Why not six? Or more? Or just four? Here's why.
By
Jack Scanlan

1 Aug 2016 - 11:03 AM  UPDATED 1 Aug 2016 - 11:04 AM

At the risk of sounding high: when was the last time you really looked at your hands? (Whoa, dude.) We use them every single day for countless tasks, yet they often slip our minds. So why are they the way they are?

One of the basic facts about our hands is that they each possess four fingers and a thumb: five digits in total.

But why not four, or six? Cartoonists often reduce the number of digits they draw for convenience’s sake —just look at The Simpsons — but it appears that, for humans at least, evolution hasn’t had the same priority. 

Not quite enough fingers there, Grampa Simpson.

Five digits for everybody

Human pentadactyly (the technical term for ‘possessing five digits’) isn’t unique. In fact, the ancestor of all modern tetrapods — mammals, reptiles, amphibians and birds — had five digits on each of its four limbs back in the Devonian period, 420 to 360 million years ago. Even bats and whales have the bony remnants of five digits in their wings and flippers, respectively, even though they no longer have need for proper hands.

Essentially, we have five digits because our ancestors did.

Why this ancestral tetrapod had specifically five digits is still a mystery, though, according to Dr Justin Adams, a palaeontologist at Monash University.

“We know that in the Devonian there were many [fish-like] tetrapods with different digit ray numbers — five, seven, I believe up to 13 — but by the end of the period, we had only pentadactyl tetrapods,” he says.

Were five digits simply better than any other number? Any hypotheses “would be highly speculative without a lot of data on the types of selective pressures organisms at the time were facing,” says Adams.

“I would err on the side of caution and suggest that we simply don't 100% know the 'why' or 'how' of the narrowing of hand morphology to five digit rays in the Devonian”. 

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Losing digits

Curiously, many tetrapods have lost some of the five, such as horses, whose hooves are large, single digits, and cats and dogs, which have five digits on their front paws, but only four on their back paws. Not so in primates, where the dexterity of five digits comes in handy for picking things up, so losing a digit would be detrimental. 

But if five digits are good, then more would be even better, right?

Turns out it’s not easy to evolve more: while extra fingers or thumbs can be found on the hands of people with the congenital condition polydactyly (literally ‘many fingers’), they’re never properly functional. In fact, “there has never been a single case of adding a 6th digit” in modern tetrapods, says Adams.

“You would need to have structural changes in the hand, as well to the muscles of the forearm and the bones of the wrist, to make a 6th digit work.” And that’s probably been too hard for evolution to bother with. 

Special thumbs

While we humans might have a ‘standard’ number of digits, what sets us apart from other tetrapods are our thumbs, a particularly interesting twist in our hands’ evolutionary story.

“We have a specific muscle that flexes the first digit: the flexor pollicis longus,” says Adams. “When it occurs in apes, it is often tied in with other flexing digit muscles, which means that while humans can powerfully flex their thumbs and do so independent of their other digits, other apes have an extremely limited ability to do so.” 

Koala thumbs! Photo by Mike Hauser / CC BY 2.0 / Flickr

Our flexible thumb makes working with tools incredibly easy and probably helped early humans develop rudimentary technology.

Some other mammals, like koalas and possums, also have opposable digits, which they use to keep a firm grip on tree branches. But these ‘thumbs’ evolved independently from ours, and ours are a lot more versatile – a koala couldn’t text nearly as fast as you can.