Professor Michael Trimble, from the University College London's Institute of Neurology, suggests that there must have been a time in our evolution when tears took on a meaning beyond their simple bio-mechanical function - keeping the eyeballs moist.
Biologically, tears are needed to keep the eyeball moist and they contain proteins and other substances to keep it healthy and fight infections.
In every other animal that seems to be extent of their function, but in humans, crying takes on a whole new, additional significance, 'Daily Mail' reported.
Researchers said humans can shed tears of joy and tears of anger and for a whole range of other emotions. But, most commonly, we shed tears of sadness.
Trimble said it was this uniquely communicative nature of human crying that led him to investigate the phenomenon.
"Humans cry for many reasons. But crying for emotional reasons and crying in response to aesthetic experiences are unique to us," he told Scientific American.
According to Trimble, tears are a natural response to not only suffering, but also to feeling compassion for others. He suggests that the crying in this way must have emerged in humans at a specific evolutionary turning point.
He believes that the emergence of emotional crying is connected with the dawning of self-consciousness and the development of a theory of mind - when early humans first realised their peers were also self-conscious beings.
This, Trimble claims, led to the realisation that the self and others can suffer, feel sadness and disappear.
"Attachment emotionally to others, with the development of sophisticated facial gestures associated with suffering, and with loss and bereavement ensued," he said.
