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September 23 - October 1, 2021
When three out of four Americans can’t name the three branches of U.S. federal government—an important bit of information drilled into us annually over twelve years of schooling—there may be little hope of people recalling the finer points of the Krebs cycle from high school biology.
We have an intuitive cultural sense of how strange we are, but in our typically anthropocentric way we get it flipped around. Our pets, abiding by the normal mammalian schedule, live their lives at what feels to us like an accelerated rate. We talk about dogs living in “dog years,” with each year of their life equivalent to seven of ours, as though it’s the other animals that are different.
Steve is an incredibly friendly, positive, and helpful guy, which makes sense because he’s Canadian.
Aiello and Wheeler did the calculations and found that, in humans, the energy saved by having a smaller gut and liver perfectly offset the energy cost of our larger brain.
If evolution favors the expansion of some traits—say, powerful hindlimbs and a big head full of nasty teeth—others have got to give, like the forelimbs . . . and voilà, you’ve got Tyrannosaurus rex. Or, as Darwin put it in On the Origin of Species (quoting Goethe), “to spend on one side, nature is forced to economise on the other side.”
There are hundreds of amino acids on Earth, but only twenty-one are used to build proteins in living plants and animals. Nine of these are considered essential for humans, meaning our bodies can’t make them on their own; we need to get them from our diet (don’t worry—if you haven’t died yet, you’re getting them).
Imagine what else Lavoisier might have discovered if he hadn’t been guillotined in the throes of the French Revolution just a few years later.
Using the equation for walking cost, you’d find that an average 150-pound person burns 54 kilocalories to walk a mile (0.36 × 150 = 54). A smaller person, at 100 pounds, would burn 36 kcal. (These are the costs above and beyond the costs of resting, which we’ll discuss below.) If we want to factor in the effort to carry a backpack or a baby, we just add the weight of those items to the “Weight”
The brain and liver share the title of “costliest organ.” Your brain weighs a little less than 3 pounds but burns about 300 kcal per day, accounting for 20 percent of BMR.
Work by Christopher Kuzawa and colleagues has shown that in children three to seven years old, the brain accounts for over 60 percent of BMR, three times more than in adults. So much energy is channeled to the brain during these early critical years that it actually slows down growth in the rest of the body.
I’ve got a buddy who works deep in the rain forests of Indonesia studying orangutans and gibbons. Inspired by bird watchers who keep a record of all the species they’ve seen over the years, he keeps a “Life List” of every tropical disease he’s ever contracted. It’s not a short list. Inevitably, coming home from a field season means a prescription for Flagyl to kill the beasts that have turned his guts into a frat house.
A 220-pound caribou female will produce one 14-pound calf each year, equivalent to 6 percent of her own body weight. In that same amount of time, a 1-ounce female mouse will produce about five litters of seven pups each, equivalent to 500 percent of her body weight. The difference corresponds fairly well with the mouse’s ten times higher rate of cellular metabolism.
Since the heart has to pump enough blood to all tissues in the body to meet the demand for nutrients and oxygen, heart rates (beats per minute) match the cellular metabolic rates: they are faster in small species and slower in large species. But since small animals also die earlier than large animals, the total number of heartbeats in a lifetime are the same across species, from the tiniest shrews to the mightiest whales. We all get about one billion heartbeats.
If you want to ruin a paleoanthropologist’s day, suggest that the fossil hominin species that he discovered, named, and has dedicated his life’s work to, is in fact just a local variant of some other, previously described species.
It’s a general rule of ecology that plant-eaters don’t travel very far each day, because plants are plentiful and don’t run away. Living apes rarely cover more than a mile or two in a given day.
By 500,000 years ago, hominins are controlling fire. (This breakthrough may have come a good deal earlier. Debate on the subjects is . . . heated.)
Why they went extinct and we persisted—why we’re the only hominins left on the planet today—remains one of the great mysteries. It’s often been argued that we were simply smarter or more creative, but it’s not at all clear that was the case. Neanderthals had brains a bit larger than ours and were making cave art, playing music, and burying their dead long before we showed up. Perhaps it was just dumb luck, a cosmic roll of the dice where chance happened to favor us. Perhaps we brought new diseases into Eurasia with us as we expanded across the globe, wiping out the Neanderthal and Denisovan
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Brian Hare and Vanessa Woods describe in their book Survival of the Friendliest
Obesity, type 2 diabetes, and heart disease don’t evoke the same moral horror as genocide, but they kill more people globally each year than violence. These diseases aren’t inevitable. The Hadza don’t get them. They are what people in public health call “diseases of civilization,”
The World Health Organization puts the worldwide number of deaths each year from inactivity at 1.6 million.
If the quintessential Hadza outlook on life were to be boiled down to its fundamental essence, it would have to be hamna shida. No problem. There’s rarely a conversation with a Hadza man or woman that doesn’t end with this catchall upbeat assessment. Want to stay in our camp for a few weeks and hang out? Hamna shida. You’d like to measure our food and follow us around? Hamna shida. Wondering about the hyena that’s been lurking around camp? Hamna shida.
And what about the Hadza in camp? They couldn’t simply pick up and move their grass houses out of the path of the fire. There was no fire department to call for help. Instead, the women and kids had a dance party. They cut boughs from the bushes around camp and used them to beat down the fire, pushing it around camp as the wind swept it along, singing and smiling and laughing the entire time. Dave and I helped and sang along, learning how to stare down destruction the Hadza way, with some hard work and a song.
Exercise will keep you healthy and alive. It just won’t do much for your weight.
Overconfident interpretation by shouty people who don’t know as much as they think they do has a name in science, the Dunning-Kruger effect. In 1999, David Dunning and Justin Kruger, psychologists at Cornell University, had a brilliant insight that seemed to explain why incompetent people are so annoying: their very incompetence blinds them to how incompetent they are.
Happily, we can tune out these diet extremists and have a look at the data ourselves. There are three lines of solid evidence that tell us something about the diets our ancestors ate: the archaeological and fossil record, ethnographies of living hunter-gatherers, and functional analyses of the human genome. The details differ and it’s easy to get lost in the weeds, but the overarching message from each is clear: we evolved as opportunistic omnivores. Humans eat whatever’s available, which is almost always a mix of plants and animals (and honey).
Focusing on the average suggests that there’s one “true” natural human diet, and anything else leads to disease. That makes as much sense as arguing there is one “true” human height, and anyone who deviates from it is pathological.
Mark Haub, a professor at Kansas State University, followed a junk food diet for ten weeks to make the point that calories are all that really matter for weight, and tracked his progress on Facebook for the world to see. He ate a Twinkie every three hours instead of normal meals, and rounded out the diet with chips, sugar-rich cereal, and cookies. The diet sounds like a health disaster (and I’m not recommending it!), but the key piece of the puzzle was the calories: Haub limited himself to 1,800 kcal per day, well below his daily energy expenditure. At the end of ten weeks, he had lost
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deprivation, can cause dysregulation in our neural reward systems that can lead to overeating. Our brains can also learn to substitute food reward for the emotional and psychological rewards we crave when we’re feeling isolated, scared, or sad. The result is stress-eating, and it’s a real thing: even in a laboratory setting, people eat more after a stressful experience.
Constrained daily energy expenditure changes the way we think about the role of exercise in our daily energy budget. With a fixed energy budget, everything is a trade-off. Instead of adding to the calories you burn each day, exercise will tend to reduce the energy spent on other activities. You can’t spend the same calorie twice.
When a large portion of the daily energy budget is spent on exercise, the body is forced to be more frugal with the remaining calories at its disposal. Suppressing the inflammation response, limiting it to target real threats rather than sounding the alarm constantly, reduces the energy spent on unnecessary immune system activity.
The modern bicycle was invented in 1885, and in less than a decade, drug use in competition was widespread and generally accepted. People became understandably concerned when riders started dying in the 1890s. Apparently, the preferred performance cocktail at the time—a mix of cocaine, caffeine, strychnine, and heroin—had some nasty side effects.
physical activity changes the way the brain regulates hunger and metabolism. Regular exercise seems to help the brain match appetite to caloric needs.
Of course we’d like to avoid heart disease. But first we’d like to check our phones. Maybe get a snack. Relax a little. If exercise isn’t going to make me look hot, it can wait.
Sports scientists and physiologists still argue about the mechanisms in our body that set the limits (for ringside seats to this scientific brawl, check out Alex Hutchinson’s excellent book, Endure).
When we’re resting and during low-intensity activity (reading this book or strolling around the park), our bodies burn fat as their primary fuel. That makes good sense as a biological strategy: you’ve got virtually unlimited energy stored as fat, and, although it takes longer to process and burn fat to make ATP, at low levels of energy expenditure, we don’t require anything faster.
faced with an enormous exercise workload, the runners’ bodies were reducing energy expenditure on other tasks to try to keep daily energy expenditures in check.
John’s work pointed toward thermoregulation as the critical issue: if metabolic rates get too high, the body overheats. In one memorable study, he shaved mouse mothers with nursing pups to show they could burn more calories and produce more milk when they were able to lose body heat faster.
no matter the event or circumstances, the maximum amount of energy the body can absorb is around 4,000 to 5,000 kcal per day. Beyond that, and you’ll be in negative energy balance, burning more fat and glycogen than you can replenish each day and slowly melting away.
But natural selection rarely targets one trait in isolation, and most traits have multiple effects that all contribute to evolutionary success or failure. The obvious utility of a trait today might not be the reason it arose. We think of feathers as adaptations for flight, but they began in the earliest avian ancestors as insulation.
Richard Wrangham lays out in great detail in his excellent book Catching Fire, cooking completely changed our diets and in turn changed our bodies.
As James Watt, inventor of the steam engine, deduced through experiments at the dawn of the Industrial Revolution, a horse can comfortably produce around 640 kcal of work per hour (the definition of horsepower) and sustain that output for ten hours, day after day.
Mammals in the wild typically get around 40 calories of food for every calorie they spend foraging. Humans in hunter-gatherer societies like the Hadza or mixed foraging-farming societies like the Tsimane fare a bit worse, with each calorie of work spent on food production yielding around 10 calories of food. Our modern food production system violates the fundamental laws of ecology. When we include the fossil fuel energy consumed in food production, we burn 8 calories for every calorie of food we produce.
In 2015, more than 39 million Americans with low incomes lived in food deserts, defined as areas where grocery stores are more than a half-mile walk if you’re urban or a ten-mile drive if you’re rural.
