Daniel Dennett’s example graphically illustrates the fact that brains are expensive in terms of energy. This is why most creatures on Earth get by without one, and even a creature with a brain like the juvenile sea squirt ditches it once it is no longer useful. “Basically there are two types of [[Animals]],” says the neuroscientist Rodolfo 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.” Incredibly, the human brain does all of its mega-computing on roughly 20 watts, the power requirement of a very dim light bulb. By comparison, a supercomputer capable of a similar rate of computation requires 200,000 watts. In other words, the brain is 10,000 times more energy-efficient than a supercomputer. Despite this, it is very energy-hungry compared with other tissues. Although the brain accounts for only 2-3 per cent of an adult’s mass, it uses about 20 per cent of the body’s oxygen. The huge energy need of the human brain compared with the rest of the body is not surprising given the fact that the brain contains about a hundred billion brain cells – as many as there are stars in our Milky Way galaxy. Each brain cell, or “neurone”, may connect to 10,000 others via finger-like extensions called “dendrites”. This provides the potential for a thousand trillion connections. And it is in these connections and their relative strengths that information such as memories is stored. Every experience you have every moment of every day changes the connectivity of your brain. In the words of the cognitive scientist Marvin Minsky, “The principal activities of brains are making changes in themselves.” the human brain has shrunk by about 10 per cent in mass since peaking in size between 15 and 30,000 years ago. One possible reason is that humans once lived in a world of dangerous predators and needed to have their wits about them to avoid being killed. Today, we have effectively domesticated ourselves and many of the tasks of survival – from avoiding immediate death to building shelters to obtaining food – have been outsourced to society. We are smaller than our ancestors too and it is a common characteristic of domestic animals that they are smaller than their wild cousins. It does not mean we are more stupid – brain size is not necessarily an indicator of intelligence – but it may mean that our brains today are wired up differently from, and perhaps more efficiently than, those of our ancestors. `Concepts:` `Knowledge Base:`