A vestigial structure is a genetically determined attribute that, through the evolutionary process, has lost some or all of its original function. Perhaps the most famous example is the appendix, which in humans has lost most of its ancestral use.
Evolutionary anthropologist Dorsa Amir started a Twitter thread explaining the phenomena. Amir is a postdoctoral researcher with Boston College.
The Twitter thread begins with a basic explanation of the structures.
It seems she just wants to watch the world learn.
Put your hand flat on a surface and touch your pinky to your thumb. Do you see a raised band in your wrist? That th… https://t.co/yq7Tg0Jvad— Dorsa Amir (@Dorsa Amir)1547573370.0
The reason we know the Palmaris longus was used for getting around trees is because we share a common ancestor with primates. For example, the orangutan still uses that muscle and it is well defined. While some of our closest relatives, such as the gorilla or chimpanzee, do not employ the muscle, it still shows up in similar rates as on us.
Check out your ear. Do you see this little bump? That’s called Darwin’s tubercle. It used to help you move your ear… https://t.co/6hcRiBy7Ml— Dorsa Amir (@Dorsa Amir)1547573370.0
Darwin's Tubercle was originally named the Woolnerian Tip, named for Thomas Woolner who depicted it in one of his sculptures. While the tubercle is possibly a vestigial structure, it's also possibly formed by environmental factors.
Here’s a more obvious one: the tailbone. This is the ghostly remainder of our lost tails, which were useful for bal… https://t.co/GKu6CX8l24— Dorsa Amir (@Dorsa Amir)1547573371.0
The tailbone is also known as the coccyx, which is just fun to say. There are some who claim we still need the tailbone, evidenced by the various bits of musculature attached to it. However, most coccygectomy studies show it produces little to no disadvantages for someone without one.
Ever wonder what this little pink thing in your eye is? This is the plica semilunaris. It used to be a third eyelid… https://t.co/CTFsCMS0bw— Dorsa Amir (@Dorsa Amir)1547573371.0
In birds and lizards, this third eyelid covers the eye for protection. While it doesn't perform this function in humans, the plica semilunaris is not without use. During eye movement, it helps us maintain tear drainage.
Oh, and you know how you sometimes get goosebumps when you’re cold or scared? That’s a vestigial reflex that used t… https://t.co/LGWF5cOvMn— Dorsa Amir (@Dorsa Amir)1547573372.0
You've likely also encountered goose bumps in connection with hearing a really good song. This is because music can react in the brain similar to tangible ingestions like food or even psychoactive drugs. The dopamine release changes your breathing, temperature and heart rate, activating the goose bump response.
Another cool reflex is the palmar grasp reflex. If you place your finger on an infant’s palm (or feet!), they will… https://t.co/eHCJx4lgE0— Dorsa Amir (@Dorsa Amir)1547573372.0
The grasp reflex is very strong in infants. Imaging has actually shown they can perform the action in utero. Nowadays, the grip is still strong, but not reliable. They may let go without warning. (Please do not try and pick up your child this way.)
A few addendums: Wisdom teeth — yes, though still ~functional for original purpose. Appendix — potentially yes,… https://t.co/YfEzYLaQSP— Dorsa Amir (@Dorsa Amir)1547599130.0
After the thread, Amir took questions from the class.
@MolemanPeter Good question! In most cases, vestigiality is determined by comparing to related animals, considering… https://t.co/tChbG38J8V— Dorsa Amir (@Dorsa Amir)1547579612.0
@IonaItalia @Victori18956942 @michael_merrick Yes, true! Some vesitigal structures still have reduced function or h… https://t.co/DTWooxI5yd— Dorsa Amir (@Dorsa Amir)1547593527.0
@DPWF0 It may still work to a small degree, as some vestigial structures still serve a reduced role or have found n… https://t.co/IXNmT0bCo7— Dorsa Amir (@Dorsa Amir)1547583243.0
Amir is a researcher for Boston College, studying human behavior. She and other researchers recently published a paper about the impact of your childhood socioeconomic status on your adult preferences.