Think about everything your hands do in a day – scrolling through your phone, typing out thoughts, or cooking something from scratch. It’s easy to feel like our hands and arms evolved specifically for these modern human tasks.

But an anatomical structure inside our wrists hint at a different origin. These tiny bones don’t owe their existence to our evolutionary history as toolmakers.

Instead, they connect us directly to a much wilder past: a time when our ancestors walked on bent knuckles across the forest floor.

A new study has reopened one of the longest-running debates in human evolution: Did the common ancestor shared by humans, chimpanzees, and gorillas walk on its knuckles?

Rethinking how ancestors moved

For decades, scientists have argued over how our ancient ancestors moved. Chimpanzees and gorillas both knuckle-walk when they travel on the ground.

Humans, of course, walk upright. That leaves one major question. Did our shared ancestor also move like a knuckle walker before human evolution took a different path?

Researchers have searched for clues in the spine, pelvis, shoulders, and hands. Different studies reached different conclusions, leaving the debate unresolved.

This new research approached the mystery from another angle. Instead of focusing on large bones, the team examined the carpus, the set of small bones that form the wrist.

Human wrist bones hold clues

The wrist is far more complex than it appears. Each carpal bone connects with several others through ridges, grooves, and curved surfaces that control movement and stability. These structures determine how the wrist handles pressure and weight.

Because the bones are small and irregularly shaped, many earlier studies relied on rough measurements or visual comparisons.

Laura Hunter and her colleagues at the University of Chicago decided to use a more detailed approach.

The researchers created three-dimensional models of nearly every carpal bone across a wide range of primates. They then analyzed the shapes using spherical harmonics, a mathematical method that converts complex surfaces into measurable data.

That allowed the team to compare wrist anatomy in far greater detail than before.

Comparing thousands of wrists

The scale of the study stands out. Researchers examined more than 2,000 carpal bones from living primates, including humans, chimpanzees, gorillas, bonobos, orangutans, gibbons, and several monkey species.

They also included fossil hominins such as Australopithecus afarensis, Australopithecus africanus, Homo naledi, Homo floresiensis, and Neandertals.

This broad comparison mattered because studying only humans and chimpanzees can create misleading conclusions. Similar features might come from shared ancestry or from separate evolutionary changes.

By adding monkeys and other apes into the analysis, the researchers gained a clearer evolutionary picture.

Philip Reno, a developmental biologist at the Philadelphia College of Osteopathic Medicine who was not involved with the study, called the project “impressive,” adding that “they bring up many interesting kinds of questions that relate to the mosaic of hominid evolution.”

Human wrist bones resemble apes

One of the clearest findings was that human wrists resemble African ape wrists far more than monkey wrists.

Several bones, especially the lunate and triquetrum, are strikingly similar between humans, chimpanzees, and gorillas. Other wrist bones beside humans, including the hamate and trapezium, also share close similarities.

The researchers believe these features likely existed in the common ancestor shared by humans and African apes millions of years ago.

That alone does not prove knuckle-walking. However, the specific shapes of these bones appear linked to the mechanics of weight-bearing on bent fingers.

Signs of knuckle-walking

Chimpanzees and gorillas use a specialized wrist structure during knuckle-walking. Their wrist bones lock together tightly, creating stability while supporting body weight.

Several features linked to that locking mechanism also appear in humans. These include the shape of the lunate bone, the fusion of the scaphoid and centrale bones, and the broad head of the capitate.

Humans no longer use their wrists for knuckle-walking, but that older structure may still remain.

“If these features remained in our lineage, it is surely not because we’re knuckle-walking,” Hunter said. Instead, evolution may have repurposed older wrist structures for newer functions.

The researchers argue that knuckle-walking currently offers the simplest explanation for why humans and African apes share so many wrist traits.

Climbing theory still survives

The study does not completely rule out other possibilities. Some shared wrist features may also support vertical climbing, an activity common among chimpanzees and gorillas.

For example, a stronger ridge on the trapezium bone may help support thumb muscles during climbing.

Still, the researchers found that climbing alone could not explain the full pattern. Orangutans climb extensively, yet their wrist anatomy differs from that of African apes and humans.

That difference strengthens the case for a knuckle-walking ancestor.

Human hands evolved later

Although much of our wrist structure appears ancient, some bones changed significantly during human evolution.

The capitate, scaphoid, trapezoid, and trapezium show major differences in modern humans, especially on the thumb side of the wrist.

These changes likely supported precise hand movements linked to tool use. Features such as a wider trapezoid surface and an expanded capitate neck improved thumb control and fine manipulation.

Interestingly, these newer adaptations seem to have built upon the older ape-like wrist structure already present in our ancestors.

The modern human hand did not appear from scratch. It evolved step by step on top of an inherited framework.

Ancient wrists looked different

Some fossil wrist bones added another twist to the story. Certain hominin capitates look surprisingly similar to those of monkeys that walk on their palms through trees.

Earlier researchers viewed this as evidence against knuckle-walking ancestry. The new study offers an alternative explanation.

According to the authors, after ape-specific features disappeared and before modern human tool-related traits fully evolved, the wrist may have passed through a more generalized stage. That temporary structure happened to resemble monkey anatomy.

Early humans adapted later

The fossil evidence also suggests that advanced tool-related wrist adaptations appeared later than expected.

Homo naledi and Homo floresiensis show considerable variation in wrist anatomy. In Homo naledi, one individual had a more modern wrist while another looked more chimp-like.

If intensive toolmaking had shaped these species for a long time, researchers would expect more consistency.

Instead, the findings suggest that early members of the genus Homo may not have depended on constant, sophisticated tool production.

Modern wrists carry ancient history

The study does not settle the debate completely. Researchers still lack crucial fossils from species such as Homo erectus and Homo habilis.

Some scientists also argue that shared wrist similarities may simply reflect close evolutionary relationships rather than specific behaviors.

“Showing morphological similarity of humans with our closest relatives is kind of the null hypothesis you would expect,” Reno said. “You need differences to really tease out whether there has been actual selection.”

Understanding our beginnings

Even so, the new evidence strengthens the idea that our ancestors may once have traveled on their knuckles before evolving into upright walkers and eventually becoming skilled tool users.

That possibility changes how we think about the human hand. Beneath the precision needed to type, paint, or shape stone may lie an older structure inherited from a quadrupedal ancestor.

“We became the human lineage, but understanding where we started from is what tells you how we got here,” Hunter said.

The study is published in the journal Proceedings of the Royal Society B: Biological Sciences.

NOTE – This article was originally published in Earth and can be viewed here