The living apes, our closest relatives, are well-behaved primates, eating fruit and leaves straight from the tree and nibbling on the occasional meal of insects or small game.

Like every other mammal, apes learn early to fend for themselves, foraging on their own as soon as they're weaned, and they know better than to give their hard-earned food away. Fossils from deep in the human lineage, the first four million years after we broke from the other apes, indicate our early ancestors played by the same ecological rules.

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Around 2. Early populations of the genus Homo stumbled onto a new way of making a living, something unprecedented in the history of life.

Instead of pursuing a career as a plant eater, carnivore or generalist, they tried a strange, dual strategy: some would hunt, others would gather, and they'd share whatever they acquired.

This cooperative approach placed a premium on intelligence, and over millennia brain size began to increase. Our Paleolithic ancestors learned to knap delicate blades from round stone cobbles, hunt large game and cook their food.

They built hearths and homes and began changing the landscape, developing an ecological mastery that led eventually to farming. These evolutionary shifts reverberate today. The cooperative foraging that pushed our hunting, gathering and farming ancestors to flout long-established ecological rules didn't just change the foods we eat.

It altered fundamental aspects of our biology, including our metabolism. The same unlikely series of events that gave us birthday cake has also shaped the way we eat it—and how we use the calories. For all the talk about metabolism in the exercise and dieting worlds, you would think the science was settled.

In reality, we've been embarrassingly short on hard data about the calories we burn each day and how we evolved to obtain them.

But recently my colleagues and I have made important strides in understanding how our bodies use energy. Our findings have overturned much of the received wisdom about the ways human energy requirements change over the course of a lifetime.

And, as we've discovered in a parallel effort, our energy needs are deeply intertwined with the evolution of our food-production strategies: foraging and farming. Together these studies provide the clearest picture yet of the inner workings of the human engine—and how our strategy for earning, burning and sharing calories underpins our extraordinary success as a species.

Our bodies are wonders of coordinated chaos. Every second of every day, each of your 37 trillion cells is hard at work, pulling in nutrients, building new proteins and doing the myriad other tasks that keep you alive. All of this work takes energy. Our metabolism is the energy we expend or the calories we burn each day.

That energy comes from the food we eat, and so our metabolism also sets our energy requirements. Calories in, calories out. Evolutionary biologists often think about metabolism as an organism's energy budget.

Life's essential tasks, including growth, reproduction and bodily maintenance, require energy. And every organism must balance its books. Humans are a striking example of this evolutionary bookkeeping in action. The traits that distinguish us from the other apes, including our huge brains, big babies and long lives, all require a lot of energy.

We pay for some of these costs by spending less on our digestive system, having evolved a shorter intestinal tract and smaller liver. But we have also increased our metabolic rate and the size of our energy budget.

For our body size, humans consume and burn more calories each day than any of the other apes. Our cells have evolved to work harder. The work our bodies do changes as we age, the activities of our cells waxing and waning in a choreographed dance from growth to adulthood to senescence. Tracking those changes through our metabolism could provide a better understanding of the work our cells do at each age as well as our changing calorie needs.

But a clear audit of our metabolism over the human life span has been hard to obtain. It's obvious that adults need more calories than infants—bigger people have more cells doing more work, so they burn more energy.

We also know that elderly people tend to eat less, although that's often accompanied by a loss of body weight, particularly muscle mass. But if we want to know how active our cells are and whether metabolism gets faster or slower as we grow up and grow old, we need to separate the effects of age and size, which is not easy.

You need a large sample with people of all ages, measured with the same methods. Ideally, you'd want measures of total daily energy expenditure, a full tally of the calories used each day. Researchers have been measuring metabolic rates at rest for more than a century, with some evidence for faster metabolism in children and slower metabolism among the elderly.

Yet resting metabolism accounts for only 60 percent or so of the calories we burn over 24 hours and doesn't include the energy we spend on exercise and other physical activity. Online calorie calculators purport to include activity costs, but they're really just a guess based on your self-reported weight and physical activity.

In the absence of solid evidence, a kind of folk wisdom has developed, cheered on and cultivated by charismatic hucksters selling metabolic boosters and other snake oil. We're often told our metabolism speeds up at puberty and slows down in middle age, particularly with menopause, and that men have faster metabolisms than women.

None of these claims is based on real science. My colleagues and I have begun to fill that gap in scientific understanding. In John Speakman, a researcher in metabolism with laboratories at the University of Aberdeen in Scotland and the Chinese Academy of Sciences in Shenzhen, organized an international effort to develop a large metabolic database.

Crucially, this database would focus on total daily energy expenditure measured using the doubly labeled water method, an isotope-tracking technique that measures the carbon dioxide produced by the body and thus the calories burned over one to two weeks.

Doubly labeled water is the gold standard for measuring daily energy expenditures, but it's costly, and you need a specialized lab for the isotope analyses.

So although the technique has been around for decades, studies are typically small. Led by Speakman, my lab joined a dozen others around the world in pooling decades of data. We ended up with more than 6, measurements of people ranging from babies just eight days old to men and women in their 90s.

In , after years of collaborative effort, we published the first comprehensive study investigating the effects of age and body size on daily energy expenditure. As expected, we found that metabolic rates increase with body size: bigger people burn more calories.

In particular, fat-free mass the muscles and other organs is the single strongest predictor of daily energy expenditure. This makes good sense.

Fat cells aren't as active as those in the liver, brain, or other tissues, and they don't contribute much to your daily expenditure.

More important, with the relation between mass and metabolic rate clearly established from thousands of measurements, we could finally test whether metabolism at each age was faster or slower than we'd expect from size. The results were a revelation, the first clear road map of metabolism over the human life span.

We found that, metabolically, babies are born like tiny adults, reflecting their development as part of their mom's energy budget. But metabolism skyrockets over the first year of life, so that by their first birthday children are burning 50 percent more energy than we'd expect for their size.

Their cells are far busier than adults' cells, hard at work on growth and development. Earlier studies measuring glucose uptake in the brain during childhood suggest some of this work is neuronal growth and synapse development. Maturation in other systems no doubt contributes as well.

Metabolism stays elevated through childhood, slowly decelerating through adolescence to land at adult levels around age Boys decline more slowly than girls, consistent with boys' slower development, but there's no bump at puberty in males or females.

Perhaps the biggest surprise was the stability of our metabolism through middle age. Daily energy expenditures hold remarkably steady from age 20 to No middle age slowdown, no change with menopause. The weight gain so many of us experience in adulthood cannot be blamed on a declining metabolism.

As a man in my 40s, I had sort of believed the folk wisdom that metabolism slowed as we aged. My body definitely feels different than it did 10 or 20 years ago. But like hunting some metabolic Sasquatch, when you actually look there's nothing there. Same for the much touted metabolic differences between men and women.

Women have lower daily energy expenditures on average, but that is only because women tend to be smaller and carry more of their weight as fat. Compare men and women with the same body weight and body fat percentage, and the metabolic difference disappears. We did find a decline in metabolism with age, but it doesn't kick in until we hit After 60, metabolism slows by around 7 percent per decade.

By the time men and women are in their 90s, their daily expenditures are 20 to 25 percent lower, on average, than those of adults in their 50s. That's after we account for body size and composition.

Weight loss with old age, especially diminished muscle mass, compounds the decline in expenditure. As with all age groups, there's a good amount of individual variability. Maintaining a younger, faster metabolism into old age might be a sign of aging well, or perhaps it is even protective against heart disease, dementia and other age-related disease.

We can now start to investigate these connections. Guided by our metabolic road map, we have a new world of research ahead of us. What is already apparent, however, is that a bite of birthday cake does different things for a seven-year-old girl, her middle-aged dad and her elderly grandmother.

Clara's bite is likely to be gobbled up by busy cells, fueling development. Mine might go to maintenance, repairing all the little bits of damage accrued through the course of the day.

As for Grandma, her aging cells might be slow to use the calories at all, storing them instead as glycogen or fat. Indeed, for any of us, the cake will end up as fat if we eat more calories than we burn.

The road map also highlights a major conundrum of the human condition. Whether they're born into a hunter-gatherer camp, a farming village or an industrial megacity, human youngsters need a lot of help getting food.

Other apes learn to forage for themselves by the time they stop nursing, around the age of three or four. Our children are wholly dependent on others for food for years and aren't self-sufficient until their teens. Someone with more muscle mass will burn more calories than someone else who weighs the same but has less muscle.

During exercise, having more muscle mass will increase your total calorie burn, because your body needs to produce more energy to support the increased rate at which your muscles are contracting. Long story short, if you want to enhance your calorie burn, consider stepping up your strength-training game.

The American Heart Association says that body fat is also essential for functions such as storing energy, absorbing nutrients, supporting cell function, protecting internal organs, and producing hormones. The American Council on Exercise ACE says that men need at least 2 to 5 percent body fat to support life, while women need a minimum of 10 to 13 percent.

But these numbers are just the bare minimum for your body to function and should not be seen as a weight loss goal. That said, the relationship between health and body fat is complex and not perfectly understood.

RELATED: 7 Reasons You Should Find an Exercise Partner Right Now. This loss of muscle mass lowers your metabolic rate — the speed at which you burn calories — at rest and during exercise. A study published in made headlines for its findings that metabolic rate may not decline throughout adulthood, but rather that it plateaus between the ages of 20 and 60 and then begins its decline.

Also, aging is accompanied by multiple changes in human physiology — not all of which may have been adjusted for, even by expert eyes, she says. The more you do a certain type of workout, the easier it seems. Overall, this is a good thing. It means that you can run faster or for longer with practice, and your muscles will be able to lift heavier weights with proper training.

But it also affects your calorie burn. RELATED: Are You Exercising Too Much? The Department of Health and Human Services DHHS says that walking and running deliver a lot of the same benefits when it comes to lowering blood pressure and reducing your risk of chronic conditions like heart disease and type 2 diabetes.

One study found that adults who walked 1 mile burned roughly 89 calories, whereas adults who ran that same mile burned around calories. A target of minutes of moderate-intensity, or 75 minutes of high-intensity, per week is enough to yield many health benefits, including reduced anxiety, better sleep, lower blood pressure, increased cardiovascular fitness, and reduced risk or slowed progression of certain chronic conditions, according to the DHHS.

Incorporating higher-intensity exercise into your routine will boost your calorie burn and magnify these benefits even further. High-intensity interval training HIIT can also be an effective and efficient way to increase workout intensity and calorie expenditure. The bottom line is that exercise has countless benefits beyond just burning calories, so the most important thing is to find types of movement that are enjoyable and feel sustainable.

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