Can you give me any examples of genetic “defects” that still exist in humans and were perpetuated through evolution?

So for example, is there something that made sense in our ancestors but don’t have much of a purpose now?

In humans this is called Evolutionary Atavism. And there is a number of potential remnants.

The tailbone - the human coccyx 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. But, all human embryos have a tail initially. 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.

Our wisdom teeth - Our ancestors needed strong molars for mashing up and chewing plants because they were herbivores. 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 teeth are almost always removed when they begin to come in.

The appendix - The vermiform appendix 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. We know appendicitis can be a potentially fatal condition, and removing the appendix has no adverse effects, but some researchers think that the appendix might have an auxiliary function, such as aiding the immune system.

Synthesising vitamin C - In humans, vitamin C deficiency causes scurvy, and can eventually cause death. We can’t synthesize vitamin C (ascorbic acid), but our ancestors were able to. So, 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.

Nipples in men - Male nipples are sometimes referred to as vestigial, although they aren’t truly, because they were never functional in our ancestors. They are most likely to occur because in the embryonic stage we are essentially sexless, only differentiating into male and female with the presence of hormones.

Goosebumps - 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 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.

I checked on Wikipedia, and one reason for the high proportion of red-green colour blindness (in males at least) could be due to its usefulness in hunting parties. It appears that having some colour-blind members in a group ensures that a wider range of camouflage/protective colouration used by prey or other predators can be discriminated.

So colour blindness, which is a genetic defect today, was once useful to our hunter-gatherer ancestors.

Morgan, M. J.; Adam, A.; Mollon, J. D. (June 1992). "Dichromats detect colour-camouflaged objects that are not detected by trichromats". Proc. Biol. Sci . 248 (1323): 291–5. doi:10.1098/rspb.1992.0074. PMID 1354367

Definitely a few come to mind:

Sinuses and branchial cleft cysts. These are lumps or draining sinuses on the side of the neck that are supposed to be remnants of the embryonic branchial arches, which develop into gills in fish, but that mostly regress in humans and mammals.

Vestigial tail. Some babies are sometimes born with a tail or a cyst at the very end of the coccyx/tailbone that is a “left over” of (presumably) tailed “ancestors.” This tail doesn’t occur in apes and humans.

Accessory breasts (and nipples). Just like dogs and cats that have two lines of nipples down the body, humans can develop accessory breast tissue and nipples, even with lactation, anywhere along this “milk line.” Male nipples are also a remnant.

Urachal cyst. This is a very rare cyst formed from leftover structures of the umbilical cord. It usually disappears or become various ligaments.

These are some of the ”defects” I could think of offhand. Cool topic!

Yes. Quite a few:

Hemocromatosis: a disorder that leads to harmful levels of iron in the blood. This disorder used to protect people against bulbonic plague.

Sickle cell anemia: a blood disorder, reduces the ability of the malaria parasite to destroy red blood cells. More prone to certain ethnicities.

Cystic fibrosis: this combats typhoid fever.

Tay-Sachs disease: this might have evolved to combat tuberculosis.

Type 1 diabetes: this causes excessive amounts of sugar, or glucose, in the blood, but prevent cells and tissues from forming ice crystals when exposed on very low temperatures.

Not sure if this was what you were after.

Sunburns and skin cancer. One of the most common forms of cancer worldwide. Aside from mammals, every animal this has an enzyme - photolyase - that can directly reverse the damage done to your DNA by ultraviolet radiation. It uses blue light to un-link two base pairs (most often two thymines linked together). We still have backup systems to repair this damage, but none as efficient as photolyase so we burn and get cancer at much higher rates than other animals.

So why don’t mammals have photolyase? This light harvesting enzyme has evolved away from mutation repair and is now a ”cryptochrome” gene which is used to set our circadian rhythms. It still responds to light, it’s just that it can’t quite fix our DNA.

They’re not necessarily defects dispite the fact that they serve (almost) no function today. But there are loads of remnants of earlier evolutionary steps, both behavioural and physical.

For example, the appendix and wisdom teeth, which are both tied to mastication and processing of plant food, especially plants rich in cellulose. Other bodyparts include the coccyx, the last remaining part of a tail. Some people have elongated coccyx and vestigial tails that are removed in childhood, so that could qualify as a “defect”. Other “defects” could include vestigial nipples – like male nipples or extra nipples on the body.

When it comes to behaviour, goosebumps are also a vestigial reflex, and the palmar grasp reflex in infants, where some infants can actually hang from a washing line or rod by grasping.

Finally, there’s the Moro reflex in infants where the child throws out boths his arms and grasps when losing balance. This is an old evolutionary response where the infant tried to grip to the fur of his mother, clinging to her while she is carrying him.

Two that haven’t been mentioned above… Hemorrhoids and the positioning of the heart below the brain.

Hemorrhois. Large veins makes the anal sphincter water-tight. This mechanism makes sense and serves a purpose in four-legged animals with their butts up in the air and, hence, good venous drainage, but it is a very bad choice for humans sitting all day. Sitting is the new smoking of our age.

The heart below the brain. Four-legged animals, can keep their arterial pressure low because the brain is only slightly higher than the heart. In humans, the brain is not only huge, it is also way above the heart, necessitating a high arterial pressure. Arterial hypertension, with all its complications, is a major killer as you know.

Our retina is backwards, with the taps and rods facing the wrong way. This explains why we have a blind spot: the hole where the nerves of the retina go out of the eye.

The laryngeal nerve, from the brain to the larynx. It goes down from the brain, under the aorta and back up to the larynx again, which is a weird detour. It gets even more weird in giraffes :))

Getting fat. Storing fat. Being fat. Fat.

Evolutionairily speaking, our brain still thinks we live on the dry plains of Africa. Foods that contain much sugar were scarce, so our body tends to keep it in the body for as long as possible.

Today we can eat those kind of foods all day every day if we want to, which messes up the way our body reacts on those foods. It’s one of the worst things, really, seeing the problem of being overweight all over the developed world.

The appendix. It’s an evolutionary relic from when our non human ancestors digested cellulose and loads of plants. It’s a defect because it can get infected, often with fatal consequences. I removed mine.