Many features we carry today are biological “leftovers,” remnants passed down from a long line of human predecessors. Although our lifestyles have changed dramatically—we don’t sleep in trees, hunt with our bare hands, or migrate with the seasons—our bodies still hold onto certain traits that once played essential roles in survival.

Take goosebumps, for example. This strange little reaction is far from random. For our mammalian ancestors, raised hair helped trap warm air close to the skin, insulating them against the cold. When their hair stood up, their overall surface area increased slightly, allowing their bodies to retain heat more efficiently. Even now, when we’re cold or startled, a tiny muscle at the base of each hair contracts, lifting the hair and creating those familiar bumps along our skin.

Goosebumps don’t serve the same purpose for humans today—we lack the thick fur that once made the reaction effective. Yet the response persists, reminding us how deeply our biology is rooted in our evolutionary past. Modern mammals still rely on this mechanism, such as birds puffing up their feathers during winter or cats bristling their fur when frightened. These instinctive reactions echo the same ancient survival strategy.

But among all these vestigial traits, one stands out as an especially vivid marker of evolution in progress: a tendon inside your wrist that only some people have. This tendon is linked to a muscle called the palmaris longus, a structure that once helped our arboreal primate ancestors—like lemurs and early monkeys—grip and swing from branches. The palmaris longus provided strength, stability, and flexibility for life high in the trees.

As humans evolved and adopted a more upright, ground-based lifestyle, this muscle’s importance gradually faded. It became less essential for movement and survival, especially for species like gorillas and humans, who don’t use their arms for swinging through canopies. Over many generations, the muscle began shrinking in function and relevance.

Interestingly, evolution happens slowly and unevenly, so about 85–90% of people still have this tendon today. Yet modern studies show that 10–15% of the population no longer possesses it at all—a clear sign that our bodies are continuing to change as certain traits become less useful.

You can easily check whether you still have this evolutionary relic. Place your forearm on a table with your palm facing upward. Bring your pinky finger and thumb together, then gently raise your hand off the surface. Look at the center of your wrist. If a prominent band pops up beneath the skin, you have the tendon of the palmaris longus.

If nothing appears—congratulations! You’re displaying a small but fascinating example of human evolution in action. While the presence of the tendon connects you to our distant primate past, its absence shows how natural selection continues to shape human anatomy today.

Either way, this simple test offers an astonishing reminder: evolution isn’t something that happened long ago—it’s still happening right inside our bodies.