Daniel E. Lieberman (1964--) The story of the human body: evolution, health, and disease. 2013
Part I. Apes and Humans
5. Energy in the Ice Age How We Evolved Big Brains Along with Large, Fat, Gradually Growing Bodies
Fat Bodies
...All animals need fat, but humans have a special need for lots of fat right from the moment of birth, largely because of our energy-hungry brains. An infant’s brain is a quarter the size of an adult’s, but it still consumes about 100 calories per day, about 60 percent of the tiny body’s resting energy budget (an adult’s brain consumes between 280 and 420 calories per day, 20 to 30 percent of the body’s energy budget. Since brains require sugar incessantly, having plenty of fat ensures our brains an unending, reliable supply of energy. A monkey infant has about 3 percent body fat, but healthy human infants are born with about 15 percent body fat. In fact, the last trimester of pregnancy is largely devoted to fattening up the fetus. During these three months, the fetal brain triples in mass, but fat stores increase one hundred-fold! Furthermore, a healthy human’s percentage of body fat rises to 25 percent during childhood, settling back down in adult hunter-gatherers to about 10 percent in males and 15 percent in females. Fat is more than an energy reservoir for the brain and for pregnancy and breast-feeding; it is also essential to fuel the endurance athleticism necessary to be a hunter-gatherer. When you walk and run, much of the energy you burn comes from fat (though as you speed up, you also burn more carbohydrates). Fat cells also help to regulate and synthesize hormones such as estrogen, and skin fat functions as an excellent insulator, helping keep us warm.
Where Did the Energy Come From?
...Regardless of precisely how archaic humans were able to acquire regular, dependable surpluses of high-quality food, these positive balances clearly set in motion a positive feedback loop. There are several different theories about how this feedback loop worked, but all are based on the same basic principle: once you take care of your body’s basic needs, you can spend surplus energy in four different ways. You can use it to grow if you are young, you can store it as fat, you can be more active, or you can spend it on having and raising more offspring. If life is chancy and rates of infant mortality are high, then the best evolutionary strategy is to be more like a mouse than an ape and plow as much surplus energy as possible into reproducing. However, if your children are thriving and surviving, then there is a strong benefit to evolve as archaic Homo evidently did: invest more energy in fewer, better-quality offspring by extending their development so they can grow larger brains. Since bigger brains permit more learning and more complex cognitive and social behaviors, including language and cooperation, these offspring have a better chance of surviving and reproducing because they develop into better hunter-gatherers. Then, when these smarter, more cooperative hunter-gatherers generate even bigger surpluses, selection will continue to favor even larger, slower-growing brains along with longer-growing, fatter bodies. In addition, mothers with adequate food supplies and strong social support would have benefited from weaning their infants at a younger age, because they could then have more children.
5. Energy in the Ice Age How We Evolved Big Brains Along with Large, Fat, Gradually Growing Bodies
Fat Bodies
...All animals need fat, but humans have a special need for lots of fat right from the moment of birth, largely because of our energy-hungry brains. An infant’s brain is a quarter the size of an adult’s, but it still consumes about 100 calories per day, about 60 percent of the tiny body’s resting energy budget (an adult’s brain consumes between 280 and 420 calories per day, 20 to 30 percent of the body’s energy budget. Since brains require sugar incessantly, having plenty of fat ensures our brains an unending, reliable supply of energy. A monkey infant has about 3 percent body fat, but healthy human infants are born with about 15 percent body fat. In fact, the last trimester of pregnancy is largely devoted to fattening up the fetus. During these three months, the fetal brain triples in mass, but fat stores increase one hundred-fold! Furthermore, a healthy human’s percentage of body fat rises to 25 percent during childhood, settling back down in adult hunter-gatherers to about 10 percent in males and 15 percent in females. Fat is more than an energy reservoir for the brain and for pregnancy and breast-feeding; it is also essential to fuel the endurance athleticism necessary to be a hunter-gatherer. When you walk and run, much of the energy you burn comes from fat (though as you speed up, you also burn more carbohydrates). Fat cells also help to regulate and synthesize hormones such as estrogen, and skin fat functions as an excellent insulator, helping keep us warm.
Where Did the Energy Come From?
...Regardless of precisely how archaic humans were able to acquire regular, dependable surpluses of high-quality food, these positive balances clearly set in motion a positive feedback loop. There are several different theories about how this feedback loop worked, but all are based on the same basic principle: once you take care of your body’s basic needs, you can spend surplus energy in four different ways. You can use it to grow if you are young, you can store it as fat, you can be more active, or you can spend it on having and raising more offspring. If life is chancy and rates of infant mortality are high, then the best evolutionary strategy is to be more like a mouse than an ape and plow as much surplus energy as possible into reproducing. However, if your children are thriving and surviving, then there is a strong benefit to evolve as archaic Homo evidently did: invest more energy in fewer, better-quality offspring by extending their development so they can grow larger brains. Since bigger brains permit more learning and more complex cognitive and social behaviors, including language and cooperation, these offspring have a better chance of surviving and reproducing because they develop into better hunter-gatherers. Then, when these smarter, more cooperative hunter-gatherers generate even bigger surpluses, selection will continue to favor even larger, slower-growing brains along with longer-growing, fatter bodies. In addition, mothers with adequate food supplies and strong social support would have benefited from weaning their infants at a younger age, because they could then have more children.