Those who take credit for what they have not achieved, along with those who blame others for their own shortcomings, are surely worth little more than our contempt, so, bearing this in mind, I will pass for the last time for a while to the genius of Bill Bryson.  Everything that follows comes from ‘The Body: A Guide for Occupants’ and I think I will just let you read some extracts for yourself with little, if any, additional comment, aside from making the point that it is one of the best examples I have come across of making something complicated into something that we can all understand, which is a genuine gift. 

Starting at the top, what is surely most curious and extraordinary about our brain is how largely unnecessary it is.  To survive on Earth, you don’t need to be able to write music or engage in philosophy – you really only need to be able to out-think a quadruped – so why have we invested so much energy and risk in producing mental capacity that we don’t really need?  That is just one of the many things about your brain that your brain won’t tell you. 

The brain takes a long time to form completely.  The wiring in a teenager’s brain is only about 80 per cent completed, which may not come as a great surprise to the parents of teenagers.  Although most of the growth of the brain occurs in the first two years and is 95 per cent finished by the age of ten, the synapses are not fully wired until a young person is in their mid to late twenties.  This means that the teenage years extend well into adulthood, and in the meantime the person in question will almost certainly have more impulsive, less reflective behaviour than their elders, and will also be more susceptible to the effects of alcohol and drugs.  The teenage brain is not just an adult brain with fewer miles on the clock: it is a different kind of brain altogether. 

The region of the brain associated with pleasure grows to its largest size in the teenage years.  At the same time, the body produces more dopamine, the neurotransmitter that conveys pleasure, than it ever will again.  That is why the sensations you feel as a teenager are more intense than at any other time of life.  But it also means that seeking pleasure is an occupational hazard for teenagers.  The leading cause of death among teenagers is accidents – and the leading cause of accidents is simply being with other teenagers, for example in a car, where one extra person multiplies the risk by a factor of four. 

Six of our facial expressions are universal: fear, anger, surprise, pleasure, disgust and sorrow.  The most universal expression of all is a smile, which is rather a nice thought.  No society has ever been found that does not respond to smiles in the same way.  True smiles are brief – between two-thirds of a second and four seconds.  That is why a held smile begins to look menacing.  A true smile is the one expression we cannot fake.  You can make your mouth smile, but a true smile needs muscles in the eyes to contract as well, over which we have no independent control, so you can’t make them sparkle with feigned joy. 

No one really knows why we have eyebrows.  One theory is that they are there to keep sweat out of the eyes, but what they do really well is convey feelings.  Think how many messages you can send with a single arched eyebrow, from ‘I find that hard to believe’ to ‘Watch your step’.  One of the reasons the Mona Lisa looks enigmatic is that she has no eyebrows.  In one interesting experiment, subjects were shown two sets of digitally doctored photographs of well-known people: one with the eyebrows eliminated and the other with the eyes themselves taken away.  Surprisingly, but overwhelmingly, volunteers found it harder to identify the celebrities without eyebrows than without eyes.    

Remarkably, even with all the improvements in care, you are 70 per cent more likely to die from heart disease today than you would have been in 1900.  That is partly because other things used to kill people first, and partly because a hundred years ago people did not spend five or six hours an evening in front of a television with a big spoon and a tub of ice cream.  Heart disease is far and away the Western world’s number one killer, responsible for the deaths of about the same number of Americans as cancer, influenza, pneumonia and accidents combined.  One in three Americans dies of heart disease and more than 1.5 million suffer a heart attack or stroke each year. 

When we started to walk on two legs instead of four, we adapted to cope with a narrower pelvis to accommodate our new gait.  This brought a huge amount of pain and danger to women in childbirth.  Until recent times, no other animal on Earth was more likely to die in childbirth than a human, and perhaps none even now suffers so much.  One consequence can be seen in the carrying of bags, with women tending to do it differently than men do.  It is thought that their wider hips necessitate a less perpendicular carrying angle for their forearm so that their swinging arms are not constantly banging against their legs.  That is why women generally carry bags with their palms facing forwards, allowing their arms to be slightly splayed, while men carry them with palms facing back. 

Life expectancy on Earth improved by as much in the twentieth century as in the whole of the preceding eight thousand years.  The average lifespan for an American male went from 46 years in 1900 to 74 by century’s end.  For American women the improvement was better still – from 48 to 80.  It is important to bear in mind that historic life expectancy figures were always skewed by childhood deaths.  When we read that life expectancy was 46 years for American men in 1900, that does not mean that most men got to 46 and then keeled over.   

Life expectancies were short because so many children died in infancy, and that dragged the average down for everyone.  If you got past childhood, the chances of living to a reasonably advanced age were not bad.  Lots of people died early, but it was no means a cause of wonder when people lived into old age.  As the American academic Marlene Zuk has put it, ‘Old age is not a recent invention, but its commonness it.’  The most heartening advance of recent times, however, is the striking improvement in mortality rates for the very young.  In 1950, 216 children in every thousand – nearly a quarter – died before the age of five.  Today the figure is just 39 early childhood deaths in a thousand – one-fifth what it was seventy years ago. 

Bryson concludes with a section about what happens at the end of our lives, which has a certain inevitable bleakness, but he always manages to raise a smile, this time with his final sentence: ‘But it was good while it lasted, wasn’t it?’