Why is Flesh So Weak?

J Bostock
Tue, 07 Apr 2026 22:13:05 GMT

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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