Wings on a flightless bird, eyes on a blind fish, and sexual organs on a flower that reproduces asexually—the casual observer might ask, what's the point? But these vestigial organs and structures, once useful in an ancestor and now diminished in size, complexity, and/or utility, carry important information and give us clues to our evolutionary past.
Though humans often think of vestigial organs as useless little fixtures that sometimes, as in the case of the appendix, cause us extreme anguish, we wouldn't know nearly as much about macroevolution as we do now without their presence. In On the Origin of Species, Charles Darwin used vestigial organs as evidence for evolution, and their presence has helped define and shape our phylogenetic trees.
Why the Leftovers?
Contrary to what most think, vestigial doesn't necessarily mean useless; in some cases, we may just not yet know exactly how the organ is used in its current incarnation. (The human thymus was once thought to be vestigial). Because these structures can be traced back through the ancestors, they essentially serve as a marker of evolution; no organism can have a vestigial organ that hasn't been found in its forefathers. For this reason, you won't ever find feathers on a mammal or gills on a primate.
Similar in concept to vestigial structures are atavisms, which are the reappearance of a structure or trait that isn't found in the immediate ancestors. For instance, whales and dolphins have been found in nature with hind limbs; this rare occurrence is due to the reemergence of a trait they inherited from their terrestrial ancestors.
Humans also contain structures that mark where we came from and perhaps, which structures' evolution will take care of over time.
Human Tail (Bone)
One striking example of an atavism is the human coccyx, or tailbone, which is a relic of the mammalian tail. Useful for mammals that use tails for balance, species-to-species signaling, and support, the tail is missing in apes and in humans. However, all human embryos initially have a tail. Normally, they regress into four to five fused vertebrae (the coccyx). However, there have been numerous case studies of human children being born with an extended coccyx—a tail—that was removed without incident. Ranging from one inch to five, the gene that normally stops vertebrae elongation is decreased and the human tail remains at birth.
Wisdom Teeth
Our ancestors, known to be herbivores, needed strong molars for mashing up and chewing plant material. This relic is why many of us will develop wisdom teeth, also known as third molars. Theoretically, they could still be used for chewing, but in one third of people, they can come in sideways, impacted, or can cause pain and infection. This is why these vestigial structures are almost always removed when they begin to come in.
Appendix
Another leftover from our plant eating ancestors is the vermiform appendix, which is an organ attached to the large intestine. A similar sac is much bigger in other animals than it is in humans and is used to aid in digesting high cellulose diets.
While appendicitis can be a potentially fatal condition, and removing the appendix has no adverse effects, some researchers think that the appendix might have an auxiliary function, such as aiding the immune system.
Vitamin C Synthesis
In humans, vitamin C deficiency causes scurvy, and can eventually cause death. We can't synthesize vitamin C (ascorbic acid), but our ancestors, save for the guinea pig and primates, were able to do so. Therefore, it makes sense that we have a vestigial molecular structure, now defunct, that manufactures the vitamin. The gene required for vitamin C synthesis was found in humans in 1994, but it was a pseudogene, meaning it was present but unable to function. The pseudogene was also found in some primates and guinea pigs, as expected.
Male Nipples
Male nipples are sometimes referred to as vestigial, although they aren't truly, because they were never functional in our ancestors. Instead, they most likely occur because in the embryonic stage we are essentially sexless, only differentiating into male and female with the presence of hormones.
Goose Bumps
When we get goose bumps, it's the action of muscle fibers called erector pili, which cause the hairs in follicles to stand to attention. In animals, such as a cat, this causes a larger appearance and can be used to thwart an attacker, as well as trap air between feathers and fur for insulation. However, humans, with our minimal coating of fur, don't really need the raised hair; we use jackets instead. It is therefore thought that goose bumps don't really serve much of a purpose. However, the small expenditure of energy used to contract the muscles could, perhaps, cause a tiny release of heat. Or, because goose bumps are associated not only with cold, but emotional responses as well (listening to a good song, seeing a scary movie) they could now serve as a form of communication with others.
Vomeronasal Organs (VNOs)
In mice and other animals, the tiny vomeronasal organs (VNOs) are thought to be responsible for pheromone detection, helping to pick up the chemicals that signal a potential mate, reproductive status, and other social cues. Although similar structures have been found in humans, they're largely thought to be vestigial and inactive, having lost nerve connection to the brain.
There are other vestigial and atavistic structures in humans, especially when you consider the potential leftovers in our genomes. And if they don't require too much energy or resources to make, chances are they'll stick with us for the long haul.
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