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’ – 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. 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.
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. 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.
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.