4 million years ago, a human ancestor called australopithecus anamensis wandered the woodland habitat of East Africa. Many environmental challenges surrounded her and defence against harm was critical for her to survive: her number one priority.

Fortunately, millions of years of evolution had equipped her body with sensory neurons to detect harm and motor neurons in order to trigger a response (ideally in her legs). Known as survival circuits these sensory neurons are as old as life itself - even single celled organisms sprout sensors to feel around for harmful chemicals and can physically retract from a threat if necessary. The problem was that her response to danger was still rather unsophisticated. Survival circuits are useful at the moment of attach to help you run.

What she really needed was an intelligent warning system, a way to scrutinise threats rigorously and react before it was too late. And that is exactly what evolution provided. First appearing in nearly 500 million years ago, the human brain formed a region called the amygdala.

Because evolution is an excellent recycler, always building on what is already available, the amygdala actually developed from our most primitive sense - the sense of smell. Sitting just beneath the amygdala is a region called the olfactory bulb which controls smell. As our brain grew more complex, evolution simply layered the amygdala on top. This was a smart move by the brain: in positioning itself this way, it could be hooked up to receive signals from our other four senses: sight, sound, taste, and touch.

One might think that processing so much information would be slow, a cumbersome task for the brain. But the pathway linking the amygdala to the senses is shorter than the pathway linking it to the neocortex, the home of higher-order thinking that evolved much later. This means that we don't have to think or reason our way through a situation that demands an instant emotional response. It is why you know a stranger following you down an alleyway doesn't feel right; why unexpected sounds in your house at night ring alarm bells in your head.

Medical literature is replete with examples of what can happen when the amygdala is jeopardised. One woman who had amygdala damage became completely immune to fear. She was unfazed by exposure to snakes, spiders, and horror films. Patients with amygdala damage report less fear and can develop something called affective blindness, the inability to identify the emotional expression of faces correctly.

For australopithecus anamensis starting to have amygdala meant they could finally learn to associate certain things in their environment - a rustle in the trees, the scampering of monkeys, the smell of a bif cat - with imminent peril. As humans evolved, the amygdala adapted, generating five more primal emotions: anger, surprise, disgust, joy and sadness. And millions of years later, australopithecus anamensis' descendent homo sapiens generated culture and civilisation, the amygdala adapted again sparking sophisticated kinds of emotional processing such as forgiving the wrongs of those we love, or confronting and accepting loss. It is thought that the neurons in the amygdala modulate the activity of the neocortex which in turn modulates our emotional behaviour. The relationship between the amygdala and the neocortex allowed our brains to experience new kinds of emotional complexity.

Amanda Walker, a 48 year old designer, experienced a stroke which tore through her amygdala. And now she lacks the capacity to feel joy or love or sorrow. Asked how she feels about her predicament, she responds 'I can't answer that. There is nothing to feel'