Nature exhibits a seemingly endless array of patterns – from the cow’s spots to
the surgeonfish’s stripes to the cheetah’s spots and stripes – but Alan Turing (the
guy who invented computers) thought those patterns might not actually be all that different.
To show what he meant, he came up with a simple set of mathematical rules that could give
rise to all manner of patterns.
The rules go something like this: Inside each theoretical organism, there are
two substances called an activator and an inhibitor.
The activator stimulates production of both substances, while the inhibitor slows production
down.
In other words, it’s kind of like a predator-prey relationship: the more activator bunnies there
are, the more new bunnies they make, but a bigger bunny population also means more food
for the inhibitor foxes, which means more foxes, which leads to fewer bunnies, which
leads to fewer foxes.Except that Turing’s rules say that, as the populations of predator
and prey increase, their ranges expand – only, the foxes need more room, so their range expands
faster than the bunnies’ range.
Soon, the foxes dominate the surrounding areas, leaving the bunnies in the middle to keep
multiplying.
So what we end up with is a pool of activator surrounded by a moat of inhibitor . And wherever
the activator is more abundant than the inhibitor, it triggers some kind of change – like the
production of pigment – which, in this case, gives rise to a spot of color.
The beauty of Turing's theory is that, by adjusting the variables, like the spreading
rate of the activator and inhibitor, or the total area of the system, you can get all
kinds of patterns.
For example, if you start out with slightly more activator than inhibitor in a bunch of
spots, you might end up with...well, a bunch of spots.
Or, if the system is just a narrow little strip – like a snake or a tail – you might
end up with stripes.
Or, if the activator spreads a little faster, it might leak out and join up with other patches
of escaping activator, creating a labyrinth pattern.
What’s more, Turing’s rules could be used to simulate patterns that closely mimic the
splotches on cows, the stripes on fish, the mosaic patterns on giraffes, and even the
tentacles on hydras.
But the fact that his mathematical rules worked on paper didn't prove that nature followed
them.
And now, several decades later, scientists are still trying to figure out whether some
of the patterns and structures in nature arise from real-life activators and inhibitors.
On the one hand, we’ve found some patterns that happen in a very un-Turing way, like
the segments in a developing fruit fly, which come predetermined by the fly’s genetic
blueprint..
But on the other hand, we’ve also come across some tantalizingly Turing-like systems; in
developing mice, for example, a protein called “sonic hedgehog” inhibits another, activator
protein with a less awesome name, producing stripey ridges on the roof of the embryo’s
mouth.
And in its nubby limb ends, three different proteins activate and inhibit tissue growth
to generate the stripe-like appendages known as digits.
But regardless of how perfectly or imperfectly Turing’s theory describes what we see in
the real world, the coolest thing about it may be that it inspired biologists to go looking
for evidence of Turing’s ideas in living creatures.
So observations inspired a theory that inspired observations that are bringing us a little
closer to understanding how the cheetah got its spots – and its stripes.
This video was supported by Audible, where you can find the largest selection of audiobooks
available anywhere, including "The Information," an awesome book by science writer James Gleick
that explores how luminaries like Alan Turing have helped us figure out what information
even is, and how we can go about creating, transmitting, and storing it.
To download The Information or another book of your choice, and show your support for
MinuteEarth, go to www.audible.com/minuteearth.
Oh – AND, a while ago we put out a survey to find out about your personal experiences
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