Have you ever wondered how you get five digits on each hand and foot?
Why did you not develop four, six, or another number entirely?
Turns out there are three molecules you should thank.
The genes, known as Bmp, Wnt, and Sox9, work together to tell the body where to grow fingers.
As an embryo, you start to develop limbs at around week four.
A network of molecules that work together to create patterns in nature was first proposed by World War II codebreaker Alan Turing.
He became interested in computational biology later on in his career. In 1952 Turing wrote a paper predicting that chemicals could spread out over developing plants and animals, creating a pattern and determining the final form of the organism.
Now scientists have shown that Turing's network is in fact how fingers form.
Why (or even if) five is the optimal number is a more difficult question to answer.
"It's the ultimate meta-problem on top of everything," Sharpe told Ed Yong at Not Exactly Rocket Science.
Of course, it's not just humans that have five fingers and toes. In fact, this number dates back to before amphibians split from birds, mammals and reptiles 340 million years ago. Since then, some animals lost digits through evolution, but none seem to have gained them. (Moles and pandas have adapted a wrist bone for use as a sixth finger, but there are no examples of true six-fingered animals today.)
Biologist Michael Coates explains in Scientific American that this might reveal a constraint in digit numbers.