What a Wonderful World (7 page)

One of the most profound questions in science is: why is the Universe constructed in such a way that it acquires the ability to become curious about itself? The question presupposes the existence of an objective Universe
out there
. Yet everything we know about reality, including our model of the Universe, is a construct of the human brain. ‘The brain’, as poet Emily Dickinson wrote, ‘is wider than the sky.’ Before we can truly address any of the really deep questions about the Universe, we first need to understand the filter through which we perceive that Universe.

Captain James T. Kirk of the starship
Enterprise
called space ‘the final frontier’. But he was mistaken. It is not space that is the final frontier. It is the human brain: the ultimate piece of ‘matter with curiosity’.

Our brain – ‘the apparatus with which we think that we think’
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– processes information from our senses, using it to update its internal model of the world. It then decides, on the basis of that information, what action to take. The brain is responsible for art and science and language and laughter and moral judgements and rational thought, not to mention personality, memories, movements and how we sense the world. ‘It is in the brain that the poppy is red, that the apple is odorous, that the skylark sings,’ wrote Oscar Wilde.
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Not bad for a chunk of unprepossessing matter with the consistency of cold porridge. The question is: how did something as
complex and amazing come about? The answer is inextricably bound up with the origin of the nervous system – and with the harnessing of lightning.

In the beginning, there were simple bacteria – microscopic bags of gloop with the complexity of small cities. They faced a serious problem: how to orchestrate their internal ‘factories’ to make the micro-machinery of life – the Swiss-army-knife molecules known as proteins. The solution they hit on was to release molecules such as glutamate, which diffused throughout their liquid interiors. When such a chemical messenger docked with a molecular receptor – fitting into a cavity like a key into a lock – it triggered the cascade of chemical reactions needed to make a protein.

After almost 3 billion years stalled at the single-cell stage, life made the giant leap to multicellular organisms. But it continued to use its ancient, tried-and-tested system of internal communication. Take sponges, for instance. These colonies of cells pulse in synchrony in order to pump food-laden water through channels in their bodies. Sponge cells achieve this feat of coordination by detecting chemical messengers such as glutamate, which are released by other sponge cells. It is nothing more than what happens inside a single bacterium writ large. If it ain’t broke, don’t change it, as far as nature is concerned.
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The chemical messengers of a sponge take many seconds to diffuse to all of its cells and trigger a response. This is acceptable for a creature living in surroundings that are constant and predictable. However, in a rapidly changing environment, where a quick response to threats is essential for survival, a faster method
of internal communication is imperative. Such a means is provided by electricity.

Remarkably, electricity is as ancient a feature of cells as chemical messengers. Cellular membranes are leaky and prone to let through dangerous charged atoms such as the sodium in salt.
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In order to survive, bacteria needed a way to pump out such ions. They solved the problem with the aid of tunnel-like proteins called ion channels, which span the cell membrane and can open and shut to expel ions. But, inevitably, pumping ions through such a channel creates an imbalance of electric charge between the inside and the outside of the cell. It is this voltage difference that provides a cell with a nifty communication opportunity.
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To send a super-fast signal, a cell needs only to manipulate the voltage across its membrane, which it can do simply by pumping ions rapidly through an ion channel. This causes an abrupt change in the voltage across the membrane, which has a knock-on effect on the next ion channel, and the next, and so on. Like a microscopic Mexican wave, an electrical signal propagates along the membrane, thousands of times faster than any chemical messenger,
literally at lightning speed
.

Of course, a communication system based on electricity – a true
cellular telephone system
– needs a means not only of transmitting a signal but a means of detecting it at its destination and doing something useful with it. Cells have this covered too. A type of channel known as a voltage-gated ion channel can open in response to an electrical signal, allowing ions such as calcium to pass through the membrane. These then trigger a cascade of cellular processes, effectively turning the incoming electrical signal back into a bog-standard chemical messenger, which can do something useful such as trigger the building of a protein.

Voltage-gated ion channels, just like regular ion channels, are present in bacteria. Cells that use them for internal communication simply borrowed them and adapted them to the new and specialised task.

An internal cellular telephone system was in existence even before the first multicellular animals. In fact, it can be seen in action in a water-living, single-celled creature called
Paramecium
. When
Paramecium
is swimming along and bumps into an obstacle, a voltage is created across its membrane. This causes a Mexican wave of ions to pulse around its body. Lightning fast, the wave reaches hair-like extensions on the surface of the cell, which, when they ripple in synchrony, can propel the cell. Instantly, these cilia reverse their beating, causing
Paramecium
to back away from the obstacle.

A useful trick for a single-celled creature such as
Paramecium
turns out to be indispensable for a multicellular organism. After all, as creatures grew ever larger, it became likely that the place on their bodies where they sensed a dangerous touch was a
long way
from the place where a muscle had to be contracted in response. Sending a signal via a chemical messenger was far too slow. Long before an animal could take evasive action, it might be eaten. Electricity was the only solution. And nature responded by creating a specialised electrical cell – the nerve cell.

A nerve cell has a cell body with a nucleus like a normal cell. But there the similarity ends. One side of the cell is extended like a long, thin wire, while the other side sports a number of finger-like extensions. The long, thin wire, known as an axon,
transmits
an electrical pulse to another nerve cell, whereas the finger-like extensions, known as a dendrites,
receive
electrical signals from the axons of other nerve cells.

Crucially, the axon of one nerve cell does not touch the dendrite of another. There is a gap – known as the synapse. Here, the electrical signal from the axon is converted into chemical messengers.
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These diffuse across the gap and dock with receptors, which open ion channels and thus trigger a new electrical signal. Sound familiar? It is the very same molecular lock-and-key system that bacteria came up with almost 4 billion years ago. Life, far from discarding its ancient and sluggish communication system, mediated by chemical messengers,
integrates
it into its super-fast and modern communication system, mediated by electricity.

The mediation of the electrical signal by chemical messengers is not just an unfortunate hangover from the beginning of life. It makes it possible for an almost infinite array of responses from a nerve cell. This is because there are a host of different chemical messengers, or neurotransmitters, each of which has an effect on a dendrite if and only if the dendrite possesses a receptor for it. Some trigger, or excite, an electrical current in the dendrite whereas others prevent, or inhibit, a current.

The two most important neurotransmitters in the human brain are glutamate – the fossil relic of the system of chemical messengers used by bacteria billions of years ago – and gamma-aminobutyric acid, or GABA. Virtually all communication between nerve cells, or neurons, in the brain is mediated by these two simple amino acids. Other neurotransmitters such as dopamine and acetylcholine merely moderate their action. Most drugs that affect behaviour work by blocking or mimicking a particular neurotransmitter, thus stimulating a receptor site and generating
the same effect as the neurotransmitter. For example, lysergic acid diethylamide, or LSD, a mere speck of which causes dreamlike psychedelic hallucinations, has a chemical structure very similar to the neurotransmitter serotonin.

Because a nerve cell has extensions capable of both
sending
and
receiving
electrical signals, it can join together with others in a network, with each nerve cell connected via its dendrites to the axons of many other nerve cells. Such a network can behave in a complex way.

Even a single nerve cell can exhibit memory. Say an electrical signal from a sense – perhaps touch – comes in along a dendrite and triggers the nerve cell to send a signal along its axon to contract a muscle. If, in addition to going to the muscle, the axon splits and part of its signal feeds back into a dendrite of the nerve cell, it triggers contraction again. And again. And again. A nerve cell can refire about every hundredth of a second. In this way, the nerve cell
remembers
the stimulus. If four nerve cells are connected together, they can exhibit complicated behaviour such as contracting a muscle to move away from either a stimulus on the left side or on the right side of an animal. This gives some hint of the complex behaviour possible if nerve cells connect together not in quartets but hundreds or thousands or even
hundreds of billions
.

The earliest nerve cells, though connected to each other, were also connected to the external world – receiving an input signal directly from senses or providing an output signal to, for instance, contract a muscle. There was no computation in between. However, at some stage in the history of life, nerve cells began to connect
only to other nerve cells
. This enabled such neurons to process the input information from the environment in new and
complex ways in order to decide on an appropriate response. It was an epochal moment in the history of life. It marked the birth of the
brain
.

‘Basically there are two types of animals,’ says Columbian neuroscientist Rodolfo R. Llinás. ‘Animals, and animals that have no brains; they are called plants. They don’t need a nervous system because they don’t move actively, they don’t pull up their roots and run in a forest fire! Anything that moves actively requires a nervous system; otherwise it would come to a quick death.’
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A neuron is often likened to a logic gate of a computer.
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A logic gate, built from transistors, can be wired together with other logic gates to create a circuit that, for instance, adds together two numbers. But, whereas a logic gate has only two electrical inputs and spits out a signal that depends on the current flowing in those two inputs, a neuron can have 10,000 or more dendritic inputs, and spit out a signal that depends on the complex interplay of all those electrical inputs on numerous neurotransmitters and receptors at the nerve cell’s synapse. So, although it is true that a neuron is the fundamental building block of a biological computer, just as a logic gate is the basic building block of a silicon computer, it is more than this. A neuron is a
computer in its own right
.

Brains, built of neurons, are expensive to run. The human brain accounts for a mere 2–3 per cent of the mass of an adult yet guzzles about a fifth of the body’s energy when resting.
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Having said this, the brain does all of its mega-computation on roughly 20 watts of power, the equivalent of a very dim light
bulb. By comparison, a supercomputer capable of an analogous rate of computation requires 200,000 watts – it is 10,000 times less energy-efficient than the brain.

For some creatures, however, the energy expense of running a brain is simply too great. The juvenile sea squirt has a rudimentary nervous system that enables it to wander through the sea searching for a suitable rock or hunk of coral to cling to and to make its home. ‘When it finds its spot and takes root, it doesn’t need its brain any more,’ says American cognitive scientist Daniel Dennett. ‘So it eats it!’
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Despite this rather disturbing example of autocannibalism, the benefits of having even a simple brain usually appear to outweigh the costs. For instance, the nematode worm,
Caenorhabditis elegans
, has a brain with a mere 302 neurons – so few that its brain is completely encoded in its DNA. The nematode worm, unlike the sea squirt, does not eat its brain. It must therefore provide the worm with an important competitive advantage.
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The human brain weighs about three pounds and has about 100 billion neurons – by sheer coincidence, roughly the same number of stars in our Galaxy, galaxies in our Universe, and people who have ever lived. ‘The human brain is the most complex object known in the Universe,’ says Edward O. Wilson, ‘
known, that is, to itself
.’
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According to a theory developed by American neuroscientist Paul MacLean, in the course of evolution three distinct brains have emerged, accreting one on the other. ‘With modern parts atop old ones, the brain is like an iPod built around an eight-track cassette player,’ says American journalist Sharon Begley.
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