The anthropologist Barry Bogin has made the case that only humans have, in addition to infancy and juvenility, two new developmental stages before reaching full maturation. In mammals, infancy is defined as the period of maternal nursing. Juvenility is the stage after weaning and before sexual maturity. Primates in general have a relatively extended infancy period characterized by rapid growth, which stops with the eruption of the first permanent molars and the end of mother’s lactation. In chimpanzees these events occur around age four. In humans, however, breastfeeding typically ends by about age two to three (or earlier), yet the first permanent teeth only emerge around age six. The period between weaning and first permanent teeth Bogin calls “childhood”—perhaps a less well-established term could have avoided confusion. In any case, Bogin’s proposal is based on clear biological markers.
Human childhood in Bogin’s sense of the word is characterized by a slowing of the rate of growth, immature dentition, immature motor control, and, importantly, dependence on food and care from others. Brain growth during childhood is severely advanced relative to skeletal and muscle growth, with the brain peaking in mass at around age seven. In other primates, after infancy, offspring can find food for themselves and have the teeth to show for it. When human mothers stop lactating, however, they still need to provide food for their child (or need others to do that job). Transition to this stage makes it possible for the mother to breed again. Chimpanzee birth intervals are nearly twice as long as those observed in typical human hunter-gatherers. Therefore, even though humans reach sexual maturity at a much later stage than apes, humans can reproduce much more frequently.
Juvenility is the next stage. It is characterized by the capacity of the youngsters to fend for themselves and yet be sexually immature. In most animals this period is short—in mice, for instance, it is a few days—but in primates it is quite long. In baboons, for example, there are three to four years between weaning and puberty. Juvenility in humans begins around age seven, with the increased secretion of androgens leading to the growth of pubic hair and a changed sweat composition. This phase is known as adrenarche and also occurs in gorillas and chimpanzees. Juvenility is characterized by a decline in growth rate and typically ends with the achievement of sexual maturity and adulthood. In humans, however, there is another stage that Bogin argues to be unique: adolescence.
The life cycle of animals is typically characterized by an initial acceleration and subsequent deceleration in growth rate. Human adolescence, however, is marked by a renewed skeletal growth spurt after years of decreasing growth rates. Adolescence starts typically around age eleven or twelve in females and a year or two later in males. Females may fully complete this stage as late as age nineteen and males a couple of years later. The period is characterized by exploration, sensation seeking, and various changes in social relationships, as well as by vulnerability to mental disorders such as schizophrenia and depression. As is widely known, in the process of achieving social and economic maturity, adolescents often challenge established cultural practices.
While brain size and basic structure are in place by human juvenility, throughout adolescence cortical grey matter becomes thinner, while white matter increases.6 In line with these changes in the brain, the adolescent mind becomes increasingly capable of controlling behavior: adolescents improve in focusing attention, discipline, and resisting temptation. Executive self-control gradually improves. Even on very simple tasks such as “when a light appears on the left of the screen, look to the right; when it appears on the right, look left,” inhibition errors only gradually decrease across adolescence. Adult levels of control are reached at the end of this period, at which point physical growth is complete.7
Whether or not one agrees with Bogin’s definitions of developmental stages, it is clear that human development takes dramatically longer than that of any other primate and that it is characterized by unusual growth patterns. This developmental path provides ample opportunity for our second inheritance system to gain traction and transmit local culture.
A key to making these developmental changes possible must have been cooperative breeding. People other than the mother—including the father, uncles and aunts, grandparents, or even unrelated group members—help teach, protect, and provision offspring. Studies on hunter-gatherer groups show that families benefit from social norms of food sharing. Adults often intentionally acquire more food than they can consume to distribute among the group. The wider family typically helps raise the kids.
Human child-raising structures are not limited to Western nuclear families. The Musuo in Chinese Himalaya, for instance, have a system in which men do not invest in raising their own children but instead look after the children of their sisters and aunts. In this way they solve a fundamental problem of paternal parental investment. Before genetic testing, males could never be entirely certain that their purported children were in fact theirs. The Musuo avoid the problem of potential cuckolding altogether by supporting their less closely related, but more certain, kin—the children of their female relatives. There are diverse human child care arrangements, but what they all have in common is the support of nonmother members of the group.
Cooperative breeding is not a uniquely human trait per se.8 However, humans provide unusual amounts of care to their children. In hunter-gatherer societies, prior to modern prophylaxis and medical care, about 50 percent of newborns survived into adulthood. Although this figure might strike us as horribly low, it is high compared to figures from other animals. Many animals do not invest at all in their offspring and have accordingly low survival rates. Fish, for instance, lay millions of eggs for only one to reach maturity. Lions, on the other hand, have few offspring, invest significantly in them, and are lucky to see 15 percent survive to adulthood. Primates with their extended development invest even more than most other mammals and tend to achieve a survival rate higher than lions, with chimpanzees reaching a figure of about 38 percent. Parental investment and survival rates appear to go hand in hand.
While some great apes outlive their fertile period, most stay reproductively active until they die. Chimpanzees start reproducing from about age thirteen onwards, and females, of whom fewer than 10 percent live to age forty, tend to reproduce until the end. In humans, on the other hand, females go through menopause and often live for several decades without reproducing. Human groups often contain a sizable cohort of postreproductive elders—posing an evolutionary mystery. Postreproductive individuals draw natural resources away from those of reproductive age. Such a phenomenon makes no evolutionary sense, unless these individuals contribute in some other way to the survival and reproduction of their genes.
The explanation is that postreproductive elders increase inclusive fitness. Grandmothers provide a range of support to grandchildren and are often crucial in raising them. Evidence from traditional societies suggests the presence of a grandmother reduces mortality rates of infants. In some populations increase in the postreproductive period was found to be associated with increased numbers of grandchildren. Clearly, this novel life stage helps propagate genes.
Humans generally live longer lives than apes, surviving to age seventy and beyond even in hunter-gatherer societies. The elderly can draw on a greater wealth of experience and knowledge—or memes, if you like—than younger individuals. They act as living links to previous generations and so may harbor crucial information about challenges the group may face, in areas such as foraging, predation, natural disasters, and enemies. In a sense they function as the libraries of nonliteral cultures: they are critical to retaining its “meme pool” (recall that crystalized intelligence does not typically decline with age). All human societies have notions of wisdom they respect. Those regarded as wise are typically thought to be compassionate, knowledgeable, experienced, and reflective. They have superior insight in significant life matters and are therefore consulted in times of trouble or when far-reaching decisions are to be made. This is another way through which the elders can
contribute critically to the success of their children and children’s children. Younger group members, in turn, revere and support the elderly, even when they eventually fall into the protracted “imbecility” Herschel lamented.
WE ARE AN EXTRAORDINARILY COOPERATIVE primate. Children learn that others around them are prosocial and helpful. Humans impart vital mental skills and knowledge across generations. And so we managed to accumulate wisdom and technologies, passing them on from one mind to another over immense spans of time. This practice allowed us to exploit what John Tooby and Irvine Devore called “the cognitive niche”: through reasoning, planning, and cooperation we overcome the defenses of plants and prey, as well as the threats of predators and competitors. Our mental scenario building and rapid exchange of useful information have enabled us to flexibly deal with new challenges and outwit other creatures that could only adapt to us through the much slower traditional process of natural selection.9 Wherever humans traveled, they could quickly accumulate critical information to compete with other large animals. Indeed, there is evidence that humans frequently caused mass extinctions of prey animals shortly after arriving on new shores.
Which leads me to our prehistory and the creation of the gap: What steps occurred on our path from the last common ancestor with chimpanzees to modern humans? And what forces brought these changes about?
1The theater metaphor does not suggest, of course, that there is some ghost in the machine watching the mental plays. That idea is a nonstarter because it merely shifts the problem to the question of who is watching the mental play in the ghost’s head. Baars’s notion of broadcasting across the system, on the other hand, is a possibility.
2Hearing voices is quite normal. Relax. The trouble starts when you attribute these internal voices to external sources.
3A narrative can explain how we get from one point to another, and it can lead to identification of the causal links involved: the grass is wet because it rained this morning. Explanations in turn help us predict: it is raining now, so the river will rise. Crucially, they also provide opportunity for control. We often do not know the actual causal chains involved, but stories suggest what might have worked in the past. Many technologies and rituals have been invented to try to control things that matter to us, sometimes causally successful (e.g., making fire) and sometimes not (e.g., making rain).
4And social scientists sometimes underestimate the power of biology. Both factors are typically important. When asked whether nature or nurture contributes more to personality, the neuropsychologist Donald Hebb is said to have astutely replied: “Which contributes more to the area of a rectangle, its length or its width?”
5It is not entirely clear if this means that the enculturated minds are enriched or whether the captive zoo animal minds are deprived. The appropriate comparison point can only be wild apes raised in a normal social environment.
6This reflects that synapses are pruned and axons become increasingly mylenized.
7However, some aspects of the brain, such as white-matter connectivity, continue to mature even after adolescence. In particular, a white-matter tract called the uncinate fasciculus, implicated in socio-emotional processing linking orbitofrontal cortex, amygdala, and temporal regions, matures surprisingly late. It is found to peak in the mid-thirties.
8For example, marmosets breed in small groups in which all adults, males and females, contribute to carrying and supporting the group’s offspring.
9Fast-reproducing smaller organisms, such as ants and bacteria, remain difficult to control and are arguably more successful than humans, at least in terms of number of individuals, diversity, and distribution.
ELEVEN
The Real Middle Earth
WHEN MY MOTHER DIED, SHE was buried in the local cemetery in Vreden, the town in Germany where she was born and where she gave birth to my siblings and me. Around the time of her funeral, an archaeological dig found eighty graves nearby—dated at over three thousand years ago. Hearing this made me think of the many generations before me that eked out a living in our hometown. Did my ancestors live in the area for all this time? Perhaps. But it’s likely that my lineage is more varied than that and possibly includes famous forebears such as Genghis Khan and Cleopatra.
Let me explain. In many cultures one’s surname is passed on through the male line. My father’s father’s father was called Suddendorf, as is my son. Through church records we can trace back eight generations to an ancestor called Dirk. If I had a time machine and visited Dirk, would I recognize him as my forefather? Names can be misleading—you are, of course, just as closely related to your mother’s family as to your father’s. You have 2 parents, 4 grandparents, and 8 great-grandparents. Perhaps you know all their names. If you assume, for simplicity’s sake, that people reproduce by around age twenty-five, then this means that your ancestry includes 16 great-great-grandparents one hundred years before your birth and 32 great-great-great-grandparents twenty-five years earlier. Do you know all their names? When you move back eight generations, you should find 256 direct ancestors. Besides Dirk there were 255 other ancestors to whom I am equally closely related. Shared name or not, if one of them had not had children, I would not be here today.
If I traveled back four more generations, I would have 4,096 direct ancestors to visit. The numbers increase rapidly as we move back further in time. Following the same logic, four hundred years before your birth there are 65,536 ancestors. Go back six hundred years, and the figure is a staggering 16,777,216. Genghis Khan died in 1227 CE, and the number of my direct ancestors in his generation would appear to be over two billion. You see, even if he hadn’t spread his seed across Eurasia, I would have a good chance of being his descendant. So would you, if you have Eurasian ancestry. And if this calculation was correct.
There is a serious flaw in this number crunching, of course. These exponential increases eventually become nonsensical. If you go back to Cleopatra some two thousand years ago, such logic would mean you had more than a septillion ancestors (a thousand billion times a thousand billion or, more precisely 1.2 x e24). The numbers exceed not only the number of people alive but even the number of people who ever lived. Why? The calculation wrongly assumes that ancestors are themselves not related to each other, when in reality they often are. Sometimes they are closely related, as is evident in the famous inbreeding of the royal families of Europe. Yet slightly more removed inbreeding happens regularly when, for example, people find lovers in their own village and its environs. Nonetheless, if Jesus had a surviving bloodline, many, rather than a select few, could claim direct descent. The oldest continuously recorded family tree is that of Confucius, who, over eighty generations after his death, has well over two million registered descendants.
Some human groups have been in relative isolation for long periods and would hence not have had much opportunity for mixing with other groups. Australian Aboriginals have probably been isolated for the longest. With perhaps the odd influx of Indonesian or Melanesian visitors, they have been separate from other humans for about two thousand generations (i.e., more than fifty thousand years). They can claim the purest bloodline, as it were. Most of the rest of us are mongrels.
IN ORDER TO UNDERSTAND THE origin of the gap, we must first understand where we come from. With modern genetics we are now able to examine the ancestry of people even without historical records. Most DNA gets mixed from one generation to the next, ensuring variation among offspring. However, there is some DNA that does not get recombined, which helps geneticists to reconstruct our ancestry. The Y chromosome (which females lack) is passed on exclusively from father to son. Mitochondrial DNA is in the cell body and is passed on via the female’s egg, not the male’s sperm. That is, you have the same mitochondrial DNA as your mother and her mother, and your siblings share it too. Occasionally, however, even these nonrecombining parts of DNA change through random mutations, becoming genetic markers. As a result of common descent, people of the same region often share the same markers. When
some of them migrate, they take the markers with them, distinguishing themselves from populations already present in the new land. Thus are geneticists able to trace the history of human migration through DNA. By comparing DNA from people of different regions, they can calculate where their most recent common ancestor lived. Given that random mutations occur at a relatively constant rate (though some DNA regions do attract faster mutation than others), they can also estimate when common ancestors lived.
While some have raised legitimate concerns about the potential for abuse of this new genetic knowledge by racists and insurance companies, it opens up a wonderful new avenue for reconstructing our past. For example, the finding that there is more genetic variation within Africa than outside of it tells us about our common origins. In mitochondrial DNA, for instance, all non-Africans can be subdivided into two lineages, or so-called haplogroups (M and N). Other lineages exist only in Africa. In other words, Swedish, Japanese, Aboriginal, and Maya DNA are much more similar—that is, they are more closely related—than the DNA of two different groups of Africans. There is a simple explanation for this pattern: people evolved in Africa, and a sub-group of Africans moved out and populated the rest of the world.
Analyses of the Y chromosomes of men from around the world have yielded an estimate for the most recent common ancestor—Adam, if you will—of about 60,000 years ago.1 All humans alive are patrilineally descended from him. Analyses of mitochondrial DNA have suggested that our most recent female common ancestor—we can call her Eve—lived between 150,000 and 200,000 years ago in East Africa. The first thing to note here, then, is that she lived a long time before “Adam.” A reason for this time gap is that males and females are not equivalent in their reproductive potential. Women bear between none and about a dozen children during their lives, whereas men can sire from zero to hundreds of offspring. This means that a few men can be grandfather to a great many people. Though hundreds of people may share the same grandfather, they cannot all have the same grandmother. And so the last common grandmother is found much further back in the family tree than the last common grandfather. This is not to say that only an individual male or female was alive at these times. There would likely have been tens of thousands of individuals alive, many of them producing descendants. The bottleneck represents only the most recent common ancestor to all of us, even if most of us are also related to Adam’s and Eve’s companions.
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