Toward the end of Bill Bryson’s hugely enlightening and entertaining “A Short History of Nearly Everything” (2003), an early model of modern man emerges from Africa a little over 100,000 years ago with primitive stone axes. Smaller-brained and more fragile than their stout, adaptable Neanderthal competitors, modern humans, improbably, prevail.

Later, we name ourselves Homo sapiens because we think we’re smart. What do we do with our wits? Mostly, kill off other species and destroy their habitat and our own. And that predilection long predates our more recent zeal for torturing ourselves and our fellow life-forms by recklessly raising the global thermostat because Hummers are cool and we don’t like drinking out of faucets.

Still, among this generally arrogant species, some, like Bryson, have always appreciated the glorious complexity and privilege of our unlikely existence.

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Having described the physical nature of our world and beyond, from the atomic to the intergalactic, in “The Body” he now turns inward to explain — in his lucid, amusing style — what we’re made of. Along the way, as he has before, he weaves in stories of the astonishing characters who have been figuring humans out.

Consider John H. Gibbon’s quest, in the early 1930s, to oxygenate blood to make open-heart surgery possible: “To test the capacity of blood vessels deep within the body to dilate or constrict, Gibbon stuck a thermometer up his rectum, swallowed a stomach tube, and then had icy water poured down it to determine its effect on his internal body temperature.”

You’re committed to your job? Really?

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Bryson also chronicles myriad examples of medicine gone wrong. Founding Father and esteemed surgeon Benjamin Rush, during a yellow-fever epidemic, “bled hundreds of victims and was convinced that he had saved a great many when in fact all he did was fail to kill them all.”

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“Part of the problem,” Bryson explains, “was that he believed that the human body contains about twice as much blood as it actually does and that one can remove up to 80 percent of that notional amount without ill effect.”

Why didn’t Bryson subtitle “The Body: A Guide for Occupants” as “An Owner’s Manual” instead? Wouldn’t that have been terribly clever?

No, because, as Bryson makes clear, we don’t own our bodies. Evolution owns our bodies. We occupy them — although the more you read, the less clear it is how to define these roving, rickety, skin-slathered bone towers of electric impulses, chemical cocktails and micro-organismic colonies as “we.” And, as you know, we take pitiful care of these fleshly loaners, filling them with crud, parking them in chairs and laying them out on sofas to the point of reversing the mortality gains modern science has enabled. Then again, there are exceptions, such as Jeanne Louise Calment, who ate two pounds of chocolate a week, smoked until she was 117 and died at 122. (Bryson doesn’t mention it, but she also enjoyed red meat, cheap wine and hunting.)

Shorter than “Nearly Everything” and with more snack-size chapter lengths, “The Body” nonetheless draws on dozens of experts and a couple hundred books to carry the reader from outside to inside, from up to down and from miraculous operational efficiencies to malignant mayhem when things go awry. In some cases, that biological ingenuity and chaos are two sides of the same coin.

Cancer, for instance, as one expert puts it, “is the price we pay for evolution. If our cells couldn’t mutate, we would never get cancer, but we also couldn’t evolve.” There are more than 200 cancers, and age is usually a major factor, with an 80-year-old 1,000 times as likely as a teenager to get it.

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Despite the body’s harrowing malfunctions, you will marvel at the brilliance and vast weirdness of your design. The brain, holding its “200 exabytes of information, roughly equal to ‘the entire digital content of today’s world.’ ” The heart, beating some 3.5 billion times in a lifetime. The bones, “stronger than reinforced concrete, yet light enough to allow us to sprint.” The lungs, processing 4,000 gallons of air a day.

“You are pretty seriously perforated,” Bryson writes, with “two to five million hair follicles and perhaps twice that number of sweat glands.” And you are “exquisitely fine-tuned,” with nerve receptors able to detect movement of 0.00001 millimeters.

You grow 25 feet of hair in a lifetime. You host 40,000 species of microbes, and when you kiss you transfer some 1 billion bacteria to your beloved. While that statistic might not enhance the mood, the special sharing is thought to be helpful in sampling the partner’s histocompatibility genes involved in immune response. Oh, baby!

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In a lifetime, you eat 60 tons of food, extracting the nutritional necessities and then producing seven tons of poop. That’s almost the weight of three Hummer H3s. (Not that I have some weird thing about Hummers.) We produce enough flatulence that before laparoscopic insertion of carbon dioxide became the norm, patients undergoing anal surgery sometimes literally exploded.

Bryson dispels some long-held biological myths. That “taste map” on your tongue you learned about in middle school — sweet on the tip, sour on the sides, bitter on the back? Not true. What is true is that we have taste receptors not just in the mouth, throat and gut, but in the heart, lungs and testicles too, perhaps so that they can send signals to the pancreas to adjust insulin output.

And no, men don’t think about sex every seven seconds. They think about it 19 times a day, about the same rate they think about food.

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While we’re timing things, let’s note that our sex acts take about nine minutes (“in Britain at least”). Men burn 100 calories during those encounters, women 70.

Older people have an increased risk of heart attack for three hours after coitus, but Bryson, ever the practical life coach at age 67, wisely points out that the risk “was similarly raised for shoveling snow, and sex is more fun than shoveling snow.”

Alexander C. Kafka has written about books for The Washington Post, the Boston Globe and the Chicago Tribune.

The Body

A Guide for Occupants

By Bill Bryson

Doubleday. 464 pp. $30

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