The fifth and final major resource that is useful for studying the evolution of the human brain is the cognitive development of children. For many years, it was thought that the physical development of human fetuses precisely mirrored the evolutionary development of the species. Thus, human fetuses were said to have a tail and pharyngeal pouches that resembled the tail and gill slits of ancient vertebrates from which mammals evolved. Based on such observations, generations of biology students were taught that “ontogeny [the physical development] recapitulates phylogeny [the evolutionary development].”
The rigid interpretation of this maxim has been discredited by Harvard University biologist Stephen Jay Gould and others. The physical development of individuals does not precisely recapitulate the evolutionary development of the species. However, there are broad parallels, and this appears to be especially true for human cognitive development. Sir John Eccles, a British neuroscientist and Nobel laureate who devoted his life to the study of mammalian brains, believed that “the progressive development from the consciousness of the baby to the self-consciousness in the child provides a good model for the emergent evolution of self-consciousness in the hominids.” Child development specialist Jean Piaget also believed that “the development of thought in children closely parallels the evolution of consciousness in our species.” More recently, Daniel Povinelli, a University of Southwestern Louisiana psychologist who has specialized in comparing chimpanzee and human cognitive processes, noted that “comparing the ontogeny of [human] psychological capacities should allow evolutionary psychologists to reconstruct the order in which particular features of mental state attribution evolved.” A symposium on this subject concluded that “the sequence of cognitive development in humans roughly parallels the sequence of its evolution in ancestral forms.” Thus, the cognitive development of children can be used as a clue to help reconstruct evolutionarily the cognitive development of hominins, including Homo sapiens.15
FIGURE 0.3 White matter connecting tracts important for skills making us uniquely human.
Although much has been learned about the evolution of the human brain, much more is yet to be learned. As stated in a recent assessment of the field, “our understanding of the relation between the structure and function of the brain remains primitive, especially when compared to other organ systems.” The broad outlines are reasonably clear, but the finer details of brain evolution are still being sorted out. Over the next decade, we can expect additional progress as brain neuroimaging techniques become increasingly sophisticated. We will then have a much better understanding of the function of specific brain networks as well as the evolution of the connecting white matter tracts, which should lead us to an even better understanding of the emergence of the gods.16
PARALLEL EVOLUTION
There is one additional critical concept that underlies the argument of this book. When it is said that neurons, glial cells, and brain connections evolved over millions of years, what is actually meant? Genes are stretches of DNA and can be altered by a number of factors, including errors in cell division, radiation, viruses, and some chemicals. Evolution of a brain occurs when the molecular structure of a gene associated with the brain undergoes an alteration that provides the organism with some reproductive advantage. For example, when Homo sapiens acquired what is called “autobiographical memory,” as will be described in chapter 5, they were able to plan for the future much more skillfully than other hominins living at that time. Some altered genes are disadvantageous to the organism, and these genes die out. Other altered genes provide some reproductive advantage, and these genes are more likely to be passed on. Evolution is thus figuratively the attempt of genes to get ahead in life. Darwin called this process natural selection:
It may metaphorically be said that natural selection is daily and hourly scrutinizing, throughout the world, the slightest variations; rejecting those that are bad, preserving and adding up all that are good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked the lapse of ages.
Our brains are thus the remodeled products of 200 million years of such trial-and-error natural experiments. It should therefore not surprise us to find that our brains include many features of unintelligent design, features that make no sense today but that probably evolved to prevent some ancestral mammals from becoming an hors d’oeuvre for a brontosaurus.17
A special aspect of evolution that is important for the theory being proposed in this book is the existence of parallel evolution. This occurs when organisms that have had a common genetic origin continue to evolve along similar lines even though they have been separated from one another for thousands, or even millions, of years. The separated organisms develop along similar lines either because they are subjected to similar external selection pressures, such as climate or food supply, or because they have internal constraints, such as common anatomical structures that limit the number of developmental possibilities. Parallel evolution has been defined as “the recurrent tendency of biological organization to arrive at the same ‘solution.’ ” The products of parallel evolution have both intrigued and perplexed observers. Harvard historian Daniel Smail called them “some of the eeriest features of Postlithic human society.… Agriculture was independently invented on different continents, as were writing, pottery, priestly castes, embalming, astronomy, earrings, coinage, and holy virginity.… We celebrate the diversity of human civilizations, but it is the similarities that are the most startling.” Such phenomena become comprehensible if understood as products of continuing brain evolution.18
EXAMPLES OF PARALLEL EVOLUTION
The most widely cited example of parallel evolution is the evolution of mammals in Australia. More than 100 million years ago, the continents drifted apart and Australia became isolated from other continents. However, Australian mammals and the mammals on other continents had shared common ancestors before the continents drifted apart, so some of the descendants continued to evolve along remarkably similar lines. Examples of such parallel evolution include the Australian crest-tailed marsupial mouse and the European mole, the Australian sugar glider and the North American flying squirrel, and the Tasmanian wolf and the North American gray wolf. There are, of course, other mammals that evolved along different lines because of genetic mutations and different external selection pressures, such as climate, food supply, predators, or other factors.
Studies that have compared the brains of marsupial mammals in Australia with the brains of placental mammals on other continents have demonstrated the anatomical underpinnings of parallel evolution. The brain areas governing vision, hearing, and sensory stimuli are said to be remarkably similar in both types of mammals. The authors of one study concluded that “marsupials have evolved an array of morphological, behavioral, and cortical specialization that are strikingly similar to those observed in placental mammals occupying similar habitats, which indicate that there are constraints imposed on evolving nervous systems that result in recurrent solutions to similar environmental challenges.”
Another example of parallel evolution in brain development comes from a study comparing the brains of Old World and New World monkeys, which have evolved separately for 30 million years. One New World monkey, the cebus, uses a precision grip in which “the thumb and forefinger are brought into contact with one another to manipulate small objects or engage in goal-directed tool use.” An Old World monkey, the macaque, also uses a precision grip. When the brains of both monkeys were examined, remarkable anatomical similarities were found in the part of the parietal lobe that governs hand use. The authors concluded that “evolutionary changes involving skeletal, muscular and neural features proceed in parallel and, thus, features of the body and brain are linked.… The similarity of these [anatomical] fields in cebus monkeys and distantly related macaque monkeys with sim
ilar manual abilities indicates that the range of cortical organizations that can emerge in primates is constrained, and those that emerge are the result of highly conserved developmental mechanisms that shape the boundaries and topographic organizations of cortical areas.”
J. Karlen and L. Krubitzer, “The Functional and Anatomical Organization of Marsupial Neocortex: Evidence for Parallel Evolution Across Mammals,” Progress in Neurobiology 82 (2007): 122–141; J. Padberg, J. G. Franca, D. F. Cooke et al., “Parallel Evolution of Cortical Areas Involved in Skilled Hand Use,” Journal of Neuroscience 27 (2007): 10106–10115.
Parallel evolution of brain development can explain many of the remarkably similar developmental trajectories described in this book. For example, it seems likely that the initial genetic brain changes that enabled us to fully place ourselves into the past and future (autobiographical memory) took place before Homo sapiens left Africa. Because these brain developments were already underway, Homo sapiens continued to evolve cognitively along roughly similar lines for thousands of years whether they ended up in Portugal, Pakistan, Peru, or Papua New Guinea. Insofar as the widely disparate groups experienced similar selection pressures, such as increasing population pressures following the domestication of plants and animals, it should not surprise us to find disparate geographical groups arriving at similar outcomes. For example:
About 40,000 years ago, the first examples of visual arts appeared in cave paintings in places we now call Spain and Indonesia, and in sculpted ivory figurines in Germany.
Between 11,000 and 7,000 years ago, plants and animals were independently domesticated in southwest Asia, China, Papua New Guinea, Peru, and probably Mesoamerica.
By about 9,000 years ago, ancestor worship had apparently become widespread in both southwest Asia and China.
Between 6,500 and 5,000 years ago, higher gods had independently emerged in southwest Asia, China, and probably Peru.
Psychologists Mark Leary and Nicole Buttermore similarly suggested that “the neurological substrates for conceptual-self ability were in place before H. sapiens began to disperse out of Africa.… This may reflect a case of parallel evolution in which cognitive changes that occurred before the dispersion from Africa had evolutionary momentum.”19
Thus, the cognitive evolution of the human brain made possible the emergence of gods and civilizations. This would then be the starting point for a remarkable period of human development. In a mere 6,000 years, we would go, in the words of brain researcher Marcel Mesulam, “from the oxcart to Voyager, from the Sphynx to Rodin’s Kiss, and from Gilgamesh (by way of the Odyssey) to the Divine Comedy.” It has been a truly extraordinary journey. But to fully understand how all of this came about, we must begin at the beginning, with the first of the five major cognitive advances.20
1
THE MAKING OF THE GODS
1
HOMO HABILIS
A Smarter Self
The history of religious belief is rarely given centre stage in grand narratives of the evolution of civilization and of humanity and yet the urge to comprehend the human condition—the quest for soul food—may be just as great as the quest for food and reproductive success.
—Mike Parker Pearson, The Archeology of Death and Burial, 1999
The gods were born following a pregnancy lasting approximately two million years. It took that long for hominin brains to evolve structurally and functionally from being primate-like brains to being brains that possessed the cognitive faculties of modern Homo sapiens. Insofar as an evolutionary origin of deities is correct, the concept of a god would not have occurred to hominins prior to about 40,000 years ago, and the gods themselves would probably not have become fully visible prior to about 10,000 years ago. The human brain, and thus the self-aware human world, would not have been ready for them before that time.
Mammalian brains had, of course, been evolving for 200 million years prior to that time. For the first 140 million years of their existence, mammals were insignificant “small creatures living in the nooks and crannies of a dinosaur’s world.” During those eons, evolution was experimenting with the development of the three-part brain—the forebrain, midbrain, and hindbrain—that forms the central nervous system chassis for all mammals.1
About 65 million years ago, an asteroid apparently struck earth, producing a cataclysm that killed the dinosaurs and many other creatures. Mammals not only survived but thrived in a world now devoid of Jurassic predators. As Stephen Jay Gould noted, “We must assume that consciousness would not have evolved on our planet if a cosmic catastrophe had not claimed the dinosaurs as victims. In an entirely literal sense, we owe our existence, as large and reasoning mammals, to our lucky stars.” Our origin, added Gould, makes Homo sapiens “a kind of cosmic accident, just one bauble on the Christmas tree of evolution.”2
With the disappearance of dinosaurs, mammals rapidly diversified, grew larger, and became the new lords of the earth. The mammalian forebrain increased disproportionately in size compared to the midbrain and hindbrain and eventually occupied most of the space within the skull. As the forebrain grew, it differentiated into the four lobes (frontal, temporal, parietal, and occipital), basal ganglia, hippocampus, amygdala, thalamus, and hypothalamus. Most significantly, the brain developed a thin layer called the neocortex, which has been said to be like a 13-inch pizza covering the four lobes of the brain. According to Georg Striedter’s Principles of Brain Evolution, “The neocortex was the key innovation of mammalian brains,” because it included six layers of neurons, compared to the three layers in the cortex of earlier animals. Since neurons are connected three-dimensionally, both horizontally and vertically, to other neurons, the additional three layers increased neuronal connections exponentially, thereby making possible the processing of much more complex information and thought.3
As part of the diversification of mammals, the first primates appeared approximately 60 million years ago. They proliferated rapidly into hundreds of species, of which 235 species still exist. About 30 million years ago, a group known as New World monkeys (for example, cebus monkeys and marmosets) went their separate evolutionary way, and 25 million years ago the Old World monkeys (for examples, baboons and macaques) did the same thing. The great apes, the group most closely related to us, began dividing about 18 million years ago, with the orangutan, and then the gorilla, starting down separate evolutionary paths. Finally, about six million years ago, the hominins separated from chimpanzees, our closest hominid ancestor.
It is important to note that the hominins did not evolve from chimpanzees as we know them. Rather, both hominins and chimpanzees evolved from a common ancestor that lived about six million years ago. During the intervening time, both the hominin line and the chimpanzee line continued to evolve. Among the chimpanzees, for example, one group became geographically isolated in West Africa about two million years ago, and that group evolved into bonobos, also called pygmy chimpanzees. Insofar as the evolving chimpanzee line was subjected to similar evolutionary pressures as the evolving hominin line was during the 6 million years, it would not be surprising, given the principles of parallel evolution, to find that chimpanzees would develop some cognitive abilities similar to those developed by hominins. Awareness of self, to be discussed in chapter 2, is an example of such parallel development.
THE FIRST HOMININS
The evolution of one species into separate species is usually a gradual process. Thus, Sahelanthropus tchadensis, a fossil found in 2001 in Chad and thought to be at least six million years old, has been classified by some as the first bipedal hominin but by others as a chimpanzee. Its brain capacity was less than 400 cubic centimeters, equal in size to the brain capacity of modern chimpanzees.4
Sahelanthropus tchadensis was followed during the next four million years by Ardipithecus kadabba, Ardipithecus ramidus, and several species classified as Australopithecus—anamensis, afarensis, africanus, garhi, boisei, robustus, aethiopicus, and, from fossils discovered in 2010, sediba. There is mu
ch discussion regarding which hominin descended from which other hominin, but in fact there are not yet a sufficient number of specimens to make such determinations with any certainty. The study of early hominin fossils has been said to still be in “the stamp-collecting phase that begins most branches of science.”5
What is clear is that these early hominins had a brain capacity of approximately 400 to 475 cubic centimeters, only slightly larger than that of chimpanzees, and their behavior was quite similar to that of chimpanzees. They spent their days foraging for fruits, nuts, roots, and tubers and retreated to trees to escape predators and to sleep. Some researchers have claimed that some species of Australopithecus used stone tools, but other researchers have been doubtful. The most famous examples of Australopithecus are “Lucy,” whose fossils were found in 1974 in Ethiopia, and three sets of footprints embedded in volcanic ash in Tanzania. We occasionally romanticize Australopithecus and tell ourselves that they were not very different from us, but except for walking upright, they were in fact very different. Because of their rudimentary brain development, they could not think about themselves, they could not boast of their accomplishments, they could not gossip about other australopithecines, they did not worry about what might happen after they died, and they did not worship gods. Thus, it is generally believed that Australopithecus individuals “differed from other African apes (just as the other African apes differed from one another) but they were still apes, in mind if not in body.”6
Evolving Brains, Emerging Gods Page 3
