The Earth is a garden in space and we are some of its blooming life. How odd to be a sack of chemicals that can contemplate itself, and how much fun.
—Diane Ackerman1
For thousands of years, we humans ascribed Earth's rich biodiversity to a Creator who had both designed and fashioned the world around us. In the Judeo-Christian tradition, this grand creation had taken place about six thousand years ago. Later, medieval Christian philosophers suggested that we might understand God better if we were to investigate his handiwork directly through the study of nature. After all, if God had given humans commandments to properly live our lives, perhaps God had provided nature with laws as well. Indeed, in 1687 Isaac Newton proposed a simple mathematical framework—gravitation—to explain why an apple falls to the ground and how the planets circle our Sun. Natural philosophy made clear that God's universe was both lawful and comprehensible!
Biology and geology, rather more intricate than planetary motion, took longer to reveal their inner workings. It was not until the early 1800s that Earth's long history was elucidated. The importance of cellular organization in biology became apparent at around the same time, while the nature of disease was not understood until the late 1800s. Darwin had proposed a natural mechanism for the origin of biological design in 1859, but his ideas were not successfully integrated with genetics until the 1930s. In 1953, the double helix of DNA was identified as the mechanism for replicating hereditary information and carrying this information across generations. Now, with an understanding that fruit flies, mice, and people all use similar genes to build themselves, the glory of morphological development is yielding its secrets. Science has given us an evolutionary epic spanning almost four billion years of biological history—an odyssey of increasing complexity over time.
Ecosystems themselves have become richer in species and more complex over time. Terrestrial vegetation and the trophic levels it supports are far richer today than in ages past. The great coal deposits of the Carboniferous period were the consequence of having far fewer decomposers than today's tropical forests. Over the last hundred million years, flowering plants have further expanded terrestrial species richness, both with their own numbers and with those who feast upon them.
One of the unusual features of flowering plants is that they appear to spend less energy on building defensive chemistry than do other seed plants. The result is that they can sustain many more herbivores. Peter Ward claims that the world's biomass may have peaked over 300 million years ago (mya).2 Very unlikely, I believe, considering what's been happening on land surfaces! By developing a vegetation far richer in both species numbers, structural variety, and nutritional resources, flowering plants created new environments in which insects, mammals, and birds have also proliferated.3 Best of all, and from our own perspective, these increasingly more nutritious environments allowed one particular animal lineage, the primates, to build bigger brains. All told, this is a story of progressive advance. However, for many scholars, the notions of progress and increasing complexity over time have been seen as highly suspect, culturally biased social constructs.
THE CONCEPT OF PROGRESSIVE CHANGE
Natural selection therefore works like a ratchet, which turns the operation of random variation into a trajectory.
—Nick Lane4
Paleontology—the study of fossils and their history—paints a picture of escalating numbers and increasing complexity over time. This differs little from our views regarding human cultural history. First we were hunter-gatherers, then settled agriculturalists, and soon building grand civilizations. Though many larger city states may have risen only to collapse, human knowledge and technology have continued to grow and expand over time. With dramatic innovations changing people's lives during the nineteenth century, the notion of progress became a central tenet of Western thinking. Since the science of paleontology expanded as the Industrial Revolution unfolded, progressive change was understood to have begun at the very dawn of life.
Nevertheless, and for over much of the last century, discussions of progress have been unwelcome in the biological sciences. Stephen Jay Gould was especially disdainful of progress in any of its manifestations.5 He argued that natural selection is totally devoid of any progressive drive and, in this view, he was perfectly correct. But here again we stumble into nature's many-tiered complexity. Just because individual reproductive success is not, in and of itself, progressive does not mean that progressive trends cannot arise along the way. Accidental gene duplications allow for increasing developmental complexity; predation pressure drives counter-adaptation. From a broader perspective, progress appears to be an emergent consequence of evolutionary dynamics: the inevitable consequence of many interactions over time and space.
Academic hand-wringing over whether there really is such a thing as progress in the history of life continues to be fashionable. Writing about verifiable trends in the increase of complexity over time, Daniel McShea notes, “Given the historical background and the power of culture to penetrate perception, it is reasonable to wonder whether this impression of large-scale directionality is anything more than a mass illusion.”6 He himself presents evidence for increasing complexity within the vertebral column over time—from fish to human—but worries that trends of simplification can “offset” such increases. Humbug! Imagine a world overrun by simplified parasites. Would this erase the fact of increased complexity in other lineages over time? I don't think so.
Looking backward into time, and impressed by the rise of successful lineages, it is easy to see “directionality” in much of life's history. Looking forward, however, provides no way of discerning future directions; there is no way of predicting success or failure. Worse yet, extinction is the inevitable endpoint for most lineages. Centuries ago, natural philosophers had envisioned God's purpose at work in the slow unfolding of the natural order, with our own species (Homo sapiens) as the culminating glory in this odyssey. Such anthropocentric self-adulation has been banned from the halls of modern science. While abandoning a deity-directed progression of life, scientists also came to avoid the concept of evolutionary progress. However, if we accept the Big Bang scenario, the universe itself has progressed mightily. Starting with hydrogen, helium, and a bit of lithium, we are told that aging and exploding stars helped forge all the heavier elements now gracing the cosmos. Surely beginning with three elements and ending up with more than ninety represents a profound increase in complexity. A few of these heavier elements sit inside our most essential enzymes, absolutely critical in maintaining complex life forms. Clearly, the history of both our universe and ourselves has entailed progressive advance. Since Darwin's time, the metaphor of progressive evolutionary change has proven fundamental to our understanding of the living world.7
The idea of progress is supported by studies of fossil marine lineages; in many genera, extinction rates have tended to diminish with time. Many genera—as they move through time—produce more species, and these species tend to cover larger geographical areas or invade a wider variety of ecological niches. Such trends should diminish the probability of generic extinction over time. In addition, genera having special qualities decreasing their extinction rates should also increase over time.8 This is natural selection operating beyond the level of individual species.
Many scholars conflate the word progress with the notion of betterment. Progress, however, has wider meanings. When your doctor informs you that your disease is progressing, betterment is not what comes to mind! Though progressive change may be the icing on evolution's many-layered cake, there are exceptions. Speciation and lineage-splitting may not involve substantive increase in complexity. Such trends give us many more species, but no new organs, no new abilities. This is diversification in a “lateral,” not an “upward,” direction.9 In addition, selection can result in evolutionary stasis, where a lineage persists over long time periods without evident improvement. As boys exploring Long Island's Great South Bay, my friends and I were fascinated by horseshoe crabs (Limulus polyphemus). Some were up to two feet long. And all had a long spike for a tail. Resembling fossil trilobites, their front end is a single rounded U-shaped carapace with two wicked-looking eyes on either side. We would usually encounter their translucent exoskeletons, shed when they molted. Once in a while, we engaged with the living beast along the shore. But here's the point: a recent fossil discovery suggests that horseshoe crabs—in much their present form—have been around for 400 million years. That is a very long time, and not very progressive. More amazing is the fact that we can put a human gene into a bacterium, in order to have that bacterium synthesize human insulin—an astonishing example of how deeply conservative some biochemical functions really are!
To make matters more confusing, there are animals that combine highly “advanced” traits together with “primitive” characters, all within the same body. The rear end of a platypus has a cloaca, rather like reptiles, from which females discharge a leathery egg—just like reptiles. But the front end of the duck-billed platypus is an extraordinarily sensitive device, highly attuned to seeking prey in muddy water. In addition, and almost unique among mammals, the male platypus has poison glands! Here we find ancient and advanced traits in the same mammalian body. All told, evolutionary history is full of both conservative stasis and dynamic advance. But there are also parasitic lineages that have become much less complex. Using their hosts as resources, parasites can jettison a lot of excess baggage. Such exceptions do not negate a general pattern of progress in the history of life.
Philosopher of science Michael Ruse argues that a strongly progressive philosophy of human history, developed in the eighteenth and nineteenth centuries, has both inspired and debased evolutionary biology from its earliest beginnings. “Complexity is a poor guide [for assessing progress] since ancient forms (trilobites) were more complex even possibly than humans,” claims Ruse.10 While trilobites may have had lots more legs and body segments than we do, one glance at our nervous systems will make clear who's way out in front! In addition, some mammals have become larger over the last fifty million years.11 Called “Cope's rule,” this may be the result of larger males commandeering more territory, attracting more females, and fostering more offspring.
More generally, life's three-billion-year history has documented increases in the maximum size of living things. This has been a pulsed history, with eukaryotic cells representing the first grand enlargement beyond their smaller bacterial antecedents. Becoming multicellular produced a second pulse of increasing size. And finally, as oxygen pressure rose, larger animals began to roam the seas.12 Paleontological evidence is clear: our planet and its biota have become progressively more complex over geological time. In the words of Richard Dawkins, “if you let the animals bring their own definition—you will find progress, in a genuinely interesting sense of the word, nearly everywhere.”13
Though many lineages have participated in a variety of progressive trends, it is only among land vertebrates, and the placental mammals in particular, that the evolutionary drive toward ever-greater organic complexity has reached its earthly zenith. Though our kidneys and our livers may be highly complex organs, essential to our proper functioning, they are far simpler in structure than our most complex organ: the brain. And though the human life-trajectory may not be as elaborate as that of a butterfly, we humans have impacted our planet like no other species—ever!
BUILDING BIGGER BRAINS
Brains, with millions of neurons and billions of interconnections, are the most complex organs in biological organisms. Among terrestrial animals, a number of mammalian lineages have enlarged their brains significantly over recent geological time. However, we must be careful in our comparisons. To make anatomical trends more meaningful, we will follow Harry Jerison, who compared brain/body mass ratios.14 Ratios are a better metric than simple brain size because ratios avoid the “big animals got bigger brains because they're bigger animals” conundrum. (Elephant brains are about three times as large as ours.) Comparing vertebrate animal brain/body mass ratios across lineages, Jerison found three differing data sets. First, birds and mammals sit well above the averages of brain/body mass ratios found in fish, amphibians, and reptiles. Secondly, primates stand a bit above most other mammals, and, third, we humans occupy a further step beyond the primate norm.
The historical escalation of brain volumes is one of the clearest examples of increasing organic complexity in the fossil record. But before we discuss the fossil record, let's take a minute to make clear what we mean by intelligence. Information processing is what brains really do. A pragmatic definition states that intelligence is the “ability of an organism to sense its surroundings and make appropriate responses.” An appropriate response will help the organism find food, avoid predation, recognize a potential mate, and deal with other challenges. Complex animals with eyes, ears, and other sensory organs linked together in a neural network do the job of monitoring their environments. Animal brains take this sensory input from many organs to build representations of the outside world. Dragonflies, darting back and forth in their pursuit of mosquitoes, are a superb example of eye-brain-flight coordination, and they do this with a brain the size of the head of a pin!
In discussing higher intelligence, however, we are focused almost entirely on animals with backbones. Early in their history, fish developed an insulating fatty (myelin) sheath around their nerve fibers, allowing electrical signals to zip quickly along those fibers. Insulated nerve fibers allowed these early vertebrates to react more quickly to the challenges they faced. However, just as there is more biodiversity on land, brain power expanded more grandly on land. Clever dolphins and smart whales are mammals that first evolved on land, returning only later to the sea. The smartest invertebrates, eight-legged octopi, are marine creatures—but they are few in kind and number. Those few fish species that do have large brains generate electrical fields to seek their prey in murky water, using brain power to analyze the data they perceive. Despite these exceptions, larger brains are mostly found in birds and mammals. Why should this be?
Simply stated, building and maintaining a more complex structure requires more energy. We humans use about 20 percent of our resting energy just to keep our brains going. When we are deprived of oxygen, our brain is the first organ to suffer damage. That's the maintenance issue, however there is also the problem of building a bigger brain. Human infants utilize about half their food intake to expand their growing brain in the first year of life. In addition, maintaining a larger brain means burning more energy faster. Mammals require five to ten times as much food to sustain themselves as a reptile of the same weight.15 Your pet lizard can survive a month without food; your pet gerbil will not. As we've noted, sharply differentiated teeth allow mammals to cut, tear, and chew their food, allowing more rapid digestion. Many toothless birds fill their crops with pebbles to grind up their chow. Both mammals and birds have longer intestines than reptiles, digesting their food more quickly, and both have a four-chambered heart to distribute oxygen more efficiently throughout their bodies. In addition, mammals and birds are warm-blooded, keeping their bodies at higher temperatures, where metabolism is quicker and brains work more efficiently. Just as significantly, both birds and mammals must support the early development of their young. Surely building a bigger brain is a bigger expense. And yet the fossil record indicates a steady increase in relative brain size among many mammalian lineages over the last fifty million years.
Ethiopia's ape-like fossil “Lucy” (Australopithecus afarensis) lived around 3.3 million years ago and had a brain volume of about 450 cc, similar to that of a chimpanzee. (A cubic centimeter or cc equals 0.061 cubic inch.) Over the following three million years, our lineage expanded that brain volume to the one we're carrying around today: averaging around 1,400 cc. Here's a three-fold increase in only three million years! Surely one of the most dramatic examples of escalation in the fossil record, this scenario has an additional twist. Our fossil cousins, the Neanderthals (Homo neanderthalensis), were isolated from other humans in seasonally frigid Europe and western Asia for half a million years. During that time, they also evolved big brains! Their skulls are longer across the top, while ours are more rounded. Recent analyses of Neanderthal newborn skulls indicate that their brain growth was similar to our own.16
In addition to being brainy, contemporary humans exhibit a grand variety across the globe, while having considerable genetic diversity within each and every hamlet. Our species is hugely polymorphic! Unfortunately, paleoanthropological practice assumed that our ancestors were much less varied, giving new and different names to many different fossil remains. This practice has populated our past with a “shrubbery of species.” From my readings, it seems that Homo erectus may have been as variable as we are, and can serve as most everybody's ancestor between one and two million years ago. A group of five crania recently excavated in Dmanisi, Georgia (including a fully preserved large male skull), support this view.17 However, and with time, something new and different did arise in Africa, perhaps 200,000 years ago. This lineage had a more slender frame, a higher forehead, a less protruding chin, smaller teeth, and was the first hominid to throw spears with stone spear points! We've given this distinctive form our own Latin name: Homo sapiens.
Spreading out of Africa and around the world, this is the lineage that provided most of the genetic heritage we humans carry around today.18 However, even as Neanderthals were on their own trajectory in Europe, something very special was happening on the island of Flores in Indonesia. There, a population of early hominids adapted to their island home by becoming much smaller. Nicknamed “Hobbits” by their discoverers, these short, small-brained people lived as recently as 18,000 years ago and appear to have become distinctive (called Homo floresiensis).19 These fascinating “little people” are a reminder that the evolution of humans has included a lot more diversity than we imagined earlier.
Returning to the two main strands of modern hominid evolution, both H. neanderthalensis and H. sapiens enlarged their brains significantly over the last million years. Neanderthals expanded their brain power in ice age Europe and western Asia, while our ancestors, Homo sapiens, expanded their brains in Africa and spread out from there. Despite the huge costs in food procurement and loss of mothers in birthing, both modern humans and Neanderthals increased their brain volumes independently! Here is one of the most extraordinary examples of escalating complexity in the fossil record, and it was done in parallel by two closely related lineages in very different environments! How did this unusual trajectory begin?
WHY PRIMATES GOT SO SMART
Early mammals had a brain/body ratio equivalent to that of today's opossums and hedgehogs, about three times the weight of the reptilian average. For most mammals, this level of brain enlargement (encephalization) was maintained well after the dinosaurs went extinct (65 mya). Fifty million years ago, the dog-sized ancestor of horses (Hyracotherium) had a brain/body mass ratio similar to that of modern opossums. Becoming larger over time, horses reached the average modern mammalian brain/body mass ratio by twenty million years ago. Luckily for us, there was one mammalian lineage that did not fit these general patterns.
From their beginning, primates tended to have larger brains. While most mammals allocate about 5 percent of their metabolism to maintaining brain function, primates utilize 9 percent or more. What made the monkey lineage smarter? The initial step may have been shifting from a nocturnal lifestyle on the ground to the daytime pursuit of nourishment in treetops. With the expansion of the flowering plants, trees bearing flowers and fruits became an important resource for a variety of animals. Pursuing insects and fruit in the high canopy, early arboreal primates required better three-dimensional vision and more physical agility. Over millions of years, muzzles became shorter and eyes came closer together, allowing visual fields to overlap. A larger brain now processed two slightly incongruent images for accurate depth-perception, important when jumping from branch to branch. Binocular stereoscopic vision had additional benefits, such as finding tasty but camouflaged bugs to eat or watching for predators in the dead of night.20
Because their lives were now spent among the tips of branches, where blossoms and fruits reside, primates evolved flexible arms and fingers to catch insects and reach for yummy fruits. Longer rear legs with grasping toes helped hold on tight. Separate digits, with a flexible thumb and big toe, were important in grasping slender twigs. Claws became transformed into more useful nails, while the grasping surfaces of hands and feet developed a ridged skin, helping hold onto smooth branches, and giving us our fingerprints. At the same time, arms and shoulders were developing new kinds of flexibility. Hand-to-mouth feeding—after careful examination—is characteristic of all primates. Surely, none of this monkey-business would have been possible without the flowering plants. Neither tree ferns, cycads, nor conifers provide such a variety of nectar-filled flowers, juicy fruits, or nutritious seeds as do the flowering plants. These features attracted many insects into the treetops, quickly followed by the earliest insect-eating primates.
With forward-pointing eyes, primates lost much of their lateral vision. Thus, it became important to travel in small groups, where extra pairs of eyes could survey all directions, watching for eagles, snakes, and arboreal cats. These social interactions promoted increased brain size. The better you understand the intentions of your associates, the better you can interact with them and make the troop an effective social unit. In general, the larger the number of interacting females in the troop, the better developed is that species’ neocortex.21
At all stages of pregnancy, a primate fetus consistently has about twice as much brain tissue as a similar sized fetus of any other mammal. In other words, brain development is specifically privileged in primates.
—Robert Martin22
As primates continued evolving, there were two instances where brain size expanded even further. The first increase took place around twenty million years ago, as seen in the fossil called Proconsul. These animals were among the earliest of the apes. Not only did Proconsul have a slightly larger brain, it lacked a tail, as do all the living apes (the hominoids: gibbons, orangs, gorillas, and chimps). Spending more time in an upright position, both legs and back became stronger. More significantly, apes became the only mammals to be able to rotate their forearms rather like a windmill, allowing them to swing through the canopy. This is another instance where flowering plants made a singular contribution to our ancestry. Few non-flowering trees have broadly spreading branches in their upper crowns. Flowering trees do, giving rise to tropical forests with a canopy of many spreading branches, where a larger animal can swing from tree to tree. Called brachiating, our swinging ancestors developed broad shoulders, long rotatable arms, flexible elbows, and strong hands. Flowering trees not only fostered the origin of primates, they are responsible for our remarkably flexible arms!
WALKING ON TWO LEGS
After the origin of apes, the next critical event for our lineage was spending more time on the ground. Chimps and gorillas spend a lot of time on the ground, and both have developed a four-limbed, knuckle-walking mode of travel when on the ground. Human ancestors instead became more upright and two-legged, probably sometime around six mya.23 By three mya we have better fossil evidence, as represented by Lucy and her kin.24 Though Lucy's brain was the size of a chimpanzee's, her skeleton makes clear that she walked upright on two feet. Her arms were still long and ape-like, indicating that she spent nights sleeping in trees. (You need to be careful where you sleep in the African savanna—what with lions, leopards, and hyenas prowling around.)
Walking on two feet—bipedality—was a crucial advance for the hominid lineage. Our feet, in fact, are the most distinctive element of our skeletons. No other primate has feet with little toes up front, a big toe alongside the others, a tough padded heel, and an arch that translates impact into forward propulsion.25 Standing upright gave us a better view across the grass savanna, reduced our exposure to the noonday Sun, and freed our forearms from having to support us. The fact that Lucy had modern feet while still carrying around an ape-like brain makes clear that our upright gait came well before we built our larger brains.26 A further advance, developed in Homo erectus, was a more flexible neck, allowing us to survey the landscape quickly, and longer legs to expand our range. Much earlier, our ancestors had abandoned the muscularity of chimps and gorillas to become lighter, covering ground ever more nimbly.27
Bipedality proved transformative, freeing our forelimbs to carry things around, throw rocks, and fashion tools. Pressing our opposable thumb against a fingertip gives us a strong “precision grip,” useful for everything from basket-weaving to using stone flakes as if they were razor blades. Opposing our thumb against all our fingers gives us a “power grip” for wielding clubs and hammers. Just as significant, our hands were now free to signal to our companions. By using arms and hands to make gestures, then coupling those gestures with vocalization, humans gradually developed their premier talent: language!
Language is a trick that allows the mind to question itself; a magic mirror that reveals to the mind what the mind thinks; a handle that turns the mind into a tool.
—Kevin Kelly28
Our minds are marvelous devices; we use our brains so effortlessly that we fail to appreciate what's really going on inside. Our vision appears to be simple and straightforward, but it is amazingly intricate. Visual information-processing protocols begin within the eye itself, where the signals from a hundred million rods and cones must be collated and processed before being transmitted by an optic nerve having “only” a million nerve fibers. The image projected on our retina is upside-down; our mind makes things right-side-up. Years of experiments with domestic cats and small monkeys have shown how different batteries of neurons and different areas of the brain contribute to the creation of what appears as a simple moving image. Thanks to clever brain processing, tilting our heads does not tilt the horizon! We notice unusual movements in the environment immediately, even when they are at the edge of our peripheral field. The macula, a small central area where we see sharply, scans the moving element repeatedly as the brain analyzes the object. Failure to recognize a predator could be fatal, but over-reacting to gusts of wind wastes precious energy. Decisions need to be made quickly and economically.29 Distinguishing the intermittent cadences of a gusting wind from the interrupted rhythm of a quietly stalking predator may have created in our mind those faculties we now use when making music or exploring the structure of mathematics. The “human mind” is the emergent property of a large and complex brain.
HELPLESS BABIES GAVE US LARGER BRAINS!
A critical step in escalating hominid intelligence came only after the bipedal stone-throwing troop could defend itself on the ground and through the long tropical night. Defense during the night and on the ground may have involved constructing encampments enclosed by a barricade of thorny branches. It was not until this level of group defense had been achieved that females could give birth to helpless infants; infants unable to cling to their mothers’ bodies. Once on-the-ground-security was achieved, mothers were protected through the long and dangerous night. Though we have no fossil evidence, fiber-woven slings—holding baby close to mother—was an essential early innovation. Though the birth of helpless babies placed our ancestors at greater risk, these infants were born with their cranial sutures unfused. Because these sutures are open, baby humans double their brain size in the first year of life! This little detail, coupled with a much longer childhood, built our larger brains.30
The human brain is hugely expensive; though only 2 percent of our body weight, 20 percent of our resting energy is devoted to maintaining the brain. No other organ burns so much energy on a continuing basis. (Though sleep gives us some respite, nerve cells operate continuously; they cannot be turned off.) While expanding human intelligence involved costly increases in neurologic hardware and interconnectivity, there was an even greater cost. The human birth canal has lagged behind brain expansion. Tragically, women suffer the most difficult and dangerous birthing among vertebrates. Childbirth has been a dreaded killer of women throughout recorded history. Even in today's world, as many as 10 percent of Afghanistan's women are likely to die during or shortly after birthing. What forces of selection could possibly have countered so costly an expansion of the human brain? Whether in human economies or in natural selection, benefits must equal costs! What manner of selective pressures might have driven the costly expansion of our brain volume?
BUILDING THE LARGER HUMAN BRAIN
Not only are our babies helpless; born to healthy mothers they can be called “obese.” Human females themselves, unlike any other primate, averages 20 percent of their body weight in fat, helping the fetus develop and nurse the infant during food shortages. In contrast, men's bodies average around 7 percent fat. In addition, and unlike other primates, the female pelvis differs considerably from that of the male, allowing for a larger-headed baby to pass through the birth canal. Homo sapiens is a sexually dimorphic species, with plump females having the essential responsibility of birthing and nurturing our young.31 Males, on the other hand, must secure and defend territory with sufficient resources to sustain the group. In fact, dangerous birthing and helpless babies may be the reason why women experience menopause. Released from the dangers of further birthing, older mothers can contribute significantly to the care of their grandchildren. Basically, motherhood is the second most important job in the world; acquiring nutritious food is the first.
A troubling conundrum in biology has been the question of how Mother Nature actually goes about building something as complex as the human brain. This is a question very similar to asking how complex animals develop from a single fertilized egg, and the answers are much the same. Not too many genes, coupled with carefully cadenced developmental protocols, can create very complex, yet fully integrated, structures. The fact is that there simply aren't enough genes in the human genome to specify a brain having billions of neurons, all nicely interconnected by trillions of synapses. Genes do not carry sufficient information to specify the complex brain. Clearly, human brains build themselves! Genes may provide the necessary scaffolding, but developmental protocols and environmental cues build the basic structures. More than that, “synaptic plasticity” allows changes to be made in neural connections. Parts of the brain that are used expand and proliferate, while unused parts of the neural network wither in a process called “cognitive sculpting.” Scientists came to these conclusions after doing some rather nasty experiments. By covering the eyes of kittens for a period of time during their early development, researchers discovered that these kittens would never learn to see. Though appearing perfectly normal, these kittens had been blinded! Visual input is necessary at a critical time in the kitten's development for the visual areas of the brain to form. The visual system builds itself to see!
Human infants deprived of social contact during their early years cannot learn to speak. Clearly, the brain uses external inputs to configure itself at critical points during development. Playfulness is characteristic of all smart animals. Play helps refine behaviors, practice social interactions, learn from accidents—all in preparation for the challenges of adulthood. With input from our environment and a kind of “natural selection of neurologic development,” our brains build themselves!
But why might human intelligence have advanced in the face of huge energy costs and an appalling loss of mothers in birthing? The literature of anthropology has many suggestions and hypotheses explaining the escalation of human intelligence. Increased and more complex social interactions must have been important. Parents and their young cannot survive without a supportive social network. Born as helpless infants and with many years to reach adulthood, we require an altruistic social setting in which to mature. We are the only primates with white sclera around the iris of our eyes. In this way, we can see who is eyeing whom—clear evidence for our deep sociality. Fossil remains confirm that early humans had strong social bonds; our ancient ancestors cared for their afflicted brethren, both the toothless old and the compromised young.32 Unlike other primates, human mothers share the care of their infants! This explains why hunter-gatherer mothers averaged a birth every three years, in contrast to chimps and gorillas, where the birth interval is longer. Sarah Hrdy claims that a more socially interactive childhood made it imperative for children to understand the mental states and intentions of those around them.33 In the words of David Barash, “The more conscious our ancestors were…the more able they were to modify—to their own benefit—other's impression of them. Those who possess an accurate theory of mind can model the intentions of others, and profit thereby.”34 Complex social maneuvering, coupled with cooperative child-rearing and shared intentionality, were critical elements in building successful human groups and an expanding consciousness.35
Tool use must have been another factor in driving the expansion of the hominid brain, especially the crafting of better hunting, cutting, and digging implements. Slings of woven fiber allowed mothers to carry their babies long distances. Surprisingly, early stone tools remained much the same over more than a million years, even as our brains were enlarging. Perhaps the gradual development of our most distinctive talent—language—was the driving force in the evolution of our deepening intelligence. Considering that tongue, mouth, and vocal tract had to work in exquisite synchrony with our mind, human speech undoubtedly developed slowly over thousands of generations.
A much less convincing argument for the cause of our escalating intelligence is that of climatic cycles during the ice ages.36 If responding to climate change made humans smarter, why didn't other social hunters, like lions and wolves, also get smarter? More telling, how might climatic oscillations be responsible when two hominid lineages, one living in the cold of the north and the other in tropics and subtropics, expanded their brain volumes independently? None of the selective forces we just mentioned seem strong enough to counter the costs in energy, birthing, or early child-rearing that a large brain requires. Though rarely discussed in the literature of anthropology, there was another powerful “selective force” in the costly escalation of human intelligence!
TRIBALISM: HOW TERRITORIAL CONFLICT MADE US REALLY SMART
For good and for ill, Homo sapiens is inescapably a tribal animal.
—David Berreby37
Though receiving little press, a singularly powerful factor drove the expansion of human intelligence. Excepting only pathogens and parasites, competition with other humans was the most dangerous aspect of our environment.38 Not individual humans, but other packs of humans! As hunter-gatherers requiring a high-quality diet, we survived only in small groups. Local biomes, whether in African savannas or northern forests, can support only a few dozen mature humans and their young. Such environmental constraints kept our ancestors living in small bands over a very long time. Surviving on unreliable resources through each year and across the millennia was an incessant challenge.
Small independent human groups required safe shelter, drinkable water, and habitats providing sufficient nutrition. During a long drought, or simply with the expansion of our numbers, competition for limited resources became inevitable. For a local band of hominids, continued access to good habitat was the only way to avoid starvation. Competition between local tribes would determine who had access to good territory and who would be driven elsewhere—or expire.
Discussing the importance of conflict in human affairs has not been popular in the literature of anthropology. Introductory texts have surprisingly little to say about conflict or warfare; it's not part of the liberal imagination.39 In strong contrast, a few fossil localities bear witness to a harrowing past. Fossil human bones mixed with those of tasty herbivores—all with similar butchering marks—make clear that, on occasion, our ancestors ate each other!40 Forget your modern sensibilities; survival was paramount. Moreover, inter-clan conflict is not unique to humans. It has been witnessed among chimpanzees, and is common in those other super-successful animals operating in small colonies: the ants. As their most incisive student, E. O. Wilson, pointed out: “The greatest enemies of ants are other ants.”41 And so it was for us.
In the context of selective forces driving the evolution of an ever-more powerful brain, the most significant factor was that humans were being stalked by an increasingly clever predator, operating in small packs. As Richard Alexander argued, “the only way to account for the striking departure of humans from their predecessors…is to assume that humans uniquely became their own hostile forces of nature.”42 Under such circumstances, one can see why language might play an especially critical role. Clear oral signals, alerting your comrades to approaching attackers, could spell the difference between life and death. A long history of inter-group conflict is consistent with an unusual aspect of both chimpanzee and human societies: we are patrilocal. Unlike gorillas and most other primates, human males usually remain with the natal group, where they become part of a defensive coalition. (Brides get exchanged; warriors do not.) Also, our clever brains work best when we operate in small groups of shared purpose. Whether hunting, gathering, or plotting, small groups of humans display an effective “collective intelligence.” Indeed, defensive male coalitions with effective leadership gave rise to patriarchal societies around the world. Just as significant, inter-tribal conflict resulted in our becoming more altruistic!
A tribe including many members who, from possessing in a high degree the spirit of patriotism, fidelity, obedience, courage and sympathy, were always ready to aid one another, and to sacrifice themselves for the common good would be victorious over other tribes: and this would be natural selection.
—Charles Darwin43
Altruism would have facilitated the coordination of raiding and ambushing on a scale known in few other animals, while parochialism fuelled the antipathy towards outsiders. Additionally, with the development of projectile weapons, humans became adept at killing from a distance, which would have reduced the costs of aggression.
—Samuel Bowles44
Keen devotion to your comrades coupled with intense animosity toward your enemies is the only way of winning and securing territory. Getting yourself killed defending your clan may be necessary to the survival of your clan. (We draftees got this message in the US Army's Basic Training!) Behavior benefiting the group results in “positive group selection,” despite the costs to individual fitness. It sounds ironic but it makes sense: greater conflict with outside groups promoted greater sacrifice within the group.45 Females standing on the sidelines became part of the victor's spoils, further adding to group fitness. Surely quick and precise oral communication, shouted to your comrades in the midst of conflict, would provide a powerful selective advantage. More significantly, inter-clan conflict can account for the independent expansion of brain size in both Neanderthals and modern humans.46 Though the ice age environments of H. sapiens and H. neanderthalensis were very different, by operating in small bands their behavior had to be the same.
Contrary to current fashion, I see our species as being hard-wired for tribalism, in much the same way as little humans are able to acquire language. We also carry “innate instructions” for love, friendship, and self-sacrifice within our community. There is absolutely no contradiction here; these are complimentary aspects of our innate tribalism! Over many millions of years, human survival required access to territory providing adequate food and water. Such territory was often limited, resulting in intense competition, and driving some groups into ever more challenging environments. Not only did inter-tribal conflict disperse us across continents, it made us the smartest and meanest critters on the planet!
Armed with a truly sophisticated computational device between our ears, and a highly versatile body, humans began elaborating their cultural skills. Language had given us rapid and sophisticated communication. Tool use became a major survival skill and was easily taught to others. The regular use of fire is documented as early as a million years ago in southern Africa.47 By firing the landscape, early humans expanded grasslands, together with their protein-packed grazers. Sleeping around a campfire made the long African night less dangerous. Cooking over a fire was another significant advance, making food easier to chew and to digest.48 Perhaps that's why both our teeth and jaw muscles became smaller over time, allowing our brain to expand more easily. By setting fires and hunting big game, humans began their transformation of the biosphere.49
Survival requires that cooperative systems be efficient. This meant that human males and females would come to play distinctive but complementary roles.50 Birthing helpless babies and nurturing children over many years allowed human females to gather foods and fiber as well. Males hunted precious protein on long forays, garnering a resource essential for nourishing brainy little humans. Two very different sexes, both in form and behavior, gave our species a division of labor foundational to our success.51 However, individual parents and their children cannot survive alone; they require a larger band to secure territory, share food, and provide continuing care in the event of a parent's death. Because of our need for energy-rich sustenance, humans survived only in small bands, closely bound together by altruistic cooperation. Born completely helpless and requiring many years of social nurturing to become effective adults, we became a deeply moral species. Regrettably, this morality was largely confined to members of our own tribe. Arguing for a wider metamorality, philosopher Joshua Greene states, “Once again, morality evolved (biologically) to promote cooperation within groups for the sake of competition between groups.”52 Incessant tribal warfare was the crucible from which human dominion has emerged; it continues under the banners of nationalism, religious factionalism, and ethnic conflict—just watch the news!
A NEW EVOLUTIONARY DYNAMIC: CULTURAL ADVANCE
A singular advance within our planet's long fossil record, humans tripled their brain volume over the last three million years. This splendid progression paved the way for new advances in terrestrial complexity. But it wasn't just a big brain that helped make our species the master generalist we have become. We are extraordinary in our physical abilities as well. We can walk, run, jump, and swim. Free and flexible arms allow us to carry things for long distances, beat with sticks, and throw rocks. No other animal on Earth can throw things the way we do, whether spears to take down game in times long past or a baseball at over 90 mph. Strong versatile hands with four slender fingers and an opposable thumb allow us to weave, whittle, sew, and fashion tools. Best of all, hands and flexible forearms allowed us to gesture to our comrades. This activity, coupled with a greater variety of vocal utterances, gave us speech! Verbal communication allowed us to warn each other over short distances, share thoughts within social networks, and formulate new tactics.
Operating in small bands, humans quickly invented new contrivances that would determine who prevailed and who fell by the wayside. Arriving in Eastern Europe around 40,000 years ago, Homo sapiens came armed with more versatile technologies. Fishing hooks and sewing needles made of bone, together with a greater variety of camping styles, were new to the European landscape. Homo neanderthalensis and their predecessors had been living successfully in this same region through many glacial cycles, but there is no evidence of even a single fish hook or sewing needle having been fashioned in Neanderthal times. Within 5,000 years of the arrival of “sapiens,” the Neanderthals of Europe had vanished. A few fossils suggest limited gene exchange between the two species.53 Recent DNA analyses confirm a bit of gene interchange between the muscular Neanderthals and their new neighbors.54 Pale skin and reddish hair were likely Neanderthal adaptations, important to survival in seasonally frigid regions. (Paler skin is essential for producing vitamin D in one's skin when you're covered in animal skins under cold and cloudy skies. Dark skin protects against excessive UV irradiation and skin cancer in the sunny tropics.)
Because of their heavier skeleton and more erectus-like facial features, Neanderthal had been considered less intelligent and unable to speak by earlier scholars, despite having brains a bit larger than ours. But what were Neanderthals doing with such big brains if they weren't talking? Also, 300,000-year-old wooden spears found at the bottom of a lake in Central Germany, together with the butchered bones of wild horses, make clear that Neanderthals were effective hunters. Most likely the horses were killed and butchered on an ice-covered lake in winter, with spears and bones sinking to the bottom in springtime to become preserved under water. More significantly, these ancient Neanderthal spears lacked stone spear points; they created stone spear points later. It is not until around 40,000 years ago that sapiens-type stone spear points are first found in Northern Italy, linked with teeth having sapiens-type DNA. With advanced cultural skills, Homo sapiens grew to larger numbers, numbers that would replace the Neanderthals. Asian populations were also confronted by the slender “out of Africa” immigrants, and they too were transformed. Nevertheless, long distances have delayed genetic mixing; even today some New Guinea and Australian natives carry snippets of ancient DNA not found in other living humans.
Landscapes with limited resources, in which groups of humans came into competition, gave rise to progressive cultural dynamics. Within-group cooperation set the stage for between-group competition, throughout our range. Recent evidence from southern Africa, dated at 70,000 years ago, suggests that the modern mind was fully functional at that time. Here, spear points had been fashioned of three very different materials. A glue-like plant resin hardened with ochre and heated secured the stone point to the haft of the spear.55 This skill required a shared tradition, dedicated apprenticeship, and careful execution. Such sophisticated weaponry is consistent with yet another fact: the human brain has not increased in size over the last 50,000 years.
Sophisticated technology, highly developed science and elaborate social or religious rituals are products of a cumulative process of cultural evolution, whereby each generation builds on the achievements of their predecessors in a gradual, approximately monotonic, ratcheting up of complexity and functionality.
—K. Smith et al.56
No longer confined to random mutations or genetic reorganization, cultural cooperation produced new behaviors, new technologies, and new possibilities. Most important, cultural inventions could be quickly adopted by succeeding generations or co-opted by competing societies. Here was a real-world example of “Lamarckian Evolution.” The French evolutionist, Jean-Baptiste Lamarck, postulated that parents who became adept at a particular skill would beget children better able to carry on that same skill, but this doesn't occur in higher animals.57 In contrast, cultural evolution does allow newly acquired skills to be passed on to succeeding generations.58 Tool making was probably our earliest skill, quickly followed by food processing. Language—our premier talent—allowed us to share both timely and abstract concepts with our kin. Imitation, another fundamental human trait, was the basis of the “conservation and dissemination of innovations in ways that allow technologies and practices to improve over time.”59
Though human numbers remained small, cultural evolution became dynamically progressive and ecologically impactful. Humans reached Australia over 50,000 years ago; by 40,000 years ago nearly all the large animals of that continent were gone! Spores of fungi specific to the dung of large herbivores decline rapidly about 45,000 years ago, followed by frequent fires and major changes in Australia's vegetation. This was not “climate change” but the impact of humans. Our species first entered the Americas from northern Asia around 15,000 years ago. Finely crafted stone spear points are first recorded in North America around 11,000 years ago, followed by the extermination of mammoths, mastodons, giant ground sloths, wild horses, and other nutritious treats. In 1910, A. R. Wallace suggested that the extinction of large mammals at the end of the ice ages were “actually due to man's agency.” A few obtuse scholars still claim that “climate change” accounted for these extinctions, oblivious to the fact that these very same animals had survived many earlier glacial cycles. Clearly, our species, operating in small bands, had become the most widely distributed and dangerous mammalian species on Earth.
Then, quite suddenly, and beginning around 10,000 years ago, at least five human societies in several distant areas of the world initiated an extraordinary advance. We developed agriculture and animal husbandry, a kind of symbiosis that would grandly expand both our numbers and our potentialities! With cultural innovations, increasing complexity on planet Earth had now become purposeful: humankind was beginning to alter the planet!