Over millions of years of evolution, during which we modified, discarded, and created physiological and behavioral traits, we became who we are today. Despite our apparent perfection, evolution overlooked some things, such as eliminating traits that no longer serve their original function. These traits constitute the "residual elements" of the human organism, or, more simply, evolutionary remnants. Although they appear nonfunctional, some may function at a lower level or develop new, minor, unrelated functions. Here are a few of them.
1. This pointed protrusion on the upper or middle part of the ear is called "Darwin's tubercle." It indicates a common ancestry with pointy-eared primates.
The auricular tubercle, or Darwin's tubercle, was first described by Charles Darwin in his book «"The Descent of Man and Sexual Selection.". Darwin originally named it "Woolner's tip" after the British sculptor Thomas Woolner, who depicted it in one of his sculptures. The tubercle is found in 58% Swedish children, 40% Indian adults, and 10.4% Spanish adults. It is usually found on both ears, although it can occur on only one.
The tubercle isn't the only vestigial part of the human ear. In macaques and other apes, the ear muscles are much more developed and can be used to move the ear, unlike in humans, chimpanzees, and orangutans, where these muscles, although large enough to be visible, are nonfunctional.
However, there are people who can wiggle their ears at will, and this can be learned. The inability to move the ears in humans and other animals is compensated for by the ability to turn the head horizontally. This is rare in most monkeys. (source)
2. The "palmar grasp reflex" is a relic of the days when babies had to grasp their mother's fur to hold on. This is why a newborn's hand reflexively grasps a finger or object.
Babies are born with a palmar grasp reflex, which develops between 11 and 15 weeks of gestation. This is one of the important, spontaneous movements and reflexes a newborn must demonstrate to be considered neurologically healthy. The reflex can be tested by placing a finger on the baby's palm: if you try to pull the finger away, the baby will grip it much tighter—hard enough to support its own weight. It's also worth noting that the grip can be unpredictable, as the baby may release the finger without warning.
The reflex will disappear within two to three months, or at most six months, as the child gradually gains voluntary control over their body. If the reflex persists, the child may have difficulty learning to use their hands to hold or release objects. The reflex can also be observed in their feet and toes. (1, 2)
3. The Jacobson's organ is an accessory olfactory organ present in many mammals, snakes, and lizards, used to detect pheromones. This organ is also present in humans, but is considered vestigial.
Jacobson's organ, or vomeronasal organ (VNO), first discovered by Danish anatomist Ludwig Levin Jacobson in 1811, is located at the base of the nasal cavity, just above the roof of the mouth. The organ contains receptors capable of detecting certain types of non-volatile or liquid organic compounds that may originate from prey, predators, or potential mates.
Unlike the olfactory bulb, which sends signals directly to the olfactory cortex, the vomeronasal organ (VNO) sends signals to the amygdala and ultimately to the hypothalamus, which is responsible for regulating hormone production and body temperature. This organ is present in all snakes and lizards, as well as many mammals.
However, in humans, the organ and its nerve fibers develop in the embryo at a certain stage. After this, the organ regresses, and the neural connections disappear. Cavities in the VNO can still be found in some adults, although without a functional nervous system, the organ simply does not function. (1, 2)
4. There is a gene that produces an enzyme that helps animals synthesize vitamin C. Although this gene is present in humans, it is vestigial, so we cannot on one's own synthesize Vitamin C.
Vitamin C is an essential nutrient responsible for tissue repair, neurotransmitter production, enzyme function, and the proper functioning of the immune system. Most animals can synthesize this vitamin themselves from glucose via gluconic acid in their liver, while plants obtain it by converting mannose or galactose into ascorbic acid.
Unlike these animals, humans, many other primates, guinea pigs, and fruit bats cannot synthesize vitamin C on their own due to the lack of the enzyme L-gulonolactone oxidase. The gene for this enzyme became inactive approximately 63 million years ago in all species. However, remnants of this mutated, nonfunctional gene still exist in the genomes of humans and guinea pigs. Because the vitamin is readily available in fresh fruits and vegetables, there has never been evolutionary pressure to activate it. (source)
5. The pink, fleshy fold in the inner corner of the eye is actually a remnant of the so-called "third eyelid." In birds, reptiles, and fish, it is transparent and closes horizontally, protecting and moisturizing the eye.
In addition to fish, reptiles, amphibians, birds, and sharks, some mammals, such as camels, polar bears, aardvarks, and seals, have a fully functional nictitating membrane, also known as a "third eyelid" or "tongue." In animals that spend a lot of time in water, such as crocodiles, beavers, and manatees, the eyelid protects the eyes from water.
Birds can voluntarily control the movement of their retinas. They close them to protect their eyes from the beaks of their chicks while feeding, and to clear debris and lubricate their eyes during parachute jumps, as peregrine falcons do. Woodpeckers close their membranes milliseconds before striking a tree to protect the retina from injury caused by the impact.
In humans, it is present only in the form of a small fold known as the semilunar fold. (plica semilunaris) , connected to the conjunctiva—the tissue that covers the whites of the eyes and the inner surface of the eyelids. Although it no longer performs its original function as a nictitating membrane, semilunar fold It helps direct tears for drainage. Furthermore, without it, the conjunctiva would have to attach directly to the eyeball, limiting the eyeball's ability to rotate. (1, 2)