Human Instinct: How Our Primeval Impulses Shape Our Modern Lives Paperback - 2003

The Origins of Survival

Fight or flight

You are walking home late one dark, wet and misty winter evening. It's been tiring today, so you are keen to get inside, close the door behind you and put the stresses of the day to rest. As you amble along, thinking in neutral, you realize you can hear measured but quickening footsteps behind you. A snatched glance over your shoulder reveals a man approaching in the gloomy street-light. He is moving rather quicker than you are walking, and he is looking at you all the while. There's no-one else in sight — no-one on the street except you and the stranger. The house suddenly seems a long way away. In less than an instant, you suddenly feel very afraid. Your heart starts beating wildly, your mouth goes very dry and you have a huge urge to start running towards the safety beyond your front door.

There is a very simple reason why you feel so terrified. Inside your body, all hell has broken loose. Biological sirens and alarms are wailing. Perceiving the threat of the potential mugger with lightning speed, your brain and autonomic nervous system — the automatic controller of the gut, heart, vessels and lungs — have gone into overdrive and produced a huge surge of adrenalin. This triggers a hormonal cascade inside you, an incredibly fast and powerful chemical relay-race designed to propel you away from a threatening situation. Just a fraction of a second later, the hypothalamus in your brain begins pumping out a substance called corticotropin-releasing hormone, or CRH, which in turn sends alarms to the pituitary gland in your brain to pour out adrenocorticotropin, or ACTH. Finally, the abnormally high levels of ACTH in your bloodstream are the warning signal for the adrenal glands, near your kidneys, to start producing cortisol.

Imagine the speed at which these precise yet complex combinations of hormones are produced - your body's reaction to fright and attack is virtually instantaneous. Almost immediately these chemical alarm bells are set off inside us, we are forcibly shoved into the (all too familiar) feeling of being acutely afraid. The adrenalin makes your heart pound faster, increasing its normal resting rate by as much as two or three times. You would have to cycle really vigorously for maybe fifteen minutes to produce that kind of rise in your heart rate under normal circumstances, but in the sudden grip of fear, the rate can triple in just a matter of seconds. You are also breathing much faster now and the blood is being rapidly redistributed around your body. The blood vessels in unimportant areas like your stomach and your skin constrict, shunting blood away and into the now dilated vessels of the muscles of the limbs. Here the extra oxygen and fuel gained by your increased breathing can be best harnessed to flee from the threat, or even fight it. There wouldn't be much sense in your stomach busily digesting that lunchtime sandwich right now, when every drop of your body's available energy needs to be used to save you from the approaching threat.

As the adrenalin and cortisol continue to gush out into your blood, your pupils dilate, allowing you to see better in darkness and shadows and to perceive any movement around you more keenly. A kind of pain-dampening effect is switched on so that you won't be distracted from getting away by any injuries. Emergency reserves of glucose are released inside you to allow for especially intense bursts of muscular activity. Even your immune system is mobilizing to cope with the possibility of dealing with a serious wound. In just a matter of moments, your body has propelled you into a state of extreme physical and psychological readiness to run or fight — whichever course of action best suits the threatening situation.

As the stranger, now just feet away from you, holds out the single, familiar glove you now realize you'd obviously dropped some way back, you may ask whether all your body's efforts were really worth it. Whether it's 'butterflies' in the tummy before an interview, the dry mouth and throat we feel minutes before we have to make a speech, or even the quickening pulse and sudden jump as we hear an unexpected bump in the night, it often seems that our bodies are overreacting. So where does this physical and psychological reaction come from? It's not as if we were taught as children to start breathing faster in threatening situations, nor can we consciously make our heart beat so much faster or force our body to produce adrenalin. What we are actually experiencing is our very own personal link to our most ancient human ancestors - a reaction which hundreds of thousands of years ago almost certainly made the difference between life and death, but which now, in most cases, simply serves to remind us of the remarkable fact that while living in a very advanced modern world, we all do so with Stone Age brains and bodies.

Indeed, this reaction in response to stress hormones goes way back in time, well before our immediate ancestors. Even animals that aren't mammals react basically in a similar way. Try startling your goldfish as it meanders around its bowl. If you place a net or a threatening object into its water, you will immediately see a very similar kind of reaction. Its fins stand out ready to flee and the gills and mouth start opening and closing in overtime. That fright reaction is caused by the same hormone, inherited down the ages: adrenalin.

Our early human ancestors lived in a very dangerous and threatening environment. When they first made it out of the trees to try their luck on the grassy plains of the east African savannah, they were vastly outnumbered by vicious and hungry predators. They lacked the brute strength of the great apes and many other large land mammals, especially the big cats. Nor were they particularly fast or agile like the antelope or gazelles. They could not fly, nor were they especially well designed for life in water. Their senses were poorly developed: no night vision, no extra-sensitive hearing to detect prey rustling in the grass hundreds of feet away, and an extremely unsophisticated sense of smell. Ape-man infants were helpless and dependent, and parents were distracted from practical matters of survival by having to care for their young. But these naked and defenceless prototype humans had to contend with the searing African heat as they traversed the vast distances of the plains in search of food, shelter and mates. If they stayed in one area, they risked starvation and attack from a stalking predator; if they were on the move, they faced the test of the unknown, of coming face to face with some terrible beast. And terrible they were.

While the most probable threat you may experience today is a brush with a suspicious person in a local street, our ancient ancestors had to face the reality of encounters with violent sabre-toothed cats and other predators. One variety was Smilodon, a sabre-toothed cat whose remains show it was almost a foot shorter than a modern-day lion, but weighed almost twice as much. Instead of the long, graceful tail cheetahs and leopards use for balance as they race across the African plains, Smilodon had a short, stumpy bobtail. This beast was a simmering hunk of muscle, designed for quick and furious violence.

Smilodon almost certainly hunted in packs. We know this because fossil specimens of the huge cat, dug up in California, show evidence of healing injuries. Some of these injuries were so serious that immediately after the trauma the cat would have been unable to hunt, so it couldn't have survived long enough for the injury to heal unless other animals from the pack had brought it food. Smilodon could roar — we know that from the structure of the hyoid bones in its throat - but like any modern feline predator it would have been silent when stalking. Once it had ambushed its prey, by charging the frightened antelope or artiodactyl with an explosive burst of power, it would have used its long, curved, sabre-like teeth, viciously ripping open the belly or throat.

It's highly likely that predators such as these would have killed and eaten early humans. In a cave at Swartkrans in South Africa, palaeontologists found the skull of an early human, Homo habilis, buried deep in sediment dated to around two million years. It belonged to an eleven-year-old child and bears the mark of an African predator: the bone is punctured in two places, an exact match for a pair of leopard's canines. A child such as this stood absolutely no chance of defending itself against these powerful beasts; even a fully grown male would have been practically helpless, given the speed, power and aggression of the big cats.

To stand the best possible chance of survival all animals have to protect themselves from danger and death, so they need a means to be alert to threats at all times, to fear them and to fight or flee in response to them. The imperative is self-preservation as well as the survival of the species. In evolutionary terms, a fearless animal would be much less likely to survive and pass on its fearless genes. Six billion humans now populate the world; our species has become the most successful in the history of all life on the planet. Our early ancestors must have developed and evolved some spectacularly successful ways to protect themselves from predators and threats - physiological and psychological reactions that were so fundamental to their survival they still exist deep within us today.