Evolving Brains, Emerging Gods Read online

  When Homo habilis evolved approximately two million years ago, the world of early hominins became significantly more interesting, because of both its brain size and its behavior. Homo habilis is generally regarded as being the first hominin to have diverged significantly from its primate ancestors, although its precise relationship to other early members of the Homo species—such as Homo rudolfensis, Homo ergaster, and the recently discovered Homo naldi—is far from settled. Fossils of Homo habilis have been discovered in Ethiopia, in northern Kenya, and especially in the Olduvai Gorge in Tanzania, which Louis and Mary Leakey made famous.

  Homo habilis is thought to have lived between 2.3 and 1.4 million years ago, although recent finds in Ethiopia suggest that it may have existed as early as 2.8 million years ago. Its average brain size is estimated to have been about 630 cubic centimeters and thus to have been one-third larger than the brain of Australopithecus.

  The larger brain of Homo habilis made it smarter than Australopithecus, and it demonstrated this intelligence by making crude stone tools. This was mostly done by breaking rocks to produce sharp stone edges. Crude stone tools have been found dated to 3.3 million years ago, but those made by Homo habilis were more sophisticated. These have been found in abundance in association with Homo habilis fossils. Although crude, such tools would have been effective for cutting the hides and tendons of dead animals, thus allowing the tool-user to strip meat. The stone tools could also have been used to break open animals’ long bones and extract the marrow, an especially rich source of protein. Animal bones found in association with the stone tools suggest that the tools were used in this way. The bones also suggest that Homo habilis was probably a meat eater, in contrast to earlier hominin species. There is no evidence that Homo habilis hunted animals, so they probably scavenged for animals that had been killed by other animals or had died of old age or disease.

  The use of tools is, of course, not unique to hominins. Many birds have been observed using tools, including crows, which use sticks and carefully cut leaves to extract insects from holes, and Egyptian vultures, which drop stones on ostrich eggs to crack them open. Sea otters use stones to break the shell of snails and crabs. Monkeys have been observed using sticks to kill snakes and rocks to crack open oyster shells, and it is well known that chimpanzees use sticks, from which they strip the leaves, to forage in termite mounds, and stones to crack open nuts.

  What makes the stone tools used by Homo habilis different is their complexity. According to Cambridge University archeologist Steven Mithen: “To detach the type of flakes one finds in the sites of Olduvai Gorge, one needs to recognize acute angles on the [stone] nodules, to select so-called striking platforms and to employ good hand-eye coordination to strike the nodule in the correct place, in the right direction and with the appropriate amount of force.”7

  Attempts have been made to teach chimpanzees and bonobos to make stone tools similar to those made by Homo habilis. One especially clever bonobo, rewarded by food treats, successfully made stone tools, but they were significantly inferior to those of Homo habilis. According to Mithen, the bonobo “never developed the concept of searching for acute angles … or controlling the amount of force in percussion.” Mithen speculated that Homo habilis had already developed cognitive skills superior to those of modern chimpanzees, “an intuitive physics in the mind … perhaps even a technical intelligence.” Such cognitive superiority is supported by evidence that Homo habilis occasionally used one tool to make another tool, such as using a stone flake to sharpen a stick; this behavior is unknown among chimpanzees.8

  Additional evidence of the intelligence of Homo habilis includes the fact that they traveled several miles to obtain specific types of stones superior for use as tools. They also carried stone tools to new sites, evidence of planning and anticipation of future use. Archeologist Kenneth Feder of Central Connecticut State University said that such behavior suggests “a high level of planning and intelligence.” Such planning and storage of tools for future use are occasionally found among chimpanzees. An adult male chimpanzee in a Swedish zoo, for example, regularly collected and stored stones prior to the zoo’s opening time, which he then used to throw at spectators across the moat surrounding his enclosure.9

  Thus, what was Homo habilis really like? They possessed advanced physical skills and some ability to plan and were clearly smarter than their hominin ancestors. However, despite their greater intelligence, there is no evidence that they possessed self-awareness or any of the other higher cognitive functions that would distinguish later hominins and lead to the emergence of the gods. British psychologist Nicholas Humphrey painted a hypothetical picture of what Homo habilis was like:

  Once upon a time there were animals ancestral to man who were not conscious. That is not to say that these animals lacked brains. They were no doubt percipient, intelligent, complexly motivated creatures, whose internal control mechanisms were in many respects the equals of our own. But it is to say that they had no way of looking in upon the mechanism. They had clever brains, but blank minds. Their brains would receive and process information from their sense-organs without their minds being conscious of any accompanying sensation, their brains would be moved by, say, hunger or fear without their minds being conscious of any accompanying emotion, their brains would undertake voluntary actions without their minds being conscious of any accompanying volition.… And so these ancestral animals went about their lives, deeply ignorant of an inner explanation for their own behaviour.10

  “Clever brains but blank minds” appears to capture the essence of Homo habilis.

  THE BRAIN OF HOMO HABILIS

  Why was Homo habilis smarter than its predecessors? One reason, quite simply, is that its brain was more than 50 percent larger than the brains of its predecessors. Although four million years had passed since the earliest hominins and chimpanzees had separated from their common ancestor, during that time hominin brains had grown only slightly larger than chimpanzee brains. Suddenly, two million years ago, hominin brains began growing much more rapidly, initiating a growth pattern that would eventually lead to the oversized brains of Homo sapiens, brains that have been characterized as “freakishly large for a mammal of our body size.” Specifically, “the human brain is 3.5 times bigger than expected for an ape our size.” Philip Tobias, a South African paleoanthropologist who did much of the original research on Homo habilis skulls and named the species, noted that “it was with H. habilis that there began the remarkably disproportionate enlargement of the brain that is one of the hallmarks of mankind.” Similarly, Michael Rose, an evolutionary biologist at the University of California, claimed that “the expansion of the human brain over the last 2 million years is one of the most rapid and sustained such morphological developments known in the fossil record.”11

  As a general rule, when it comes to brains, bigger is better. Tobias, for example, estimated that the increased brain size of Homo habilis resulted in an additional one billion neurons compared to the brain of Australopithecus. But size is not everything, since it is known that the brains of highly intelligent and accomplished people may vary considerably in size. For example, the brains of English satirist Jonathan Swift and Russian novelist Ivan Turgenev each weighed over 2,000 grams, whereas the brain of French novelist Anatole France weighed just 1,000 grams. As will be described in chapter 3, the brains of Neandertals were as big, and often bigger, than the brains of modern Homo sapiens. And, as noted in Appendix A, elephants have brains four times larger, and whales five times larger, than humans. However, if brain size is scaled to body size, human brains are among the largest known. Chimpanzees, for example, weigh about the same as humans, but chimpanzee brains are less than one-third the size of human brains. This contrasts with other organs, such as the heart, lungs, liver, and kidneys, which are of similar sizes in chimpanzees and humans. A large brain is thus a distinguishing feature that sets humans apart from other primates, but brain size alone is not what makes humans unique.12

  The
uniqueness of the human brain, rather, lies in the specific areas of the brain that are enlarged and in the intensity of the connections among those areas. According to Tobias, the skulls of Homo habilis suggest that “the increase in cerebral substance … is evident mainly in the frontal and the parietal lobes” and “appears to be less marked” in the temporal and occipital lobes. Specifically, in the frontal lobe there appears to be “quite marked remodeling of the lateral parts of the frontal lobe,” and in the parietal lobe the superior parietal lobule and inferior parietal lobule are both “especially well developed.” Tobias concluded that with the brains of Homo habilis, “hominid evolution attained a new level of organization.”13

  Thus, two facts seem well established. First, Homo habilis appears to have been smarter than its predecessors, and second, its brain had grown larger disproportionately in the frontal and parietal regions. It is reasonable to assume that these two facts may be causally connected, but is there any data to support this?

  In fact, there is. In recent years there has been an outpouring of neuroimaging studies attempting to localize the components of intelligence in the human brain. A summary of 37 such studies reported a “striking consensus” in localizing intelligence to a network involving areas in the frontal and parietal regions and the connections between these regions. Thus, the results of neuroimaging studies localizing intelligence in the brains of contemporary Homo sapiens coincide nicely with the brain areas that grew disproportionately large in Homo habilis two million years ago at the same time that Homo habilis was becoming more intelligent.14

  BASIC AREAS ASSOCIATED WITH INTELLIGENCE

  The specific brain areas that become activated in neuroimaging studies of intelligence differ slightly depending on the test that is utilized, as would be expected. For example, many studies have used the Wechsler Adult Intelligence Scale (WAIS), a test that measures verbal comprehension, perceptual organization, processing speed, and working memory, defined as short-term memory needed to solve immediate problems. The brain areas activated by this intelligence test include the following frontal areas: frontal pole (BA 10), lateral prefrontal cortex (BA 9 and 46), and anterior cingulate (BA 24 and 32). The inferior parietal lobe (BA 39 and 40) is also activated by the WAIS. When other tests of intelligence are used, such as the measurement of blood brain flow while people are playing chess, another frontal area (premotor cortex, BA 6) and another parietal area (superior parietal, BA 7) are also prominent (figure 1.1). The authors of these studies concluded that “there is much neuroanatomical consistency among results, which we have described as defining a specific frontal-posterior [parietal] network.”15

  FIGURE 1.1  Homo habilis: a smarter self.

  What is known about these brain areas that are apparently associated with intelligence?

  First, they are almost all part of Flechsig’s “terminal zones” and thus evolutionarily are thought to have developed more recently. In fact, the frontal pole (BA 10) and lateral prefrontal cortex (BA 9 and 46) were classified by Flechsig as being the last brain areas to have evolved. Second, most of the brain areas that have expanded the most are what are known as association areas; such areas are involved not in more simple muscle or sensory function but rather in complex brain functions, such as assessing input from multiple other brain areas and coordinating appropriate responses. For example, if Homo habilis had put his hand behind a rock and simultaneously heard a hissing noise and felt a slimy creature, his association areas would have integrated these sensory inputs and directed an instantaneous withdrawal of his hand. It is the association areas of the brain, not the primary brain areas, that have evolved most recently and produced the unique cognitive skills of Homo sapiens. This principle was clearly illustrated by Todd Preuss, a neuroscientist and primatologist at Emory University who has extensively compared primate and human brains. He concluded that “the primary areas maintained approximately ape-like sizes in human evolution, while association cortex underwent enormous expansion.” For example, when Preuss compared the size of the human primary motor or visual cortex with the analogous areas in primate brains, the human brain areas were not larger than expected. By contrast, when Preuss compared the size of human association areas with the analogous areas in primate brains, the human areas were many times larger than expected.16

  Other studies also support specific parts of the frontal and parietal lobes as being critical for intelligence. The frontal pole (BA 10), for example, is said to have “more expanded than any other part in the human brain as compared to our ancestors.” It plays a critical role in information processing, working memory, social cognition, the processing of emotions, and the planning of future actions. A recent study that compared the spacing of neurons in the frontal pole of humans with the frontal pole of great apes reported that the spacing of neurons in the former afforded greater neuron interconnectivity. The relative importance of the human frontal pole can also be assessed by the fact that it contains more than four times more neurons than does the similar brain area in chimpanzees. The frontal premotor cortex (BA 6) has many functions, including being activated by tasks involving the abstracting of rules and associative learning. The lateral prefrontal cortex (BA 9 and 46) plays a central role in executive functions, including planning and reasoning, as will be discussed in chapters 4 and 6.17

  The superior parietal area (BA 7), also known as the precuneus, performs a wide variety of cognitive, sensory, and visual functions. Both the superior and the inferior (BA 39 and 40) parietal areas play important roles in intelligence and have also been linked to other intellectual functions, such as deductive reasoning. And it is probably not a coincidence that when Albert Einstein’s brain was examined after his death, his inferior parietal area was found to be “15 percent wider than controls.” This is an area that integrates visual imagery with mathematical thinking and other cognitive skills. In the previous chapter, it was noted that Einstein’s corpus callosum, which connects the two hemispheres of the brain, was also enlarged; the part of the corpus callosum that was most enlarged was the part that connects the inferior parietal areas in both hemispheres. Thus, an enlarged inferior parietal area and its connecting fibers may have been one reason for Einstein’s intellectual prowess.18

  As the frontal and parietal brain areas were increasing in size in Homo habilis, it is likely that the white matter tracts connecting these two parts of the brain were also developing. The major connections are three tracts that together make up the superior longitudinal fasciculus. The three tracts connect the prefrontal cortex to the superior parietal (BA 7), inferior parietal angular gyrus (BA 39), and inferior parietal supramarginal gyrus (BA 40), respectively. Studies of the maturation of the superior longitudinal fasciculus have reported it to be “one of the slowest maturing white matter tracts,” thus consistent with the development of greater intelligence over the past 4 million years. According to other studies, the superior longitudinal fasciculus “can be clearly identified only in highly developed species.… This strongly suggests involvement of SLF [superior longitudinal fasciculus] in high-level brain functions.” As noted previously, it is not just the existence of white matter connecting fibers that is important but also the speed at which they conduct information. This is especially important for intelligence. For example, a study of comparative intelligence among primates and other animals reported that the two most important predictors of intelligence were the number of neurons in the brain and conduction velocity of the connecting tracts.19

  WHY DID THE BRAIN INCREASE IN SIZE?

  The rapid increase in hominin brain size beginning approximately two million years ago raises two questions: How did the increase occur, and why did the brain begin to increase in size at that time, after having remained relatively constant in size for the preceding four million years. Regarding the first question, there has been an ongoing debate among scientists about whether hominin brains grew larger simply by renovating existing brain areas or whether they grew larger by creating new brain ar
eas. By way of analogy, did the house down the street become larger because the owners enlarged the existing rooms or because they built new rooms as additions?

  Although the question is still unresolved, there is a consensus that most brain evolution occurs by the former method; in other words, “opportunistic evolution has conscripted old parts of the brain to new functions in a rather untidy fashion.” It seems clear that specific older brain areas, such as the hippocampus, cerebellum, thalamus, and anterior cingulate, were conscripted for new functions as the brain evolved. However, some researchers believe that new brain areas were also created during the course of evolution. For example, Philip Tobias called the inferior parietal lobule the “most distinctive region of the human brain … the only ‘entirely new structure’ to have appeared in the evolution of the human brain.” Other researchers have expressed skepticism that it is “entirely new” but acknowledge that this region is “virtually impossible to identify in nonhuman primates” and underwent “enormous expansion and differentiation … during the transition from the simian to the human condition.”20

  Regarding the question of why the brain of Homo habilis grew when it did, there is no widely accepted answer. Changes in climate and other environmental conditions, dietary changes such as increased meat eating, and social changes have all been proposed. One widely cited theory is the social brain hypothesis, proposed by Oxford University anthropologist Robin Dunbar. This is based on the observation that primates with larger brains live in larger social groups, and Dunbar therefore claimed that “primates evolved larger brains to manage their unusually complex social systems.” In other words, as the earliest hominins came together to live in larger groups two million years ago, their brains grew larger to accommodate the more complex social relationships necessitated by the larger groups. The cause-and-effect relationship in Dunbar’s theory, however, remains debatable. Larger brains would confer many evolutionary advantages in addition to managing social complexity. For example, larger visual and olfactory systems would make hominins more capable of detecting danger, and a larger memory system would help Homo habilis remember the location of food sources. Perhaps hominin brains grew larger for unrelated reasons, and then the larger brains allowed the hominins to manage social complexity and thus live in larger groups. The brain size conundrum is well summarized by science writer Michael Balter: “For now, just how the human brain got so big remains a puzzle. Fortunately, natural selection has already made it big enough that this is one mystery we might someday solve.”21