There can be a tradeoff between being tough and being squishy. A lot of animals — from cats to flying squirrels to octopuses — rely pretty heavily on the squishiness of their flesh to hide in narrow spaces, spread themselves out thin, or reach into places they otherwise couldn't.
Hard-shelled molluscs and crustaceans use the minerals that are cheaply available in their environment. This is mostly calcium; but scaly-foot snails that live near hydrothermal vents use iron too.
Cats' claws are sharpened by shedding in layers. When they scratch to sharpen their claws, they're not filing them down like an axe blade; they're flaking off worn layers to reveal fresh points. This depends on the microstructure of the claw in a way that could be incompatible with a tougher claw.
1pABA-based nonribosomal peptides exist -- the cystobactamids -- and wild-type E. coli ribosomes are happy to make aramids if you supply the right tRNAs. Making something structural through either of these pathways would definitely be a huge leap and unlikely to evolve.
But what gives Kevlar its strength is a whole lotta hydrogen bonds all lined up. That's hardly outside of proteins' central area of competency, which is how spider silk exists. So the hypothesis should not be flesh is weak because protein biochemistry is in a local optimum but evolution is smarter than you mere mortal and you have missed crucial dimensions of its optimization.
Now that de novo protein design is coming of age, there's a big push towards new protein-based fibers, so proteins may take their crown back from the aramids yet. But this is outside my area of expertise.
As for graphene, if you wanted to ball out and do it with synbio, you could maybe, super speculatively, do not trust me hack it enzymatically with Diels-Alder cycloaddition. Diels-Alderases are borderline nonexistent in nature, but some of the early successes in de novo enzymes were Diels-Alderases.
1The strongest spider silk is...weaker than Kevlar by density, but it's comparable, and much more elastic. Not bad at all. And it can be spun from aqueous solutions, while Kevlar is spun in pure sulfuric acid. You also have to consider UV resistance here.
hack it enzymatically with Diels-Alder cycloaddition
Yeah, no.
If you're interested in progress in aramid fibers, you could look up "5-(6)-amino-2-(4-aminobenzene)benzimidazole".
1TBC, nobody should think that making graphene enzymatically is in any way near-term plausible, or should be attempted as a project (god forbid), just that that's the only way I've heard of making it biologically that isn't an obvious type error. If someone wanted to write a sci-fi with biologically synthesized graphene, I would nod along if they said that.
Curious if you have any more thoughts on that -- "maybe this could be done if they figure out the inverse folding problem idk" was my best impression when I looked into it 5 years and a bio degree ago, but I don't know much about either the DA reaction or how carbon nanotubes are made etc.
(I'm bullish on the prospect of "Boltz/AF3/BioEmu successor models of the nearish future generalize enough that you can make an enzyme for darn near anything where you can make a theozyme/the thermodynamics work out," and completely ignorant of DA thermodynamics)
For some limits of enzymes, see these posts of mine:
Instead of using one molecule with two NH2 groups and one with two COOH groups, we can just use a molecule with an NH2 at one end and a COOH at the other end.
[...]The molecule in question---p-aminobenzoic acid---is fairly common in nature! You're basically one reaction away from having kevlar skin.
That wouldn't be Kevlar though, the structure would be different, and this has implications for the inter-strand hydrogen bonding that gives Kevlar its strength.
However I'm sure with some metabolic engineering a cell could make Kevlar biologically.
Silk and chitin are nature's substitute for Kevlar, though not quite as strong.
this has implications for the inter-strand hydrogen bonding that gives Kevlar its strength
What? It's the same strength. It's not like nylon 6 being weaker than nylon 66, the aromatic links are symmetric. Why would it be weaker?
I was expecting a discussion of Thulsa Doom's philosophy: https://youtu.be/wgN1sLcAQnw?si=D8VbsmoKD4uzbt9G&t=49
Ok, I got nerd sniped on the specific argument "Animals would be better off being made of stronger material than protein, but they don't because evolution can't find this solution".
Graphene
Graphene is (kind of) a wonder material. It's a single layer of carbon atoms arranged in a hexagonal grid, each one bonded to three others. Any damage to the structure entails breaking strong covalent bonds between the carbons. It's the strongest material per unit weight that we know of. The standard refrain is that a meter-by-meter hammock of graphene could support a 4kg cat while weighing less than one of the cat's whiskers.
(Of course, you could rip the hammock with your hands, or cut it with a knife, because per unit weight is doing all the work there. The sheet of graphene is weaker than, for example, a sheet of fabric, or a sheet of skin, because it's just so much thinner than them. Perhaps an animal could stack multiple layers of graphene, but at some point it loses its flexibility.)
The main problem for animals is that graphene is hard to build with proteins, and it's highly hydrophobic. It's also really weird to try and build on a molecular level. You have to deposit individual carbon atoms. How this might work in principle in a self-contained living organism is unclear to me. I find it very difficult to think about multilayer graphene deposition in living organisms.
But thankfully, we don't have to worry about that, because we can instead worry about why animals don't have kevlar skin.
Steel
"Why don't cats have steel claws" seems much easier to answer. There's not that much iron around, and it's too energy expensive to make into claws. Case closed here.
Kevlar
Kevlar is another very strong material. It's made of a polymer of p-phenylenediamine, and p-benzene dicarboxylic acid. Each of the two NH2 groups on p-phenylenediamine joines to a COOH group on p-benzene dicarboxylic acid.
The resulting long strands are fairly rigid and have two different ways to associate with each other. In the left-to-right direction, the C=O forms an interaction with an NH, in the other direction, the benzene groups have a kind of "stacking" interaction. Both of these are stronger than the weakest forces which hold proteins together, and the rigidity of the strands provides further strength: if you want to separate two strands, you can't "peel" them apart, you have to "lever" them apart, breaking many interactions at once. Kevlar is basically the strongest fibre which humans mass-produce.
Unfortunately, for Mother Nature, Kevlar is trademarked, and neither p-phenyelediamine nor p-benzene dicarboxylic acid is particularly common. But fortunately, we have a different option. Instead of using one molecule with two NH2 groups and one with two COOH groups, we can just use a molecule with an NH2 at one end and a COOH at the other end.
(Also, this is more like how biology likes to do things, compared to how industry does. In industry, it's easier to have two ingredients that you mix, whereas in biology it's often better to have just one)
The molecule in question---p-aminobenzoic acid---is fairly common in nature! You're basically one reaction away from having kevlar skin.
So why don't you? This should be totally doable! A kevlar matrix instead of a collagen one seems possible, at least more possible than graphene.
But Kevlar isn't good against crushing or piercing attacks (which are the more common kinds of attacks in the wild, think lions and snakes) so perhaps it's not worth the extra risk. The other problem is that that modern kevlar needs to be spun in threads from hot sulfuric acid.
Yeah, to dissolve Kevlar polymer, they have to use sulfuric acid. Then they can draw the fibers from it in a way that makes the polymer molecules line up. Then they can weave these fibers into a tight fabric which is actually strong. This weaving would also be difficult to organize on the molecular level.
Natural Kevlar would be quite a bit weaker than modern acid-spun Kevlar. It might not make up for the fact that it would slow you down a when running from lions (I mean, cellulose is stronger than collagen, and animals don't use that) and that this is more costly than being resistant to attacks.
Nonetheless, the kevlar skin argument seems somewhat stronger to me than the graphene skin one, but...
It's All Too Confusing
Unfortunately for those trying to make a clean argument, biology is too cursed for things to make clear sense. It's not a case of "biology could do X and be better" it's a case of "biology could do X if it made trade-offs Y and Z, and discarded A and B, which would probably be net better if it didn't cost too much energy and slow reproduction down too much." Sad!
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