It’s hard to envision a world where Homo sapiens could have progressed without hands. Over millennia, our species has heavily relied on palms and fingers to perform various tasks essential to our way of life, such as cooking, cleaning, and writing. (By the way, here’s what happens to your hands if you type for hours every day.)
Interestingly, a glance at your hands reveals much about your lifestyle and health. For instance, swollen fingers might indicate high blood pressure. Similarly, calluses suggest a life filled with manual labor. But have you ever pondered why humans have only five fingers?
It’s not unique to us; indeed, most mammals usually have five digits (even seals and whales, species without conventional arms) or begin with up to five in their embryonic stage (according to a 2017 review in the Philosophical Transactions of the Royal Society B).
Therefore, it’s reasonable to assume that mammals and other tetrapods share similar finger counts due to a common ancestor from millions of years ago (a concept known as homology). Essentially, a yet-to-be-identified great-grandparent of all existing tetrapods likely developed the five-fingered trait, which proved to be a significant evolutionary advantage.
What science tells us about why we have five fingers
According to a 1990 letter published in Nature, over 350 million years ago, aquatic animals might have been the first organisms to develop fingers. Fossil evidence indicates some had up to eight fingers; over generations, five likely emerged as the optimal number, the sweet spot that allowed species to survive, thrive, and pass their traits to descendants.
Given that our shared genetic heritage likely holds clues to the five-finger question, researchers have spent decades exploring the secrets literally at our fingertips. A 2016 study published in Nature identified two specific genes, hoxa13 and hoxd13, as crucial for 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).
However, this raises another question: Why five? Science has yet to provide a definitive answer.
Why don’t we have more than five fingers, then?
Some suggest we retained the five-finger trait because it became a consistent and stable genetic feature, regardless of the environmental conditions our ancestors faced. This concept, called canalization, is evident in the fact that, despite lacking a particular purpose, mammals usually have only seven cervical (neck) vertebrae. This might explain why a giraffe has the same number of neck bones as a human: Evolution has no incentive to refine a formula that already works. If it ain’t broke, don’t fix it.
Interestingly, a 2024 study in Nature reveals that polydactyly (having more than five fingers) is a relatively common mammalian mutation, requiring just a single nucleotide error in the sonic hedgehog gene (yes, that’s its real name). Experts like evolutionary developmental geneticist Kimberly Cooper (via Live Science) suggest that this finding implies five fingers may be more advantageous than having more: “If it’s that easy to have more than five fingers, why don’t polydactyl species exist?” Furthermore, gene linkages could trigger additional health issues (similar to a domino effect) if we evolved extra fingers.
(Now that you’ve explored this digit dilemma, you might be curious to learn about why your fingers sometimes swell.)
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