A leopard has spots. A zebra has stripes. A cow has patches. Yet every one of them began as a ball of near-identical cells with no picture on it. Where does the pattern come from?
In 1952 Alan Turing, better known for codebreaking and early computers, published a paper called "The Chemical Basis of Morphogenesis". He asked a simple question: could two chemicals that react and spread through a uniform tissue spontaneously break its uniformity?
His answer was yes, under one condition. One chemical, an activator, makes more of itself and makes an inhibitor. The inhibitor shuts the activator down, but it spreads faster. Tiny random bumps of activator then grow, while the fast inhibitor suppresses activator in a ring around each bump. The result is regularly spaced spots or stripes, with a built-in size that depends on the chemistry rather than on any blueprint.
Below is a classic two-chemical model, the Gray–Scott system. Bright lime is chemical V (the activator-like one); dark is U, which feeds it. Pick a pattern, drag across the canvas to drop in a seed of V, and change the two dials to see how a small change in chemistry turns spots into stripes.
Drag on the canvas to add chemical. Spots divide and settle; stripes wriggle into mazes.
Each pixel holds two amounts, U and V. Every step, V eats U to make more V (the reaction U + 2V → 3V), U is topped up at the feed rate F, and V is removed at a rate F + k. Both chemicals diffuse to neighbours, but U diffuses about twice as fast as V in this model. That unequal speed is the whole trick: it is Turing's "local activation, long-range inhibition" in another costume.
Notice what the dials do. They never mention spots or stripes. A small nudge in F and k changes which pattern the same equations settle on, which is one reason closely related animals can end up with quite different coats.
For decades Turing's idea was a beautiful hypothesis. A strong clue came in 1995, when Shigeru Kondo and Rihito Asai studied the marine angelfish Pomacanthus in Nature. As these fish grow, their stripes do not just stretch. New stripes appear and existing ones split, branch and shift to keep the spacing roughly constant. A reaction–diffusion simulation reproduced that rearrangement closely, which the authors took as strong support for a reaction–diffusion wave on the fish's skin.
Later work on zebrafish found pigment cells that interact over short and long distances in ways that fit a Turing-type system, although the actors are cells and signals rather than two simple chemicals. Biologists still debate the molecular details, and different animals may use different versions of the same logic. The honest summary is that the principle is well supported, and the specific recipe varies.
Turing-like logic is now used to think about more than skin: the spacing of hair follicles, the number and position of digits in a developing limb, and even the ridges on the roof of a mouth are studied this way. The big idea is that complex order does not need a complex plan. A simple local rule, repeated everywhere at once, can draw a picture by itself.