If a Tendon Pops Up When You Touch Your Pinky to Your Thumb, Here’s the Surprising Reason Why

Evolution is a captivating and immensely complex process—a tapestry woven over millions of years that connects all life forms on Earth. The concept that all mammals, including humans, share a deep ancestral connection is truly intriguing, tracing our lineage from early tree-dwelling primates through Neanderthals, leading up to the modern Homo sapiens we are today. Although it can be challenging to fully comprehend the vastness of this timescale, there is abundant, undeniable evidence supporting evolution, much of which can be found in the most intimate parts of our own anatomy.

The Human Body as a Window into Evolution

Many of the physical traits we possess, both inside and out, are not perfectly designed for modern life; rather, they are leftovers—vestigial features—from our distant ancestors. Although these characteristics hold little use in our daily routines today, they were once vital for survival. Even though we no longer hunt for food in the wild or live as nomads, these nearly obsolete characteristics have been faithfully passed down through generations.

Consider the common experience of getting goosebumps:

Ancient Function: This physiological reaction isn’t random. Our mammalian ancestors used this reaction, known as piloerection, to stay warm in cold weather. When the small muscle attached to the hairs on our arms contracts, it makes the hairs stand up.
Insulation and Intimidation: In fur-covered animals, this action increases the surface area to trap heat, creating a layer of insulation. Similarly, when animals feel threatened—like when you startle a cat—their fur fluffs up. This ancient defense tactic makes them appear larger and more intimidating to potential predators, helping to ward off danger.
Modern Echoes: Though this reaction doesn’t significantly warm a relatively hairless human, it’s still very much alive in the animal kingdom, visible in how pigeons puff up their feathers or cats arch their backs. This persistence is a clear, continuous sign of evolution in action.

Similarly, an even more striking piece of evidence for evolution can be found in our arms, specifically in a tendon that is subtly but definitively disappearing in a segment of the human population.

I. The Vestigial Anchor: The Palmaris Longus Tendon

The tendon that appears in the wrist during a simple maneuver is linked to an ancient muscle called the palmaris longus. This almost obsolete feature is a captivating link to our shared primate ancestry and a clear sign that human evolution is still ongoing.

The Test for the Tendon

To see if you possess this vestigial tendon, follow these simple steps:

Position: Rest your forearm on a flat surface, such as a table, with your palm facing upwards.
Action: Bring your thumb and your pinky finger together to touch.
Lift: Gently lift your hand slightly at the wrist.

If a raised band, cord, or ridge appears prominently in the center of your wrist, you possess the tendon connected to the palmaris longus muscle.

The Ancient Function: A Primate Grip

The palmaris longus muscle originates in the elbow and runs down the forearm, connecting to this tendon in the wrist.

Arboreal Life: The muscle was primarily used by tree-dwelling primates, suchoses as lemurs and monkeys, to assist with powerful grasping and climbing. It played a crucial role in flexing the hand and tensing the skin of the palm, providing the firm, strong grip necessary to move quickly and safely between branches.
The Evolutionary Shift: As our ancestors descended from the trees and adopted a bipedal stance on the ground, the palmaris longus muscle gradually became less necessary for survival. Humans and ground-dwelling apes such as gorillas and chimpanzees no longer rely on this muscle for critical locomotion or survival skills.

Vestigial Status and Anatomical Redundancy

In modern humans, the palmaris longus is classified as a vestigial structure—an anatomical feature that has lost most, if not all, of its original function through evolution.

Redundant Function: The muscle is now almost entirely redundant. Its function is effectively duplicated by other, stronger flexor muscles in the forearm. In fact, when the palmaris longus tendon is needed for a graft during reconstructive surgery (e.g., in hand, wrist, or cosmetic procedures), its removal typically results in no measurable loss of grip strength or motor function for the patient.

II. The Disappearing Trait: Evidence of Ongoing Evolution

The palmaris longus provides a visible, measurable, and easily reproducible piece of evidence that human evolution is still actively underway.

The Absent Population

While an estimated 90% of people still possess this tendon, a significant minority—around 10% to 15% of the population—are born without it.

Genetic Determinism: The absence of the palmaris longus is a genetically determined trait. Its disappearance represents a selective pressure favoring the efficient elimination of unnecessary anatomical structures. Evolutionary theory posits that if a structure is no longer beneficial, the resources required to grow and maintain it are better spent elsewhere.
The Ongoing Process: Those who possess the tendon carry a physical connection to our arboreal ancestors, effectively carrying a “backup” feature we no longer need. Those without it demonstrate a clear, ongoing trend toward anatomical simplification—the gradual, irreversible loss of a trait that has become obsolete in the Homo sapiens lineage.
Asymmetrical Loss: Interestingly, the tendon can also be asymmetrical, meaning an individual might possess it in one arm but not the other, further highlighting the sporadic, gradual nature of its genetic elimination.

The Significance of Absence

Showing Evolution: What It Means If You Don’t Have This Tendon

Whether you have this tendon or not, it provides a captivating link to our evolutionary history.

Presence: Those who possess it carry a physical connection to our ancestors, linking them directly to the age of climbing and powerful grasping.
Absence: Those without it demonstrate a clear, measurable sign of ongoing human evolution, representing the lineage that has successfully “edited” this superfluous trait from the genetic blueprint. It is a biological marker of a fully ground-dwelling, bipedal species.

III. Vestigial Features: The Body’s Evolutionary Archive

The palmaris longus is merely one of many vestigial features that turn the human body into a walking archive of our deep past. These features underscore how traits persist even after their original function has been lost.

Other Prominent Vestigial Structures

The Coccyx (Tailbone): The remnants of a tail, which our primate ancestors used for balance and communication. While no longer external, the fused vertebrae of the coccyx still serve as an anchor point for some pelvic muscles.
The Appendix: Believed to have once functioned as a chamber for digesting cellulose in plant matter (similar to the digestive organs of herbivores), it is now largely redundant in humans, though it may play a minor role in immune function.
Wisdom Teeth: These third molars were vital for our ancestors, whose diets consisted of coarse, tough plants and unprocessed meat. Modern diets and reduced jaw size have made them unnecessary and often problematic.
The Auricular Muscles (Ear Wiggling): These muscles allow some mammals (cats, dogs, rabbits) to swivel their ears to locate sound. While humans still possess the muscles, they are largely non-functional, with only a small percentage of people able to consciously move their ears.

These features, alongside the palmaris longus, provide overwhelming evidence that the human form is not a static design but a continuously evolving compromise between necessity and obsolescence.

IV. The Genetics and Anatomy of the Tendon’s Absence

The mechanism behind the palmaris longus’s absence is a fascinating topic in developmental biology and genetics.

Developmental Variance

During embryonic development, the muscle and tendon structures in the forearm are formed through a complex process of cellular differentiation and growth.

Gene Expression: The presence or absence of the palmaris longus is governed by the expression of specific genes that control the growth factors and cellular signaling pathways responsible for the formation of muscle bellies and their corresponding tendons.
Selective Advantage (Hypothesized): While the absence offers no immediate disadvantage in modern life, the advantage is the simple efficiency of not spending the metabolic energy or anatomical space required for a redundant muscle. Over millennia, this slight selective pressure has been enough to begin eliminating the trait entirely.

Surgical Relevance: The Silent Donor

The palmaris longus tendon’s vestigial status makes it a favorite among surgeons, particularly those in plastic and hand surgery.

The Go-To Graft: Because its removal causes no functional deficit, it is often referred to as the “go-to donor tendon.” It provides a surgeon with a piece of high-quality, biologically compatible tendon tissue that can be used to repair crucial structures, such as a ruptured ligament in the hand or wrist, or to reconstruct facial muscles.

The palmaris longus tendon is a compelling, living marker of our evolutionary journey. Whether it “pops up” on your wrist or not, it tells a story of millions of years of adaptation—from gripping tree branches to mastering the ground—and serves as a constant, tangible reminder that our anatomical future is still being written.

If a Tendon Pops Up When You Touch Your Pinky to Your Thumb, Here’s the Surprising Reason Why

The Human Body as a Window into Evolution