The Origin Of The Human Hand: Why Do We Have 5 Fingers?

It's hard to imagine a world where Homo sapiens could have gotten very far without hands. Over thousands of years, our species has been heavily reliant on palms and fingers to accomplish a wide variety of tasks that make life as we know it possible, including (but certainly not limited to) cooking, cleaning, and writing. (By the way, here's what happens to your hands if you type for hours every day.)

Unsurprisingly, a quick look at your hands can tell you a fair bit about your lifestyle and health. For example, if you have swollen fingers, you might have high blood pressure. Meanwhile, the presence of calluses hints at a routine full of manual labor. But have you ever taken a moment to look at your hands and wonder why humans have only five fingers? 

And it's not just us; in fact, most mammals typically sport five digits (even seals and whales, creatures that don't have conventional arms like us) or start out with up to five in their embryonic stage (per a 2017 review in the Philosophical Transactions of the Royal Society B). 

It's reasonable to assume, then, that mammals and other tetrapods have similar finger counts thanks to a common ancestor from millions of years ago (aka homology). Basically, a yet-to-be-identified great-grandparent of all extant (surviving) tetrapods likely evolved the five-fingered trait, which turned out to be a remarkable evolutionary advantage.

According to a 1990 letter published in Nature, more than 350 million years ago, aquatic animals may have been the first organisms to develop fingers. Based on fossil evidence, some had up to eight fingers; over time and across generations, five probably became the magic number, the sweet spot that enabled species to survive, thrive, and pass their traits on to their descendants.

Since our shared genetic heritage is the likeliest source of clues for answering the five-finger question, researchers have spent decades studying the secrets that are literally in our hands. Findings of a 2016 study published in Nature identified two specific genes, hoxa13 and hoxd13, as the driving force behind fin ray formation and digit development. According to first author Yacine Kherdjemil, this also "clearly establishes a molecular link between fin rays and fingers" (via Science Daily).

But this raises another question: Why five? Science doesn't have a definitive answer yet.

Some say that we retained the five-finger trait simply because it became a stable and consistent genetic feature, in spite of the environmental conditions in which our progenitors persisted. This idea, called canalization, is perhaps most evident in the fact that, even though there's no special reason or purpose behind it, mammals almost always have only seven cervical (neck) vertebrae. This may explain why a giraffe has the same number of neck bones as a human: There's no particular need for evolution to improve on a formula that already works. Or, as the saying goes, if it ain't broke, don't fix it.

Curiously, according to a 2024 study in Nature, polydactyly (the state of possessing more than five fingers) is a fairly common mammalian mutation, one that only requires a single errant nucleotide in the sonic hedgehog (yes, that's what scientists actually call it) gene. Experts like evolutionary developmental geneticist Kimberly Cooper (via Live Science) believe that this finding hints at having five fingers simply being more advantageous than having more: "If it's that easy [to have more than five fingers,] why don't polydactyl species exist?" Plus, it's possible that gene linkages could become catalysts of further health complications (akin to a row of dominoes) if we evolve extra fingers.

(Now that you're done reading about this digit dilemma, you may also be interested in knowing the reason your fingers may sometimes become swollen.)