One of the most remarkable aspects of life is its awareness of its surroundings. Unlike nonliving matter, life can monitor impressions from, and respond to, the outside world. Through life’s various senses—hearing, seeing, smelling, touching, tasting—all organisms acquire and file vast amounts of information. The extent to which beings are successful in doing so depends largely on their complexity. Organisms manifest this complexity best by means of one exquisite piece of matter—the brain. The brain is the central clearinghouse of all animated acts.

As these words are first written and then read, matter within our skulls is full of electrical impulses. Silently and efficiently, millions of nerve cells pass messages back and forth within our brains. These microscopic neurons guide our eyes along this printed line, quickly scanning the shapes of the letters. By matching them against memory, we recognized clusters of letters as words and often know their meaning.

Different sensory organs transmit signals that stimulate the brain, which then reacts by sending, in turn, instructions to the muscles. Nerve cells constantly interchange these signals in our heads, ordering our hearts to beat, our lungs to pump, and our hands to get ready to interact with this Web site. The body’s nervous system, of which the brain is the paramount part, controls all mental and physical activity. In fact, every thought, feeling, or action begins in the brain. All human behavior is controlled by it.

Most amazing of all, these silent, unfelt activities inside our heads make us aware that we are now thinking about them: The human brain can contemplate and explore the human brain. That alone makes our brains the most complex clumps of matter found anywhere thus far. The brain is Nature’s most tantalizing, talented, and versatile creation—the ultimate example of the extent to which matter has evolved in the known Universe.

Brain Structure and Function Brains are made of cells just like any other part of the human body. Each adult human has ~10⁸ neurons/cm³, or a total of ~100 billion neurons in a typical cranium—roughly the same number as stars in our Galaxy. Though neurons come in more than a thousand different sizes and shapes, their bulk properties are nonetheless similar. In addition to the main cell body that contains the biological nucleus and manufactures protein in the usual way, neurons also have numerous long and wiry extensions resembling roots of trees (Figure 6.31). The main, thick extrusion on one side of a neuron is the axon, which acts as a transmitter of information, carrying signals away from the cell body. The network of thin extrusions on the other side of a neuron are collectively the dendrites, which act as microscopic antennae, picking up signals sent by other neurons and carrying those signals to the cell body.

FIGURE 6.31 — Taken through a microscope, this photograph (right) shows a neuron ~0.01 cm from tip to tip. The axon transmitter (lower left) and the dendrite receivers (upper right) are clearly evident. A cleaner, "textbook" neuron is sketched at left. (D. Hubel)

Axons and dendrites enable neurons to “talk” to one another in order to monitor and control the many diverse functions of intelligent beings—and not just humans, rather any creature having a brain. On average, in the human cranium, each neuron communicates directly with ~1000 others. Together, neurons form an intricate network of hundreds of trillions of interconnections, each performing a function either assigned by heredity or learned by experience. That network of neurons in a single human brain, when examined through a microscope, strikingly resembles the wisps and filaments, when seen through a telescope, of the largest-scale structures in the Universe.

How does this communication system work, and how quickly? Much like a series of electrical circuits, each neuron passes along a signal from one place to another. When a neuron is stimulated by some external effect—a touch, sight, sound, smell, or taste—the charges on some of the atoms and ions in that neuron changes. This rearrangement of charges can quickly alter the voltage of a neuron, thereby launching an electrical impulse. Neurons, in effect, act like chemical batteries, discharging rapidly in a burst of electricity. They can then recharge themselves in a fraction of a second. All this electrical activity requires energy—energy that is derived by absorbing oxygen during respiration.

Modern research has proved that electrical signals travel swiftly through neurons (in mammals) with a velocity ~0.1 km/s, or ~225 miles/hour. That’s reasonably fast compared to everyday speeds, but much slower than an electric current traveling along a metal wire and slower still than the speed of light. A good analogy might imagine information traveling across a neuron, and from one neuron to another, like a rapidly lighted fuse.

Why so fast? Speed is essential to get information to the brain and then back to the appropriate muscles in order to respond to incoming signals. Since the speed of information depends mostly on the diameter of the neuron, some life forms requiring extremely quick responses in order to escape their predators, and hence survive, have developed surprisingly thick neurons. Sea squids, for example, have neurons with diameters ~100 times those in humans, enabling them to coordinate movement away from a site of danger or toward a source of food with a system resembling jet propulsion.

Not all neurons in our skulls are physically, or “hard,” wired together; in fact, none of them are. Instead, a small gap, called a synapse, separates an axon of one neuron from a dendrite of another. Magnified many times in Figure 6.32, such synaptic gaps measure no more than ~0.1 micron, or ~500 times thinner than the width of a human hair.

FIGURE 6.32 — The axon of one neuron is not physically connected to the dendrites of another. Instead the neurons are separated by microscopic synaptic gaps across which chemicals are secreted. Frame (c) is a substantial magnification of the neuron interaction depicted in frame (a).

For two neurons to communicate, information must jump the synaptic gap between the axon transmitters and the dendrite receivers. However, this information is not passed along by emitting electrical impulses across a synapse. Instead, an electrical impulse traveling the length of an axon induces the axon to excrete chemicals, known as neurotransmitters. These chemicals then spread across the synapse and cause a new nerve impulse to begin in the next neuron.

About a dozen neurotransmitting chemicals have been identified by modern medicine. Each can, under certain circumstances, inhibit or enhance the voltage on a nearby dendrite. Consequently, this type of unconnected wiring scheme engenders enormous complexity—much, much more than would be possible if the neurons were physically wired to one another. Each neuron can have as many as 200,000 synapses, and each of these might or might not trigger an electrical impulse in any given circumstance. And since nearly 10¹⁵ synapses inhabit a typical human brain, the number of possible routes any electrical impulse can take is mind boggling—no pun intended!

To sharpen the explanation yet make this topic even more snarled, experiments have demonstrated that these electrical impulses actually travel along a thin covering outside each neuron. Made of a fatty white substance called the myelin sheath, this covering apparently serves as insulation, much like the rubber wrapping around ordinary electrical wires, preventing the network of neurons from short-circuiting. Unfortunately, the myelin sheath erodes in some people; multiple sclerosis attacks myelin, causing some circuits to misfire, which, in turn, produce jerky movements due to uncoordinated timing. It is the complete lack of myelin in newborn infants that prevents their neurons from working in coordinated ways. The result is a child’s gradual ability to crawl and then walk, while the myelin grows during the first year or so after birth.

A final note of caution: The excretion of neurotransmitting chemicals largely dictates human behavior. These natural chemicals in our brains can be affected by what we breathe and eat. Poisons and drugs in particular—strychnine, tranquillizers, LSD, amphetamines, marijuana, and many others—change the brain’s firing mechanisms, thus change human behavior. Moderate amounts of alcohol do so as well, altering the way brains normally function. Even caffeine, in coffee-cup doses, lowers our synaptic thresholds so that a tired nervous system, although mostly depleted of transmitters, can keep us alert just a little longer. The chemistry of synapses might underlie many societal ills, although today’s neurobiologists are really only beginning to explore how chemicals specify a person’s response to circumstantial change—whether genetic or environmental.

Toward Intelligence With that briefest of introductions to brain structure and function, we return to explore the ways that simpler, ancestral life forms might have evolved the fantastic complexity now resident inside our human heads. Speculation about the paths along which intelligence originated and developed relies mainly on the fossil record—that remarkable evidence written in the stones.

The one-celled amoeba is the most primitive eukaryotic form of life known in the contemporary world—perhaps the most primitive of all living things, save the virus, which sometimes acts alive, as noted in the previous CHEMICAL EPOCH. Roughly halfway in size between an atom and a human, the amoeba has poor awareness and coordination. It generally responds only at the point stimulated, communicating the information sluggishly through the rest of its body. Although amoebas have developed a crude nervous system, living things that aspire to be more agile—and smarter—surely need quicker internal reactions.

Other single-celled creatures have managed to develop primitive intercom systems. For instance as shown in Figure 6.33, the microscopic paramecium has an array of oar-like hairs enabling it to move rapidly through water. The “oars” must act in a coordinated manner, for if they functioned independently, the paramecium would make little progress. The hairs are regulated by minute nerves that respond to chemicals emitted within the cell. In this way, messages can be transmitted swiftly and precisely from one part of the cell to another.

FIGURE 6.33 — The paramecium has a primitive intercom system that helps coordinate its movements. Such cells range from 50 to 350 microns in length. (Harvard Medical School)

Paramecia clearly have more “intelligence” than that of amoebas. An amoeba searches for food essentially by drifting into water-plant algae. Finding none, it often repeatedly gropes toward the same alga, even though the alga offers no satisfactory food. The amoeba has no memory. A paramecium, on the other hand, has better coordination and a memory of sorts. Having found no food near one alga, it will back off and seek resources in another direction. Paramecia momentarily retain traces of experience.

Compared to the amoeba, then, the paramecium is a genius. But it’s a genius operating in a watery world less than a few millimeters across. Even paramecia are unaware of anything beyond this range. No unicellular creature can be much smarter, for it can advance no further.

Despite having greater complexity than that of probably any inanimate object, a single cell can boast only the simplest intelligence. To become smarter—that is, to evolve an intricate nervous system—a single cell would need elaborate sense organs to inform it, as well as developed muscles to implement its instructions. Why can’t there exist, then, larger cells incorporating these added features, perhaps equipped with miniature hands, eyes, and brain? The answer is that single cells cannot become much larger than 0.01-cm creatures. Should they try to do so, their surface areas would increase with the square of their size (1, 4, 9, 16, …), whereas their masses, which must be fed through the cells’ membranes, would increase as the cube of their size (1, 8, 27, 64, …). So individual cells cannot become too large, lest they starve. The basic smarts of unicellular life forms are therefore limited; their physical size prevents them from developing the many and more complex organs needed for higher intelligence. Mutations have undoubtedly helped them try every conceivable means to do so during the past 3 billion years, but they have failed.

Multicells The road to greater intelligence required many cells, or multicells. But quality counts here too, for quantity is not the only issue. A haphazard accumulation of many independent cells will not do; clusters of hundreds of self-sufficient cells are hardly more intelligent than one cell. Consider a sponge, for instance, much like those harvested for use in our bathtubs. Though a sponge is multicellular, most of its millions of cells act independently. A sponge has no central nervous system, thus its “intelligence” is not much more than that of an amoeba. For some reason, sponges failed to profit by their multicellularity. As a result, they have produced no higher forms of life. Sponges are examples of life forms that long ago reached an evolutionary dead end.

What was needed was a favorable mutation allowing an accumulation of many cells to work together as a community. Interactive, multicellular organisms do have some clear advantages, not least of which is that they avoid the surface-volume problem just noted. More importantly, groups of cells within a multicellular organism can sprout particular functions. This division of labor was one of Nature’s greatest inventions. One group of cells might be highly sensitive to foods; others, more efficient in carrying oxygen; still others, tough muscular entities or protective skin casings. The net result was that each group of cells within a multicellular organism became more skilled in one capacity and less so in the rest. Specialization emerged. Accordingly, the total intelligence of such an organism greatly increased as cells, working as a team, became better able to protect themselves from predators and to obtain the food needed for survival. These were the first steps toward a symbiotic society.

The hydra is a good example of a primitive multicellular system that did evolve some intelligence. Sketched in Figure 6.34 and no larger than a toothpick, the modern hydra resembles a stalk of celery, being closed at the lower end and raveled into writhing appendages at the upper end. In contrast to any sponge, the hydra can move its entire body in coordinated fashion to, again and among other things, avoid danger and seek food. In short, the cells within a hydra can communicate. And communication is the essence of organized intelligence.

FIGURE 6.34 — Hydra organisms possess intercommunicating multicells, thus possess considerable coordination. One is shown here to scale for comparison with a unicellular amoeba and a (non-interacting) multicellular sponge. Absolute size is not always the most important feature on the road to intelligence; quality of cells often counts more, indeed hydra are much "smarter" than sponges. (Lola Chaisson)

Cells able to communicate—nerve cells—probably formed originally near the surface of multicellular life forms such as hydra, or, more realistically, hydra-like progenitors. Being exposed, these cells had the greatest opportunities to sample their environment. But being near the surface also made them more vulnerable. So, mutations and natural selection likely favored those hydra-like ancestors having deeply rooted nerve cells. Over the course of generations, these cells gradually retreated inside the organisms, yet kept their link to the environment by sprouting expendable tentacles that reached back to the surface of the organisms, and often beyond. These miniature octopus-like tentacles became the dendrites of modern neurons, the specialized cells that communicate information in more intelligent beings. Drawn in Figure 6.35 and acting as remote sensing devices, neurons of modern hydra and modern humans are basically the same, though their number and arrangement differ considerably.

FIGURE 6.35 — Mutations and natural selection gradually caused nerve cells to retreat below the surface of early organisms. In turn, the internal clustering of such nerve cells led to primitive central nervous systems, albeit often with external tentacle-like sensors. Such animate systems became much more complex than any life forms that preceded it. (Lola Chaisson)

As evolution advanced, the bulk of the neurons withdrew ever deeper within multicellular organisms. Eventually, the buried neurons merged, forming clumps of interacting nerve cells—the first and most important step in the building of a central nervous system. This clustering of neurons was one of the greatest of all evolutionary breakthroughs. Once that barrier was crossed, ~1 billion years ago on Earth, our hydra-like ancestors, as well as other sophisticated organisms resembling them, were on their way toward generating all of Earth’s brainy animal life forms, including humans.

Invertebrate Brains What does the fossil record say about the evolution of the brain? Figure 6.36 summarizes some of the main results to date, suggesting the ripening of the early central nervous system, with organisms branching out in many directions while trending toward greater complexity. Most of these branches, or evolutionary paths, however, represent organisms that either became extinct long ago or survived only as dead ends. The extinct ones are obvious, for their presence simply terminates in the fossil record. The dead-ended ones are just as clear, yet far more interesting. Apparently, at some point in their evolution, insuperable biological obstacles meant that some early creatures, such as the amoeba, paramecium, sponge, hydra, as well as worms of all sorts, made no further advance, yet survived. These are the invertebrates, or backbone-less organisms, many of them skilled and crafty in their own domain. Spiders, for instance, are marvelously accomplished performers within their particular environment; their nervous systems are clever and effective in their limited worlds, their sense organs even more varied and subtle. Bees, wasps, ants, and moths also have highly refined bodies for dealing with their specialized needs. Some—especially bees and ants—even have impressive social organizations that rely on symbolic communication.

FIGURE 6.36 — Schematic diagram of some early pathways that led toward greater neural complexity. (H. Jeritson)

Virtually all these invertebrate animals have reached evolutionary dead ends. They are trapped in endless cycles of perfected daily routines. Fossilized spiders of 100 million years ago show little variation from their modern descendants. Bees in the bush, spiders in the shed are, in a sense, living fossils.

Invertebrates are successes and failures at the same time. On the one hand, they are fabulously talented within their own restricted environments, such as the deerfly that outpaces the fastest animal, the flea that jumps a hundred times it own height, and the octopus whose eye is exceptional among the invertebrates. Successes certainly, for the invertebrates dominated Earth for nearly 0.5-billion years. But failures too, because they neglected to develop the vertebral column of bones so conspicuous in fish as well as humans—bones that form the spinal column and protective skull of more complex species.

As humans, we take brains for granted. But the vast majority of animals are invertebrates and have no true brain, no centralized nervous system. Most invertebrates’ neurons are diffusely spread in a network of fibers throughout their bodies, reminiscent of the distinction made earlier between simple colonies of unicells and more complex multicells. As such, they cannot be creative, adventurous, or visionary—at least not as we’ve come to know these qualities.

Vertebrate Brains Humans and our fellow vertebrates (backboned fish, reptiles, and mammal relatives) are anomalies to the great invertebrate failure. Vertebrates that did evolve skeletonized parts are but a minor offshoot from the vast, teeming world of the invertebrates. It seems that brains are the exception, not the rule in Nature. Not only did it take >2 billion years for some unicells to become multicellular, but also once those that did only very few evolved true brains.

The vertebrates’ foremost property, aside from their telltale backbone, is their central nervous system—as summarized in Figure 6.37. Even so, and like the invertebrates, many vertebrates were apparently unable to utilize their sensory and motor organs to full capacity. A vast array of fish, amphibians, and reptiles, including modern versions of many birds, lizards, snakes, crocodiles, turtles, and many other vertebrates, dead-ended long ago. A good number became extinct and even the survivors seem to have been unable to decide on a division of authority between the “sight” and “smell” neurons.

FIGURE 6.37 — Schematic diagram of the neural development among vertebrate animals. (H. Jeritson)

Skulls of primitive fish have been reconstructed in some detail from the fossil record (Figure 6.38). These fish lived ~400 million years ago and are among the simplest known true vertebrates. Though crude, their brains nonetheless contained all the essentials present in modern fish as well as humans. The grouping of small organs caused a bulge toward the snout and was the precursor of the much larger cerebral hemispheres in humans. Likewise, their eyes caused another bulge farther back, the forerunner of our occipital lobe on which we “see” images projected at the rear of our brain. Lateral-line organs also branched out to the side, antecedent to our cerebellum where our body movements are processed and coordinated. These ancient sense organs, though not in themselves rivaling those of some modern invertebrates, were employed more effectively because of their connection to a unified central nervous system.

FIGURE 6.38 — Cutaway diagram of the brain of a primitive fish, the simplest known form of a true vertebrate. Such brains are reconstructed from fossils, and this one here highlights the brain matter devoted to the smell and sight senses.

The development of specialized sense organs, and especially their integration with a centralized brain, aided the intellectual dominance of the vertebrates over the invertebrates. Complexity—as often exemplified by the eye, which originated as a photosynthetic organ whose initial purpose was to use light as a source of energy, but which eventually evolved into photoreceptors to use light as a source of information—rose ever more.

Sight certainly did play a major role in the advancement of these early vertebrates, as the fossil record documents the ripening over time of a relatively large visual brain (Figure 6.39). Mutations doubtlessly gave an advantage to certain species of fish, enabling them to utilize improved eyesight to move, survive, and reproduce better in the water. The sense of sight didn’t rule unchallenged, however. The sense of smell remained a keen rival in the ever-refining evolution of Earth’s life ~300 million years ago.

FIGURE 6.39 — Sight played a major role for more advanced fish, thus enlarging the amount of brain matter devoted to that sense. Even so, the tubes providing a sense of smell to the brain were still present in these fish ~300 million years ago.

Competition between sight and smell continued with time’s passage. When the amphibians transferred from the sea to the land, the flood of 2-dimensional sight data probably overwhelmed even the crafty brains of these gifted vertebrates. Smell input, on the other hand, more 1-dimensional by comparison, was still within the grasp of such a brain. Accordingly, the first amphibians likely found smell to be of more practical use than sight. The fossil record does show how the occipital lobe shrank while the cerebral hemispheres expanded over many generations. Gradually, the sense of sight regained greater usefulness as the brain of the mammals grew larger through continued mutations and natural selection. The multitude of out-of-water images no longer saturated once oceanic eyes, in fact it was the eyes that caused the brain to ramp up in size, speed, and sophistication in order to process the incoming information. The larger brains of the mammals were then able to cope with the full world of sight as well as sound. Those creatures having more intricate, complex brains were better suited to survive in a changing terrestrial environment.

The fossils also depict this decline of the importance of smell, as depicted in Figure 6.40. Although the sense of smell was of greatest value to the lower vertebrates, as the brain increased, other senses, such as seeing and hearing, became equally advantageous and eventually more so. The eye, in particular, seems to have played an essential role in the maturation of intelligence. Our much larger human cerebral hemispheres are indeed derived from the ancient smell brain, but the preeminence of this sense was long ago surpassed by sight, sound, and other general sensations.

FIGURE 6.40 — The sense of smell (diagonally shaded brain matter at lower left of each frame) declined in importance as life evolved toward more advanced forms.

Mammal Brians The most recent step in the evolution of the brain occurred in the mammals (Figure 6.41). Once again, the search for energy efficiency was central, as the most successful mammals developed multi-component hearts that permit more complete oxygenation of the blood, warm-bloodedness that allow sustained activity in cold environments, and external fur that effectively conserves energy. Basically, every living tissue needs a minimum amount of energy to function and the brain is the most energy-demanding tissue of all—which probably explains why true intelligence is found only in warm-blooded animals, since brains are metabolically intensive organs and do have high energy needs (per unit mass).

Also sketched earlier in Figure 6.36 and Figure 6.37, many evolutionary dead ends are evident among the mammals, the result of mutations that simply didn’t offer much advantage to some species. Even so and contemporaneously, other mutations altered for the better the traits of selected organisms, granting them clear advantage in the overall struggle for survival. Parts of this brain development can be watched as the head of a human fetus rapidly retraces much of evolution among the vertebrates: Two weeks after conception, the embryo’s (reptilian) brain resembles that of a frog with the olfactory (smell) bulb protruding ahead of a very small brain. Several weeks later, the olfactory has shrunk, while the occipital (sight) lobe has swollen. By many weeks beyond that, several additional neural layers have grown, indicative of evolution having superposed more layers of neurons needed for advanced mammalian functions.

More recent neural advances were driven, over millions of years and generation upon generation, by favorable mutations, which advantaged those mammals with longer arms and gripping paws for leaping, swinging, and reaching. Figure 6.42 captures many of these vitally important advances in a single illustration. Other mutations also gradually shifted the eyes of the early prosimians from the side to the front of the head, thereby producing the vitally key binocular, 3-dimensional sight, as noted earlier in Figure 6.28. In turn, the gradual refinement of dexterous arms and manipulative hands combined with more accurate eyesight to give those protomonkey ancestors distinct advantages along the evolutionary path toward even greater complexity.

FIGURE 6.42 — Favorable mutations, over long periods of time (flowing left to right in each row of frames), gradually helped move the eyes from the side of the head to the front (middle row), as well as change paws into hands and lengthen the arms (bottom row). The fossil record also documents that the brain got bigger (top row) from prosimians to chimps to humans. (Lola Chaisson)

The eye-hand-brain combination had a powerful evolutionary effect, not only for enhancing survival in the near term, but also for developing intelligence in the long term—one upshot being the seemingly limitless manual skills and boundless curiosity of Homo sapiens, a decidedly intriguing creature to be scrutinized in the next CULTURAL EPOCH. To be sure, it is the rapid interaction of the eye, hand, and brain that enables us to browse a Web site such as this one so actively and effectively.