We Are Our Brains (53 page)

However, while in China I did witness the power of medicinal herbs to conserve bodies over a long period of time. My family and I had returned to Hefei, to the Medical University of Anhui, where I'm a visiting professor, when I heard for the first time of a region known as the Jiuhua Mountains. A monk by the name of Wuxia, who lived there at the time of the Ming Dynasty (1573–1619), succeeded, over a twenty-eight-year period, in copying out eighty-one parts of the Buddhist scriptures using blood from his tongue and gold dust. He's alleged to have died at the age of 126, and his body is said not to have decayed at all in the three years after his death. The other monks, who believed that he was the reincarnation of the living Buddha, gilded his body and preserved his mummy, known as
“Monk Longevity,” in the Longevity Palace. Apparently, five-hundred-year-old mummies were also preserved and venerated in other monasteries in the Jiuhua Mountains. I didn't understand how that was possible, because the climate in that region is extremely damp. My first Chinese PhD student, Zhou Jiang-Ning, who had meanwhile become a professor at Hefei, suggested that if I was doubtful, I should go and take a look. My wife and daughter decided to join me. The university lent us a car and driver, and we were accompanied by a Chinese doctor, Dr. Bao Ai-Min, who interpreted for us.

After a six-hour drive in the darkness we reached the mountains at such a late hour that the monasteries and the many temples were shut, so we spent the night in the little town of Jiuhua. The next morning we returned to the monasteries, where Buddhist monks were praying around a glass case. In it we could indeed see a mummy covered in gold paint and sitting in a prayer position. The living monks praying in front of it were instructed by the head monk to make way for us so that we could inspect the mummy. The structure of the body was perfectly intact; it could have been used for an anatomy demonstration. The individual muscles were clearly visible through the dry, thin skin. Every monastery in the Jiuhua Mountains had one or more of these “flesh bodies”—the rather blunt name given to the mummies. With the help of our Chinese interpreter, I asked the head monk how it was possible that the body of this particular monk had remained intact so long after his death. “Because he is holy,” came the enlightening answer. In a jocular mood, I called Zhou in Hefei to tell him that we had found the solution to the puzzle: “He is holy.” According to Zhou, monks who felt they were nearing the end of their lives stopped eating normal food. Instead they ate special herbs, sitting in a vat in which they were submerged up to the neck in a mixture of herbal solution, carbon, and lime. In that way they could sometimes dry and preserve their own bodies before they actually died. Those who did so were deemed holy. Meanwhile, my daughter had been invited to join the monks in prayer. They were extremely kind to her, explaining the mysteries of
Buddhist prayer. The striking combination of the small, shaven-headed Chinese monks and my tall daughter, with her long blond hair, joined in prayer made everyone cheerful. To what extent her participation in the prayer contributed to the mummies' further preservation is something that only time will tell. I'm afraid I haven't yet managed to get ahold of the recipe of the herbal solution.

Over the course of evolution, our brain size and intelligence have increased enormously. Intelligence entails problem-solving ability, speed of thought, capacity to act purposefully, rational thought, and the ability to deal effectively with one's surroundings. There are many different kinds of intelligence—linguistic, logical, mathematical,
spatial, musical, motor, and social—so IQ is rather a limited way of testing it. The link between brain size and intelligence has nothing to do with the absolute size of the brain. The human brain, weighing in at three pounds, is of course by no means the largest: That record belongs to the sperm whale, with its nearly twenty-pound brain, while the brains of elephants weigh ten and a half pounds on average. In fact, an elephant named Alice who lived in Luna Park, Coney Island, had a brain that weighed thirteen pounds. But whales and elephants are by no means as intelligent as humans. The
relative
size of the brain compared to the animal's body, however, does have a clear correlation with the quality of the brain as an information-processing machine, as Darwin established back in 1871 and the Dutch neuroscientist Michel Hofman calculated a century later.

A better measure of the level of evolutionary brain development is the encephalization quotient (EQ), a relative measure of an animal's brain weight on top of what is needed to regulate body functions. Humans indeed score by far the best using this measurement. EQ is largely determined by the development of the cerebral cortex. The increase in our brain size during evolution was caused by an increase in the number of building blocks (neurons) and their connections. So the number of neurons in the cerebral cortex is a good measure of intelligence. These are grouped in functional units called columns. Although the cerebral cortex grew enormously over the course of evolution, the cross sections of the columns remained almost identical, around half a millimeter. It was an increase in the number of columns that caused our brains to grow bigger and the cortex to become convoluted in the process. Despite all these changes, the blueprint for the brain remained the same, so the difference between the brains of humans and those of other primates is largely one of size. This evolutionary in crease in brain size greatly increased our information-processing ability and went hand in hand with longer pregnancy, a longer period of development and learning, longer life expectancy, and fewer offspring. During the course of human evolution, skull content has more than tripled and life span has doubled in a “mere” three million years.

Various hypotheses have been put forward to account for the evolutionary pressure that led to larger brains. An initial theory was that primates' brains provided an evolutionary advantage through the ability to use tools, which increased food supply. It was then suggested (the Machiavellian intelligence theory) that larger brains were a response to the demands of a socially complex existence, causing individuals to invest in social strategies that promoted long-term survival of the group. A clear correlation has indeed been found between the size of primates' cerebral cortex and the size and complexity of the social group. Primates started to live in social groups around 52 million years ago, when they abandoned their nocturnal existence and it became safer to band together. The complexity of life in groups is strongly determined by pair formation and monogamy, both of which place considerable demands on the brain. They require optimal selection of a fertile partner as well as complex negotiations between partners. The intricacy and intensity of such relationships—an issue familiar to us all—appears to have placed strong evolutionary pressure on the brain to grow. The mechanism of monogamous partner choice in humans is thought to have developed as far back as 3.5 million years ago. It has proved its evolutionary advantage in terms of protecting the family, but it continues to place an enormous burden on our brains.

Humans are characterized by an amazing brain that weighs three pounds and is made up of cells known as neurons. We each have around 100 billion of them—fifteen times the number of people on earth. Each brain cell makes contact with around ten thousand other brain cells through specialized connections called synapses. Our brains contain over sixty thousand miles of nerve fibers. Yet the fundamental characteristics of the neuron, like the ability to receive, conduct, process, and transmit impulses, aren't inherently specific to nervous tissue. These functions (along with rudimentary forms of memory and attention) are also found in many other types of tissue in all living creatures, even single-celled organisms. But, as Cornelius Ariëns Kappers (who back in 1930 became the first director of what is now the Netherlands Institute for Neuroscience) observed, the nervous system has become vastly better at these functions as a result of evolutionary specialization. Whereas impulse speed in tissue other than the nervous system rarely exceeds 0.1 cm per second, the simplest neuron can transmit impulses at 0.1 to 0.5 meters per second. In fact, as Kappers calculated, our neurons can even reach conductivity speeds of 100 meters per second. And that's only one of the specialized characteristics of the neuron that provided a huge evolutionary advantage.

Sponges, the most primitive creatures, have only a few types of cells and lack both specialized organs and a true nervous system. But they do possess the precursors to neurons, and their DNA does have almost all of the genes it needs to build the proteins that are located in the postsynaptic membrane, the site of the receptor molecules between neurons. This shows how only a few small evolutionary adaptations are needed to create an entirely new system for the transfer of chemical messengers.

Primitive neurons developed as far back as the Precambrian era, between 650 and 543 million years ago. By then, coelenterates (aquatic organisms) already possessed a diffuse neural network with true neurons and synapses. We can trace the gradual molecular evolution
of the chemical messengers used by these neurons to those found in our brains today. One of the most studied organisms in this context is the tiny polyp
Hydra
, which possesses only a hundred thousand cells. Its neural network is concentrated in its head and foot: a first evolutionary step toward developing a brain and spinal cord.
Hydra
's nervous system contains a chemical messenger—a minuscule protein—that resembles two of our own: vasopressin and oxytocin. A protein of this kind is called a neuropeptide. In vertebrates, the gene for this particular neuropeptide first doubled and then mutated in two places, creating the two closely related but specialized neuropeptides vasopressin and oxytocin, which have recently become the focus of interest, partly because of their important role as messengers in our social brains (see
chapter 9
). Depending on their place of production, release, and reception, these two messengers can also be involved in kidney function (
chapter 5
), childbirth and milk secretion (
chapter 1
), day and night rhythms (later in this chapter), stress (
chapter 5
), love (
chapter 4
), erection (
chapter 4
), trust, pain, and obesity (
chapter 5
). By 2001, the Hydra Peptide Project had already isolated and chemically identified 823 peptides. These included neuropeptides that were subsequently found for the first time in vertebrates, like
Hydra
's “head-activating peptide,” which is also present in humans in the hypothalamus, the placenta, and brain tumors.

The chemical relationship between species is extremely close. An evolutionary basis for a rudimentary brain can be found in flat-worms, in the form of a clump of neurons known as the head ganglion. The small, gradual structural and molecular changes that take place during the evolution of the brain show that the unique place often claimed for man in the animal kingdom needs to be put in perspective. As Darwin said in
The Descent of Man and Selection in Relation to Sex
(1871): “No one, I presume, doubts that the large proportion which the size of man's brain bears to his body, compared to the same proportion in the gorilla or orang[utan], is closely connected with his mental powers.” And he hit the nail on the head
there: The size of our brain is an extremely important factor in determining intelligence, but it isn't the only one. Tiny molecular differences have also had a huge impact.