Animals

# Why Do We Have 10 Fingers and 10 Toes?

## As humans we could have had any number of fingers and toes, but somehow we ended up with 10 of each. What's so special about 10?

It's the kind of question that usually pops up in kindergarten class, or at 2 a.m. in college dorm rooms after popular tetrahydrocannabinol derivatives have been metabolized:

Why do we have 10 fingers and 10 toes, anyway?

As Lissette Padilla explains in this DNews dispatch, the truth is that we don't really know. But scientists have some pretty good guesses.

When in doubt, consult a theoretical neurobiologist, we always say. In 2001, author and researcher Mark Changizi proposed a mathematical hypothesis called the Limb Law. His equation attempts to suss out the ideal number of limbs an animal requires, depending on its environment and body size.

Applying the Limb Law to 190 animal species from seven different phyla, Changizi found that his math held up -- and the researcher suspects the same equation could explain why we have 10 fingers. In this case, our hands are the "animal" and our fingers are the "limbs." Crunch the number and you get 4.71 fingers per hand. Mother Nature, it appears, rounds up.

RELATED: 'Yeti Finger' Mystery Solved

Ask an evolutionary biologist, however, and you're likely to get a much simpler answer: We have 10 fingers and 10 toes because, somewhere in our species' past Darwinian wanderings, those numbers gave us an evolutionary advantage. Had events tumbled differently, we might have eight fingers and twelve toes.

An interesting side note: Harvard geneticist Cliff Tabin, who specializes in vertebrate limb development, suggests that if we ever did evolve a sixth finger, it would likely grow out of our wrists, like a panda.

Speaking of genetics, there's still another way to come at the question. In 2014, a research team led by James Sharpe from the Centre for Genomic Regulation in Barcelona, Spain, determined that fingers and toes are patterned by three specific embryonic molecules -- Bmp, Wnt and Sox9.

The basis for this discovery, by the by, was a theory first proposed by science superhero Alan Turing back in 1952. That guy thought of everything.

Science Daily: A Mathematical Theory Proposed by Alan Turing in 1952 Can Explain the Formation of Fingers

National Geographic: How Did You Get Five Fingers?

Discovery News: Human Hands More Primitive than Chimp Hands