Las Vegas Sun

March 28, 2024

5-MINUTE EXPERT:

A guide to our useless body parts

The body is a glorious thing. Humans have evolved to have straight fingers rather than curved, pelvises designed to support upright walking and brains that have tripled in bulk. But evolution also has rendered many of our parts useless. Vestigial structures no longer serve a purpose, but were retained over millions of years. Here’s a look at some of our biological leftovers.

How structures become vestigial: When anatomy works against survival, natural selection typically ensures that the trait is selected out of a population, because individual creatures with the feature typically die younger and are less likely to reproduce. Vestigial structures, however, don’t affect survival, so they haven’t been selected out of the population. They remain part of our anatomy but have long lost their function because of changes in human environment and behavior.

1. Plica semilunaris: This small flap of skin covers the human eye near the inside corner. It’s believed to be a remnant of the nictitating membrane found in many vertebrate species at the inner angle or beneath the lower lid and extending across the eyeball to protect and moisten it. Reptiles, birds, sharks and some mammals evolved this “third eyelid.”

2. Vomeronasal organ: This tiny pit on each side of the septum, the partition that separates each nostril, is lined with nonfunctioning chemoreceptors that evolutionary biologists believe once were used for detecting pheromones and played an important role in reproduction and chemical communication.

3. Wisdom teeth: Wisdom teeth, or third molars, were useful when humans ate a diet of tough meat, coarse leaves and hard nuts, which called for extra chewing power and often resulted in lost or damaged teeth. The jaw developed into a larger bone because of the constant workout, leaving plenty of space for wisdom teeth when they emerged. Now, humans favor a more processed diet and use cutlery, making their wisdom teeth functionless. Human jaws have shrunk as a result (also, to fit our growing brains).

4. Palmaris longus: Press your thumb into your pinky: If you see a tendon poking up from your wrist, you have a palmaris. Only about 10 percent of the population has this tendon in both arms. It helped human ancestors grip and hang from trees. Because the tendon serves no purpose in modern humans, it often is harvested by surgeons for tendon grafts.

5. Pyramidalis: About 80 percent of the population has this triangular muscle in the lower abdomen. The original purpose remains unclear, but the muscle now serves no function.

Did you know?

Scientists believe every piece of anatomy in the human body has evolved significantly from prehistoric times, except for one: the feet. Analyses of bones from the Homo naledi, human cousins that lived 1.8 million to 2 million years ago in Africa, showed that the species’ feet are nearly identical to modern man’s.

6. Fifth toe: The pinky toe, used by primates for clawing, gripping and climbing, is immaterial to humans’ balance and mobility. Rather, it’s our metatarsal bones in the middle of the foot that are essential to walking, running and balancing.

7. Coccyx: This small, bony protrusion in the lower back is a holdover from human ancestors who had tails and lived in trees. The coccyx, or tailbone, is the spot where the tail connected to the skeleton. Tails were used for balance and communication, but hominids lost the need before they began walking upright.

Did you know?

Vestigial structures have been noted since ancient times, but Charles Darwin, in his 1871 book “The Descent of Man,” was the first to provide a commonly accepted explanation as to why they exist.

8. Appendix: This one is controversial, but most evolutionary biologists believe the appendix is a vestigial organ. Charles Darwin theorized that it was used by primates to digest leaves. Although scientists more recently have found that the appendix appears to be a depository for good bacteria used in the colon to aid digestion and absorption, evolutionary biologists believe that function evolved as a secondary use for the organ rather than its original purpose. One of the main arguments for this idea: Removing it has no ill effects.

9. Gallbladder: The gallbladder acts as a storage facility and pump for bile, a fluid produced by the liver and used to digest fatty foods in the small intestine. Hunter-gatherers never knew when their next meal would materialize, so they lived a lifestyle of feast or famine. The gallbladder helped break down the huge quantities of protein and fat they ate quickly. Modern man’s more consistent diet means the gallbladder comes into play far less often. If the organ is removed, bile reaches the small intestine other ways.

10. Auricular tubercle: Considered a congenital defect, auricular tubercles indicate our common ancestry with primates and are believed to be a vestige of a joint that allowed our ancestors’ ears to swivel and flop down to better pinpoint distant sound. These small, pointy pieces of cartilage, often called Darwin’s points, protrude from the middle of the outer ear in a minority of the population.

11. Auricular muscles: A set of three muscles surrounding the outer ear, the auricular muscles are the reason some people can wiggle their ears. The muscles serve no real purpose now, but early humans used them for sound localization. Much like animals, early humans could move their ears to catch sound.

Behaviors

• Palmar grasp: Place a finger or an object in an infant’s hand, and the baby reflexively will curl his or her fingers around it to grasp it. In many babies, the grasp is strong enough to support the infant’s own body weight. The palmar grasp is a primitive reflex still used by apes, whose young cling to their mother’s fur when traveling. The reflex appears in humans as early as 16 weeks gestation and typically remains until a child is about 6 months old.

• Arrector pili: This band of muscle attaches to our hair follicles and causes hairs to stand on end when contracted, typically when someone is cold or scared. Most animals use the arrector pili to retain heat or look larger and more threatening. In humans, however, the contractions simply produce goosebumps.

In other animals

• Flightless birds such as cormorants, kiwis and emus have rudimentary wings even though they no longer fly.

• Boa constrictors have pelvic bones and whales have thigh bones even though they have no hind limbs.

• Many animals that live in darkness, such as moles and salamanders, have eyes that are incapable of sight.

• Pigs walk using only two toes per foot, but they have two more toes higher on each leg that never touch the ground.

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