Read Online The Origin of Humankind by Richard Leakey - Free Book Online
Authors: Richard Leakey
Ads: Link
of apes. Culture can be said to be the human adaptation, and it is made possible by the unusual pattern of childhood and maturation.
The helplessness of newborn human infants is, however, less a cultural adaptation than a biological necessity. Human infants come into the world too early, a consequence of our large brain and the engineering constraints of the human pelvis. Biologists have recently come to understand that brain size influences more than just intelligence. It correlates with a number of what are known as life-history factors, such as the age of weaning, the age at which sexual maturity is reached, gestation length, and longevity. In species with big brains, these factors tend to be stretched out: infants are weaned later than those in species with small brains, sexual maturity is reached later, gestation is longer, and individuals live longer. A simple calculation based on comparisons with other primates reveals that gestation length in Homo sapiens , whose average brain capacity is 1350 cubic centimeters, should be twenty-one months, not the nine months it actually is. Human infants therefore have a year’s growth to catch up on when they are born, hence their helplessness.
Why has this happened? Why has nature exposed human newborns to the dangers of coming into the world too early? The answer is the brain. The brain of a newborn ape, on average about 200 cubic centimeters, is about half that of adult size. The required doubling in size occurs rapidly and early in the ape’s life. By contrast, the brains of human newborns are one-third the adult size, and triple in size in early, rapid growth. Humans resemble apes in that their brains grow to adult size early in life; thus, if, like the apes, they were to double their brain size, human newborns’ brains would have to measure 675 cubic centimeters. As every woman knows, giving birth to babies with normal-size brains is difficult enough, and sometimes life threatening. Indeed, the pelvic opening increased in size during human evolution, to accommodate the increasing size of the brain. But there were limits to how far this expansion could go—limits imposed by the engineering demands of efficient bipedal locomotion. The limit was reached when the newborn’s brain size was its present value—385 cubic centimeters.
From an evolutionary point of view, we can say that in principle humans departed from the apelike growth pattern when the adult brain exceeded 770 cubic centimeters. Beyond this figure, brain size would have to more than double from birth, thus beginning the pattern of helplessness in infants who came into the world “too early.” Homo habilis , with an adult brain size of about 800 cubic centimeters, appears to be on the cusp between the ape growth pattern and that of the human being, while the brain of early Homo erectus , some 900 cubic centimeters, pushes the species significantly in the human direction (see figure 3.1 ). This, remember, is an argument “in principle”; it assumes that the birth canal in Homo erectus was the same size as it is in modern humans. In fact, we were able to get a clearer idea of how human Homo erectus had become in this respect from measurements of the pelvis of the Turkana boy, the early Homo erectus skeleton my colleagues and I unearthed in the mid-1980s not far from the western shore of Lake Turkana.
FIGURE 3.1
Homo erectus . (a), (b), and (c) show three views of the skull KNMER 3733, found east of Lake Turkana in 1975. This individual, with a brain capacity of 850 cubic centimeters, lived about 1.8 million years ago. For comparison, (d) shows a Homo erectus from China (Peking Man), which lived a million years later than 3733 and had a brain capacity of almost 1000 cubic centimeters. (Courtesy of W. E. Le Gros Clark/Chicago University Press, and A. Walker and R. E. F. Leakey/ Scientific American , 1978, all rights reserved.)
In humans, the pelvic opening is similar in size in males and females. So, by
The helplessness of newborn human infants is, however, less a cultural adaptation than a biological necessity. Human infants come into the world too early, a consequence of our large brain and the engineering constraints of the human pelvis. Biologists have recently come to understand that brain size influences more than just intelligence. It correlates with a number of what are known as life-history factors, such as the age of weaning, the age at which sexual maturity is reached, gestation length, and longevity. In species with big brains, these factors tend to be stretched out: infants are weaned later than those in species with small brains, sexual maturity is reached later, gestation is longer, and individuals live longer. A simple calculation based on comparisons with other primates reveals that gestation length in Homo sapiens , whose average brain capacity is 1350 cubic centimeters, should be twenty-one months, not the nine months it actually is. Human infants therefore have a year’s growth to catch up on when they are born, hence their helplessness.
Why has this happened? Why has nature exposed human newborns to the dangers of coming into the world too early? The answer is the brain. The brain of a newborn ape, on average about 200 cubic centimeters, is about half that of adult size. The required doubling in size occurs rapidly and early in the ape’s life. By contrast, the brains of human newborns are one-third the adult size, and triple in size in early, rapid growth. Humans resemble apes in that their brains grow to adult size early in life; thus, if, like the apes, they were to double their brain size, human newborns’ brains would have to measure 675 cubic centimeters. As every woman knows, giving birth to babies with normal-size brains is difficult enough, and sometimes life threatening. Indeed, the pelvic opening increased in size during human evolution, to accommodate the increasing size of the brain. But there were limits to how far this expansion could go—limits imposed by the engineering demands of efficient bipedal locomotion. The limit was reached when the newborn’s brain size was its present value—385 cubic centimeters.
From an evolutionary point of view, we can say that in principle humans departed from the apelike growth pattern when the adult brain exceeded 770 cubic centimeters. Beyond this figure, brain size would have to more than double from birth, thus beginning the pattern of helplessness in infants who came into the world “too early.” Homo habilis , with an adult brain size of about 800 cubic centimeters, appears to be on the cusp between the ape growth pattern and that of the human being, while the brain of early Homo erectus , some 900 cubic centimeters, pushes the species significantly in the human direction (see figure 3.1 ). This, remember, is an argument “in principle”; it assumes that the birth canal in Homo erectus was the same size as it is in modern humans. In fact, we were able to get a clearer idea of how human Homo erectus had become in this respect from measurements of the pelvis of the Turkana boy, the early Homo erectus skeleton my colleagues and I unearthed in the mid-1980s not far from the western shore of Lake Turkana.
FIGURE 3.1
Homo erectus . (a), (b), and (c) show three views of the skull KNMER 3733, found east of Lake Turkana in 1975. This individual, with a brain capacity of 850 cubic centimeters, lived about 1.8 million years ago. For comparison, (d) shows a Homo erectus from China (Peking Man), which lived a million years later than 3733 and had a brain capacity of almost 1000 cubic centimeters. (Courtesy of W. E. Le Gros Clark/Chicago University Press, and A. Walker and R. E. F. Leakey/ Scientific American , 1978, all rights reserved.)
In humans, the pelvic opening is similar in size in males and females. So, by
Similar Books
