What Are Vestigial Structures and Are They Truly Useless?

Vestigial structures are the footprints of our evolutionary past. Learn how they continue to shape the myths and truths of human biology today.

What is a vestigial structure?

More than one, vestigial structures are parts of our body left over from previous stages of evolution. They are remnants of physical features crucial for our ancestors but no longer vital for our survival. They aren't always functionless; some adapt over time to do something different than what they did in our ancient relatives.

Think of a vestigial structure as an old app on your phone that you don't use anymore. In the past, that app might have been useful, but now you’ve updated the phone, it’s mostly just taking up space. Just as technology moves on, so does biology, but we may still carry a little piece of the past with us.

How do vestigial structures provide evidence of evolution?

Modern beings have evolved over time, and in the process, we’ve held onto old organs and structures that aren’t needed anymore. These outdated biological features are evidence of how a species changes over time and evolves.

Vestigial structures can tell us a fascinating story about our evolutionary past. Though they served a purpose for our ancestral species, they’ve lost most or all their function as we have evolved. Instead of disappearing they’ve remained, often in a smaller form.

The reason they don’t disappear completely is because even though they’re not completely useful, they also don’t hinder our chances of surviving or having offspring. Instead, they remain as footprints of our evolutionary history.

Vestigial structure examples

Let's explore some examples of vestigial structures, parts of the body that once served a purpose in our ancestors but are now either unused or used differently. We'll take a look at the instances of both humans and other animals.

Vestigial structures in humans

Structures that may be considered as vestigial in humans include:

• The appendix. The vermiform (Latin for wormlike) appendix was considered redundant for many years. However, recent research suggests that the appendix may support our immune system and act as a safe house for beneficial gut bacteria, promoting a balanced microbiome.
• The coccyx. Also known as the tailbone, the coccyx is the remnant of a tail, once useful for balance in our primate ancestors. In humans, it is a small triangular bone and is an attachment point for several muscles, tendons, and ligaments.
• Wisdom teeth. The third set of molars is commonly known as wisdom teeth. They were once useful for helping our evolutionary ancestors chew foliage and digest plant material. As our ancestors harnessed fire and cooking, food became softer and easier to digest and our dependence on these teeth decreased.
• Auricular muscles. Muscles around the ears were useful for our ancestors to move their ears to hear predators and other threats. However, in humans, these muscles no longer serve any function.
• Darwin’s tubercle. On the ear itself, this small bump of cartilage is present in between 10–50% of the population and is thought to be a leftover feature from our pointy-eared ancestors.
• The plica semilunaris. This small fold in the inner corner of the human eye is a vestigial remnant of a transparent third eyelid widespread in animals such as fish and reptiles. When closed, it offers them eye protection and moisture while maintaining vision.

Aside from vestigial structures, we also have vestigial behaviors and reflexes—actions that have lost their original function. Two examples are hiccups and goosebumps. Hiccups are believed to be a remnant of our amphibian ancestors' breathing reflexes. Similarly, goosebumps mimic the way our hairy ancestors would puff up their fur to retain heat or appear larger to predators, a response that is largely unnecessary with modern human body hair. While these behaviors no longer serve their original functions, they remain embedded in our physiology.

Vestigial structures in other animals

Vestigial structures also exist in the animal kingdom, some examples of these include:

• Pelvic bones in whales. Whales have pelvic and hind limb bones, left over from when their ancestors walked on land. Although they no longer function as limbs, they remain present in whales. These structures now serve as support for reproductive organs, but they're considered vestigial because they’ve lost their previous function.
• Wings on flightless birds. Ostriches, emus, and penguins have wings, which are remnants of their flying ancestors. Though they cannot fly, their wings are vestigial because they have evolved to fulfill other functions, such as courting and swimming.
• Eyes in non-seeing animals. Animals that live in environments with a total absence of light such as certain salamanders, have degenerate eyes that no longer help them see.

Problems caused by vestigial structures in humans

Vestigial structures and organs can cause some health issues in humans. Let’s look at the most common cases below.

Appendicitis

Appendicitis is the inflammation of the appendix. This happens when the appendix becomes blocked by mucus, stones or feces. Such an obstruction leads to a buildup of bacteria and inflammation. If untreated, an inflamed appendix can burst, spilling bacteria into the abdominal cavity. As this can be life-threatening, appendicitis needs to be treated by surgical removal of the appendix.

An appendectomy is a surgical procedure to remove the appendix. Surgery can be done in two ways: open surgery through a single large incision or laparoscopically (keyhole surgery) using several small incisions. Recovery times vary, but laparoscopic surgery has a quicker recovery time, less pain, and minimal scarring.

Coccyx pain

Also known as coccydynia, pain in the tailbone is usually caused by a direct impact from a fall, excessive sitting, or childbirth. Less common causes of coccyx pain include infections and rare forms of cancer. Most cases of coccyx pain can be treated with anti-inflammatory medications, physical therapy, and the use of special cushions to protect the bone.

A coccygectomy is a procedure to remove the tailbone, usually due to persistent pain that doesn't respond to conservative treatment. This surgery can relieve pain but is also associated with a long recovery period and potential complications.

Impacted wisdom teeth

Wisdom teeth, also known as third molars, are at the back of the mouth. We all have four wisdom teeth, but they usually don’t appear until the late teens or early twenties, if at all. In some cases, wisdom teeth may become impacted due to a lack of space in the jaw. Impacted wisdom teeth may cause complications such as pain, swelling, and inflamed gums and may need to be removed by a dental surgeon.

Are vestigial structures truly useless?

The concept of vestigial structures is more complex than mere uselessness. The appendix and other vestigial organs have been called functionless. For example, research has suggested that the appendix isn't as useless as once thought, acting as a reservoir of beneficial gut bacteria and impacting immune function. Vestigial structures may have a different or reduced function rather than serving no purpose.

Research on vestigial structures in humans

It's important to acknowledge recent research that reevaluates their roles in human biology. While behaviors such as hiccups and goosebumps hint at our evolutionary past, some organs once considered vestigial may play significant roles. The appendix, often classified as vestigial, has been a focal point of such studies. It may not be the remnant of an ancestral part of the gut but a newly evolved one with a specialized function. In 2007, researchers suggested that the appendix may function as a safe house for bacteria, helping repopulate our guts with good bacteria after infection and supporting the function of the immune system.

Common myths and facts about vestigial structures

When it comes to vestigial structures, many misconceptions are floating around. Let's clear up some common myths and confirm the facts.

Myth #1: All humans have the same vestigial structures

Myth #2: Removing vestigial structures has no consequences

Myth #3: All vestigial structures in humans are internal

Myth #4: Vestigiality indicates organ failure

Ultimately, vestigial structures are an insight into the story of human evolution. These biological footprints demonstrate the complexity of our development from our ancestors to modern species. There’s a whole history written within us, waiting to be discovered, so why not take a deeper dive into human anatomy and evolutionary biology?