🦓 How would you give zebras their stripes?
If you were Mother Nature, how would you implement zebra stripes?
You can’t exactly tell each cell to “look at your neighbours, and if you’re surrounded by white, maybe turn black,” and even if you have a stripe-creation function, how do you control stripe width? You can’t tell a growing cell, “hey, this black region used to be 2cm, but now it’s 3, so maybe it’s time to turn white.”
It just seems like a ridiculously hard problem from a design perspective. Especially when you’re confined to design at the molecular level. It’s even more mindblowing to think that this has emerged from not a designer or a programmer, but random mutations and natural selection.
So I looked it up.
Turns out, the phenomenon was proposed by Alan Turing (I wonder if he figured out an algorithm before checking nature’s homework). Called reaction-diffusion, here’s how it works:
In the beginning, the zebra embryo’s skin starts with a random mix of black and white potential, physically embodied by melanin-producing cells being either active or inactive.
Then, two chemical signals are introduced: one activates melanin production (“Activator”), the other inhibits it (“Inhibitor”). Unlike cells, these chemicals can travel freely to spread across the skin.
You have a starting state with everything uniformly mixed (maximum entropy, maybe). But as cells divide and the body grows, small, random fluctuations in the concentration begin to appear. These fluctuations get amplified because of a feedback loop, where the system’s output influences its own future input:
Where the activator concentration is high, it encourages more production of both activator and inhibitor
Where the inhibitor concentration is high, it suppresses nearby activators activity
Inhibitors diffuse faster than activators
Boom: activator-concentrated areas (black) surrounded by inhibitor-rich areas (white), since inhibitors travel faster than activators and suppress neighbouring activator zones more quickly.
But wait, we’d get spotted zebras if that were the case, so diffusion must not be the same in every direction.
Turns out, a zebra's skin is made in a way that allows chemicals to travel faster in 1 direction; this anisotropic diffusion creates elongated patterns in one direction, shortening the other.
How do skin cells know which direction is which? And how does that make signalling chemicals travel faster?
Physically, it’s implemented by extracellular matrix fibres, which make up the scaffold of the skin, becoming aligned along the head-to-tail axis of the zebra embryo. Molecules diffuse more easily along aligned fibres.
As the zebra grows, this anisotropic scaffolding also makes the skin stretch more along the head-to-tail axis, further causing cell or spacing elongation and more physical anisotropy to alter diffusion paths.
Also, this anisotropy isn’t so magical - we humans have it too. Our tendons and ligaments are made of highly aligned fibres parallel to each other, making it easy to stretch in one direction and impossible in the other (thank god).
I know zebra stripes are just one of the many self-organizing structures that emerge from nature. But protein folding is also just a self-organizing structure, and that’s how life emerged. A simple recipe with simple conditions like 1) starting with randomness, 2) using small fluctuations that build up unavoidably in time, and 3) having a very simple feedback loop that amplifies certain signals to form a pattern.
Also consciousness is just a feedback loop, built, in the beginning, just to navigate…
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