“Little Kid” Science Question: Fingers and Toes

It's one of those "little kid" questions, something you probably haven't given any thought to since you were little, or if you're a parent, your kid may have asked it.  The ones that are borderline impossible to explain in terms a little kid can actually understand.

Except this time, the question in question isn't "Why is the sky blue?", it's "Why do we have five fingers and toes?"

Really?

Why not?  It is one of the odder questions about biology I can think of.  Why is five the magic number?  Sadly, that may be the only part of that question we never get an answer to.

So let's get started.

The short answer is that hands and feet evolved from fish fins, and the bony supports of those fins eventually became fingers and toes.

Now for the longer explanation.

We all know scientists like to categorize things.  One of the categories humans are grouped into is the "tetrapod" group, which literally means "four-legged" or "four-limbed", depending on how you want to look at it.  Basically, it's anything with a spine and four limbs.  In this group with us are reptiles, amphibians, other mammals and even birds.

Millions of years ago, all those now very different groups shared a common ancestor.  Whatever the environment at the time was, that common ancestor eventually grew five digits on it's limbs, and that became part of the blueprint for the species that came from that common ancestor.

And before you point out that cows, horses, and other hooved animals have four legs but not fingers and toes, you're only partially right.  Turns out they have five digits very early on, when they're embryos, that eventually grow together into hooves.