A tiny pit on each side of the septum is lined with nonfunctioning chemoreceptors. They may be all that remains of a once extensive pheromone-detecting ability.
EXTRINSIC EAR MUSCLES
This trio of muscles most likely made it possible for prehominids to move their ears independently of their heads, as rabbits and dogs do. We still have them, which is why most people can learn to wiggle their ears.
Early humans had to chew a lot of plants to get enough calories to survive, making another row of molars helpful. Only about 5 percent of the population has a healthy set of these third molars.
A set of cervical ribs—possibly leftovers from the age of reptiles—still appear in less than 1 percent of the population. They often cause nerve and artery problems.
A common ancestor of birds and mammals may have had a membrane for protecting the eye and sweeping out debris. Humans retain only a tiny fold in the inner corner of the eye.
A small folded point of skin toward the top of each ear is occasionally found in modern humans. It may be a remnant of a larger shape that helped focus distant sounds.
This small muscle stretching under the shoulder from the first rib to the collarbone would be useful if humans still walked on all fours. Some people have one, some have none, and a few have two.
This long, narrow muscle runs from the elbow to the wrist and is missing in 11 percent of modern humans. It may once have been important for hanging and climbing. Surgeons harvest it for reconstructive surgery.
Lactiferous ducts form well before testosterone causes sex differentiation in a fetus. Men have mammary tissue that can be stimulated to produce milk.
Bundles of smooth muscle fibers allow animals to puff up their fur for insulation or to intimidate others. Humans retain this ability (goose bumps are the indicator) but have obviously lost most of the fur.
This narrow, muscular tube attached to the large intestine served as a special area to digest cellulose when the human diet consisted more of plant matter than animal protein. It also produces some white blood cells. Annually, more than 300,000 Americans have an appendectomy.
Brows help keep sweat from the eyes, and male facial hair may play a role in sexual selection, but apparently most of the hair left on the human body serves no function.
Often mistaken for a nerve by freshman medical students, the muscle was useful to other primates for grasping with their feet. It has disappeared altogether in 9 percent of the population.
Our closest cousins, chimpanzees and gorillas, have an extra set of ribs. Most of us have 12, but 8 percent of adults have the extras.
A remnant of an undeveloped female reproductive organ hangs off the male prostate gland.
Lesser apes use all their toes for grasping or clinging to branches. Humans need mainly the big toe for balance while walking upright.
FEMALE VAS DEFERENS
What might become sperm ducts in males become the epoophoron in females, a cluster of useless dead-end tubules near the ovaries.
More than 20 percent of us lack this tiny, triangular pouchlike muscle that attaches to the pubic bone. It may be a relic from pouched marsupials.
These fused vertebrae are all that’s left of the tail that most mammals still use for balance and communication. Our hominid ancestors lost the need for a tail before they began walking upright.
The exposed tailbone of a male pelvis from the collection of 19th-century anatomist George McClellan rests on a humble seat at the Mütter Museum. The human coccyx is highly variable but is generally composed of three to five vertebrae. On rare occasions, infants are born either with no coccyx at all or with tails. While some have suggested that the coccyx helps to anchor minor muscles and may support pelvic organs, surgically removing it has no discernible effect on health.