4.3 — The Body's Oddities

Most of the strange things your body does are either leftovers from an ancestor who needed them, or side effects of a system solving a different problem. A few are genuinely unexplained, and those are marked as such.

Leftovers

Why do you get goosebumps?

Because a muscle at the base of each hair contracts and pulls it upright, and you are running a program written for an animal with fur.

The muscle is called the arrector pili, and in a furry mammal raising the coat does two useful things. It traps a thicker layer of air, which is insulation, so a cold animal is warmer. And it makes the animal look larger, which is why a threatened cat doubles in width.

You have the same muscles and almost none of the fur, so the contraction produces a bump around each follicle and nothing else. It fires from cold and from strong emotion — fear, and also music or a moving scene — because both are routed through the same sympathetic nervous system pathway. The emotional version is the same physiological event as an animal bristling.

It is one of the clearest vestigial traits in the human body: a mechanism that is fully intact and no longer does the job it evolved for.

What is the appendix for?

For a long time the answer was "nothing", and that has changed.

It is a narrow blind-ended tube attached to the start of the large intestine. In animals that live on leaves and bark, the equivalent structure is large and houses bacteria that break down cellulose. Human ancestors ate that way; we no longer do, and the structure shrank.

The current best hypothesis is that it acts as a reservoir for gut bacteria. It is rich in immune tissue, and its shape — a cul-de-sac off the main flow — means its contents are not flushed out during severe diarrhoea. After an infection that strips the gut, a population of the right bacteria can repopulate from it. The evidence is suggestive rather than settled: people without an appendix are somewhat more likely to have a recurrence of certain gut infections.

The reason it makes trouble is the same shape. If the opening blocks, the tube behind it fills with fluid and bacteria, pressure rises, blood supply is cut off, and it can burst. Appendicitis is a surgical emergency and is covered in Volume V.

Why do humans have wisdom teeth if there is no room for them?

Because the jaw shrank faster than the teeth did.

Human ancestors had larger jaws and a diet of tough, fibrous, raw material that required a great deal of chewing and wore teeth down substantially over a lifetime — so a third set of molars arriving in early adulthood was useful, and there was space.

Cooking, then agriculture, then soft processed food, reduced the mechanical demand on the jaw. The jaw is one of the structures whose growth responds to use during childhood, so a lifetime of soft food produces a smaller jaw, and the population-level change over thousands of years has been in the same direction. The teeth themselves are under weaker selection and have not shrunk to match.

The result is a third molar that arrives into a jaw with no room and comes in sideways, or not at all. Around a fifth to a third of people never develop some of them, and that fraction seems to be rising.

Why do some people sneeze when they walk into sunlight?

It has a name — the photic sneeze reflex — it affects perhaps a fifth to a third of people, it is inherited as a dominant trait, and nobody knows the mechanism.

The leading explanation is crossed wiring. The nerve serving the face, the trigeminal, runs very close to the optic nerve, and the theory is that a sudden burst of activity in one produces stray activation in the other, which the brain interprets as nasal irritation. It is plausible and it is not proven.

It is one of the honest gaps. A common, harmless, clearly heritable human trait, first described by Aristotle, and still without a confirmed cause.

Reflexes nobody can fully explain

Why is yawning contagious?

The yawn itself is not well understood, and the contagion is understood better than the yawn.

The old explanation — that yawning increases oxygen — has been tested and fails: breathing extra oxygen does not reduce yawning, and breathing extra carbon dioxide does not increase it. The current leading hypothesis is brain cooling. A yawn stretches the jaw, which increases blood flow through the skull, and the deep inhalation brings cooler air past the sinuses. Yawning increases when the head is warm and decreases when a cold pack is held to the forehead, which is at least consistent.

The contagion is the interesting part, and it tracks social closeness. You are most likely to catch a yawn from a family member, then a friend, then an acquaintance, then a stranger, in that order, and the effect has been measured repeatedly. It appears in chimpanzees and in dogs, who catch yawns from humans they know. It develops in children around age four to five, at the same time as other perspective-taking abilities, and is reduced in people with conditions affecting social processing.

That points to it being an automatic form of behavioural synchronisation — the same class of thing as unconsciously matching someone's posture in conversation. What it is for is still argued about.

What are hiccups, and why do we have them?

A hiccup is an involuntary spasm of the diaphragm, followed about 35 milliseconds later by the vocal cords snapping shut — which is what makes the sound and stops the breath.

The best current explanation is evolutionary. The nerve pathway is very old, and the same pattern — sharp inward gulp, closed airway — is the movement an amphibian tadpole uses to push water over gills while keeping it out of the lungs. The circuitry appears to be a leftover of that motor program, still present in the brainstem.

A second hypothesis, not exclusive of the first, is that it is a suckling reflex: the same gulp-and-close sequence prevents milk entering the lungs, which would explain why hiccups are far more frequent in babies and in the womb than in adults.

Most home remedies work, when they work, by one of two routes: raising carbon dioxide in the blood (holding your breath, breathing into a bag) or stimulating the vagus nerve (swallowing dry sugar, pulling on your tongue, drinking from the far side of a glass). Both interrupt the reflex loop.

Why do we cry from emotion?

Humans are the only animal that produces tears from feeling, and the mechanism is only partly worked out.

Tears themselves are not unusual — every animal with eyes needs lubrication, and irritant tears wash out dust. Emotional tears are chemically different: they contain more protein, and more of certain hormones including one associated with stress, which is the basis of the idea that crying physically removes stress chemicals. The evidence for that is weak.

The stronger explanation is social. A visibly crying face is an unambiguous signal of distress that cannot easily be faked and blurs your own vision, which makes it a costly and therefore credible signal that you are not a threat and need help. Consistent with that, people report feeling better after crying mainly when they were comforted, and worse when they cried alone or were judged for it.

Perception

What is déjà vu?

The feeling that a new situation has happened before. About two thirds of people report experiencing it, it becomes less frequent with age, and it is most common in people who are tired, stressed or travelling.

The most supported explanation is a timing error in memory. Recognition and recall are handled by different systems: one tells you this is familiar, the other retrieves where from. Normally they fire together. In déjà vu the familiarity signal fires without the retrieval, so you get the certainty of having seen it with no content attached — and the brain, unable to find a source, produces the eerie feeling that something is wrong with time.

The strongest evidence comes from epilepsy. Seizures originating in the temporal lobe frequently produce intense déjà vu as an aura beforehand, and electrically stimulating that region during surgery can trigger it directly. That locates the effect in the memory circuitry rather than anywhere mystical.

A second contributing factor is partial resemblance: a room whose layout matches one you have been in before, without you consciously identifying which, produces familiarity with no retrievable source.

Why does time feel faster as you get older?

Two effects, both real, and neither is that time is actually passing differently.

The proportional one. A year is a twentieth of the life of a twenty-year-old and a fiftieth of a fifty-year-old's. If your sense of duration is calibrated against your own accumulated experience, each successive year is a smaller fraction of what you have to compare it with. This is an old idea and probably part of the answer.

The memory-density one is stronger, and more useful. Your sense of how long a past period was depends on how many distinct memories you can retrieve from it, not on how it felt at the time. Novel experiences create dense, retrievable memories; routine ones are compressed together and often not encoded separately at all. Childhood and early adulthood are full of first times — first job, first city, first relationship — and middle age is often built from repeated days that the brain does not bother storing individually.

That has a practical consequence. The way to slow time down is not to relax; it is to do unfamiliar things. Take a different route, learn something, go somewhere new. A week of novelty is remembered as long; a month of routine collapses into an afternoon.

Why does your voice sound wrong on a recording?

Because you have never heard it before.

When you speak, sound reaches your ears by two routes. Some travels out of your mouth and back through the air, which is what everyone else hears. The rest travels through the bones of your skull directly to the inner ear — and bone conducts low frequencies far better than air.

So your internal experience of your own voice has the bass boosted substantially. A recording contains only the air-conducted version. The recording is what you actually sound like, and it is thinner and higher than the version you have lived with, which is why almost everybody dislikes it.

The rest

What determines blood type, and why does it matter?

A sugar structure on the surface of red blood cells.

Everyone has a basic structure called the H antigen. The A gene adds one sugar to it, the B gene adds a different sugar, the O version of the gene adds nothing. You inherit one copy from each parent: A and B are both expressed, O is not, so AB shows both and O shows neither.

Your immune system makes antibodies against whichever versions you do not have. A type A person has anti-B antibodies circulating from early infancy, even having never met type B blood, probably triggered by similar structures on common gut bacteria. Give them type B blood and the antibodies attack it immediately, clumping the cells and blocking vessels, which can kill.

That is why type O is the universal donor — no A or B sugars for anyone's antibodies to attack — and AB the universal recipient, having no antibodies against either.

The separate Rh factor, the plus or minus, is a protein. Its main clinical importance is in pregnancy: an Rh-negative mother carrying an Rh-positive baby can develop antibodies that attack a later Rh-positive pregnancy. This is prevented routinely with an injection, and it is one of the quiet successes of twentieth-century medicine.

Karl Landsteiner worked out the ABO system in 1901, which is why transfusion stopped being a lottery.

Why are some people left-handed?

About ten per cent of people are, the proportion is remarkably stable across cultures and across recorded history, and there is no single gene for it.

Handedness is a visible consequence of the brain being asymmetric, which is itself unusual — most animals have symmetric brains and no population-level preference. Human language processing is concentrated on one side, usually the left, and hand preference is correlated with that arrangement, though not simply: most left-handers still have language on the left.

Identical twins are frequently discordant for handedness, which rules out a purely genetic cause. Current models involve several genes each nudging a probability, plus developmental randomness in the womb — handedness can be detected in ultrasound by thumb-sucking preference before the brain's language areas exist.

Why any left-handers persist is the more interesting question, since a minority trait that provides no benefit should drift away. The leading explanation is a fighting or competitive advantage: a left-hander has faced mostly right-handed opponents and a right-hander rarely faces left-handers. That predicts left-handers should be over-represented in interactive sports and not in non-interactive ones, and they are — heavily in boxing, fencing, cricket and tennis, and not at all in swimming or running.

Why does hair go grey?

Because the cells that make pigment stop, and the reason they stop is that their stem cell supply runs out.

Each hair follicle contains melanocytes that produce melanin and inject it into the growing hair. Those cells are replaced from a small reserve of stem cells in the follicle. Over repeated growth cycles, the reserve is depleted, and hairs then grow out with progressively less pigment until they are white. A grey head is a mixture of pigmented and white hairs, not individual grey ones.

Timing is largely genetic. Oxidative damage contributes, and there is evidence that acute stress can accelerate it by driving the sympathetic nervous system to over-activate and exhaust those stem cells — a 2020 study in mice found stress could deplete the reserve within days, which is the first mechanism to make the folk belief about shock and grey hair somewhat respectable.

Hair does not turn grey. A hair that has grown is dead protein and cannot change colour. The story about someone going white overnight, where it is not fabricated, is usually a condition that causes rapid loss of pigmented hairs, leaving the white ones already present.

What comes next

The next page steps outside and looks up — the phases of the moon, why eclipses are rarer than they should be, what a light year measures, why Pluto was demoted, and how a detector the size of a building catches a particle that passes through the Earth without noticing it.