A team of international researchers has carried out the study found that the human nose has evolved to sniff out the smell when something unwarranted happens, leading British newspaper The Daily Telegraph reported on Friday.

According to lead researcher Dr Wen Li of Northwestern University, "It's evolutionary. This helps us to have a very sensitive ability to detect something that is important to our survival from an ocean of environmental information. It warns us that it's dangerous and we have to pay attention to it.

"The ability to discriminate between biologically meaningful cues such as the smell of a 175 kg lion and similar but irrelevant stimuli such as the smell of a three kg cat maximises an organism's response sensitivity while minimizing hyper-vigilant and impulsive behaviours."

In their study, a group of 12 volunteers were exposed to two pairs of very similar grassy smells, which they had previously been unable to tell apart.

The participants received mild electrical shocks each time they were exposed to one scent, but not when smelling the similar one in the pair. Only when the shocks were introduced were the subjects able to discriminate between similar smells.

The researchers then used MRI scans to show activity in a part of the brain called the olfactory cortex changed significantly once a person had learnt to associate a smell with an electric shock.

"Our data raises the intriguing possibility that neurobiological derangements in the ability to distinguish between salient cues and perceptually inconsequential stimuli may underlie the emergence of anxiety disorders characterized by exaggerated sensory sensitivity and hyper-vigilance.