In Evolution, vestigiality is where components of a body that have evolved to suit a certain purpose remain after its purpose is no longer needed. It could be a genetic factor or a psychological one.
For instance, let's say humans evolve the ability to sense infrared light using our skin. Our eyes may become useless as a result. If we still have eyes and they don't really have a purpose any more, they are then considered vestigial. As another example, lets say our toes become webbed for better swimming abilities but we still have toenails. The toenails may not have a purpose any more and would become vestigial. The second of these examples you can actually see in Manatees
Humans have a number of vestigial elements that you may or may not be aware of, so i'm gonna talk about some of them here.
Palmaris_longus_muscle
The evolutionary interpretation of the muscle's absence is fairly simple.[7] We inherited the muscle through common descent, and numerous animals that we share a common ancestor with (such as the orangutan) still actively employ the muscle. Close primate relatives (such as the chimpanzee and gorilla) also do not actively employ the muscle, and hence they also demonstrate the same variability.[8] The common descent principle suggests that at some stage our ancestor employed the muscle actively. The primate branch then began to evolve the thumb apparatus (and particularly the thenar muscle group) and consequently the Palmaris longus became vestigial. As there is no apparent pressure (positive or negative) concerning the muscle, evolution has largely left it alone.[9] Its persistence might possibly be due to the fact that the necessary changes to the gene(s) involved in losing the muscle may have other consequences, but this is only speculation.
The Baby Gripping Reflex
In newborn babies, you've probably experienced the baby death grip. Hold out a finger and they will latch on, even if they currently have no advanced coordination of their hands. This is called the Palmar Grasp Reflex, and it's a vestigial primitive reflex.
When we were tree-animals, we had to spend a lot of time in our mothers arms. It was high up, not a lot of room to step out and explore. Eventually though, momma ape would get tired of carrying us and have to put us somewhere; a dangerous prospect. She would have to put us on a branch so we could stay safe from predators. Once there we would grip hold of that branch tightly and just stay there until she picked us up again. We would also need the grip to cling onto her fur when suckling milk or if we had siblings to compete with.
Even in Human babies, the grasp is theoretically strong enough to suspend the child, but obviously has lost its effectiveness and usefulness over time. It's also not recommended that you try to suspend your newborn, so please don't.
Wisdom Teeth
The third molars in your mouth, the 'wisdom' teeth are throwbacks to when humans had to eat more solid whole foods. The reason that Wisdom Teeth are so painful and fail to come through for us is very simple. Wisdom teeth are vestigial. Our mouths are no longer built to house them.
As we evolved, our brains got bigger at the expense of the size of our jaws. Essentially, as we got smarter, our need for the extra teeth became less and less. We learned to break up our food with our hands before chewing and to prepare meals so that food was easier to eat. It meant we needed less biting power to survive. Our jaws shrank so much that the wisdom teeth no longer had room to come through. Evidence of its vestigiality can easily be seen in how late it takes for wisdom teeth to actually emerge, appearing much much later than any of the other teeth in our mouths.
The Third Eyelid
Did you know you have a third eyelid. It's vestigial. See that pink area on the innermost edge of your eye? That is what remains of our third eyelids. It is our 'Nictitating Membrane' or more specifically the 'Plica Semilunaris' of the eye.
This extra eyelid is normally transparent/ translucent and is a common, fully functional, aspect of much of the animal kingdom. Crocodiles utilise their Nictating Membrane eyelid to cover their eyes underwater. It provides a transparent protection that allows them to better see when submerged. Other animals use them to moisten the eye without impacting their vision (useful if you're normally prey).
As an extra bit of information regarding Humans. We actually still retain the vestigial muscles that were once used to control this third eyelid's movement.
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