: a hydrophobic side group is useful enough to our kind of life, but if the ribosome equivalenst of alien life were utterly incapable of bonding top hydrophobic AAs, those obviously would be selected against.
Secondly, the article you link to doesn’t suggest that specific AAs are subject to natural selection. All it says is that there is a benefit to life *utilising *AAs with a wide range of properties. That’s sensible enough, AAs are your toolkit, and you need a wide variety of tools to be adaptable.
However that doesn’t mean that any specific AA has been subject to selective pressure. All it means is that certain *types *have been subject to selection. If an AA with an arsenic side group has the same basic properties as an AA with a sulfur group, then which one ends up being used is entirely due to which one randomly evolves first. On Earth sulfur evolved first, on another planet it is just as likely top have been arsenic.
The same points go doubly for the issue of chirality. Added to which, the article in question notes that, if the mechanism is correct, then “other solar systems may harbour right-handed amino acids if they are subjected to the other type of circularly polarised light”.
In the end, the factors at play in selecting for AAs or chirality in life that doesn’t share any cellular machinery in common with us is going to necessarily radically different. The fact that AA preference is subject to natural selection here ion Earth actually *reduces *the chance of any shared AAs with alien life.
I agree with Blake, although as always when discussing alien life, there is the caveat that I mentioned earlier: we’re only working with one data point, Earth life, and we won’t really know for sure until we get more data.
However, as far as we can tell right now, the composition of molecules that make up life on Earth seems to be the result of a series of “frozen accidents”, where life chose one set of molecules. Other sets would have been, as far as we know, perfectly viable, and just as good as the ones we have. But for whatever reason, the ancient life form or forms that “won” evolution happened to be using the set that we still use today.
Now, it may turn out that we get out to the stars and find out that all life uses DNA, or the same set of amino acids, or the same chirality of sugars, or whatever. And that would require us to reevaluate our theories. But for now, no one has thought of a good reason to believe that that would be the case. Even if that did happen, it still seems unlikely that ALL of those frozen accidents are actually vital and inevitable decisions, so some of the details would surely vary from planet to planet. Any mismatch would almost certainly make alien life indigestible.
See, while I agree with the general thrust of what you’re saying, I don’t agree with this bolded bit - no two possible chemical combinations is ever going to be “just as likely”. One is always going to have a lower energy of formation than the other, and so be that little bit more likely, which can make all the difference to what gets selected for. So there’d need to be more looking into what the likelihoods are of various possibilities, based on atmospheric conditions, composition etc of the potential alien-life planet. It’s entirely possible arsenic side-chains might be more likely in a given environment. But “just as likely?” I think not.
No, I think it’s a valid non-semantic quibble - if some particular combinations are just much easier to make (let’s limit ourself to “on more-or-less Earthlike composition planets, not molten metal moons and gas giants” here) then you’d expect to find them with greater frequency than others. Unfortunately, I’m not an organic chemist, so I don’t know what the likelihoods are.
But I do know, by analogy, that we find generally the same silicate minerals on the Moon, on Mars and on meteorites and asteroids. Exotic crystal forms grown in low-G, sure. Skewed ratios of rare earths from the Earth norm, entirely possible. But a lunar basalt’s still very much a basalt in composition, a Martian montmorillonite’s still a montmorillonite and an asteroid’s olivine is still just olivine.
So I think that an earth-type planet (so not a chlorine planet or a dry ice planet) will have a greater than even chance of having similar biochemistry, if the energy differential for particular molecules is anything like what I think it is. No, this doesn’t preclude an entire biochemistry with unusual forms, but I happen to think that it’s not random chance that we get the molecules we do on Earth - that “natural selection” of a sort starts happening in some ways much sooner than the first replicators, just based on molecular formation energy. That any abiogenesis with our starting composition is going to land up with something similar to our bases.
In addition to needing the aliens to use compatible amino acids, we’d also have to have the right enzymes to break down whatever proteins and carbohydrates they used. Doesn’t that reduce the probability considerably?
Our digestive enzymes co-evolved with our prey and feed. I doubt they’re terribly universal. As mentioned above, we can’t even digest most carbohydrates of Earth plants. Some bacteria can, and have co-evolved to live in the guts of herbivores.
Eh…probably not. The proteases we have can handle pretty much any combination of amino acids we throw at them. It’s not like we have one enzyme set up to handle bovine collagen, and another to take care of chicken albumen, and so on. They’re pretty generic. If the alien proteins are made up exclusively of “our” amino acids, we should be able to break them down. Of course, that’s a rather massive “if”.
I like what ST:Voyager did when they introduced the Pred–um Hirogen race. They had them digesting the prey corpses into trophies … and maybe soup. Digest the organic material into simplest molecules that are still useful, amino acids, simple sugars, and the like. Separate out non-nutritive molecules. Enjoy some nutritive broth. Or if that doesn’t interest you, reassemble small molecules into tasty larger ones, sort of tofu-textured protein and sugar paste, augmented with nutrient stores. You save some space, and you waste less of the planet’s resources.
