In the 4 billion years that life has existed on Earth, modern humans have been around for about 200,000 years of that.
In that amount of time, we've had countless ancestors who lived in almost every kind of environment. This resulted a large number of adaptations that hung around even after they weren't needed anymore, or needed in a much smaller capacity. These are known as "vestigial structures."
So why do we bother making them if we don't need them?
While they aren't helping us out, they aren't necessarily hurting either. Sure, some of these structures, like the appendix, tonsils, and wisdom teeth, occasionally give people trouble. But they don't cause enough harm to stop someone from living to adulthood and passing on their genes.
Here are 5 lesser-known vestigial structures that serve little purpose beyond reminding us of our amazing evolutionary heritage.
When humans get cold or scared, muscles in the skin called arrector pili flex, causing goosebumps. For animals with a lot of body hair, this causes the hair to stand on end — making them look bigger in the face of predators, or making themselves warmer in the cold.
Human body hair is too short and fine for either of these to work, but the reflex is still deeply engrained and makes the attempt anyway.
2. Plica semilunaris
Ever wonder what that pink fold of flesh near the corner of the eye is for? On humans, it's a remnant of a membrane (called a nictitating membrane) we no longer have control over. For some other mammals, fish, birds, and amphibians, however, this membrane acts like a third eyelid, covering the eye horizontally.
This trait atrophied as humans didn't really need it anymore.The membrane is translucent, so while it does provide and added layer of protection against water, dirt, and high winds, it also obscures vision slightly.
3. The outer ear
Humans still do heavily depend on the ear for hearing, but we don't need it as much as some species do. Our outer ear is attached to muscles that, for many people, don't do much anymore. Our ancestors, like many of the animals that are still around today, had a lot of control over turning the ear in order to listen for predators or pick up the sound of prey. While some people are still able to wiggle their ears slightly, those muscles largely have no purpose.
Another feature on the outside of the ear is a little point of skin known as Darwin's tubercle, which relates to the pointier ears of other species. It is more prominent in some populations of humans than others.
4. Male nipples
Women need breasts to feed milk to their young, but since men don't do that, why do they even have nipples?
Estrogen present in the earliest stages of embryonic development causes the nipples to begin to form, based on cues from the X chromosome. When certain genes affiliated with male features from the Y chromosome kick in, the production of the breasts stops. Some men are able to lactate a little bit, but overall, they don't have a purpose.
5. Palmar grip reflex
Touching a newborn's palm will cause them to grab tight, and touching the bottom of their feet causes them to curl their toes, while rubbing the back of the hand or top of the foot causes them to open them up.
This phenomenon is known as the palmar grasp reflex and came about when our ancestors, who had much more body hair, needed to have their babies hang on for dear life as they traveled through the trees. Because they were tree-dwellers, their feet were much more adept at grabbing things as well.
Which one of these did you find the most surprising? Let us know in the comments!