Michael Trimble, a professor 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. He has written a new book - Why Humans Like To Cry - that attempts to shed light on the mystery of why our species is the only one in the animal kingdom to shed tears of anguish.
The work, according to its publisher, offers a wide-ranging discussion of emotional crying, looking at its physiology as well as its evolutionary past. 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. We 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.
Professor Trimble, of University College London's Institute of Neurology, said it was this uniquely communicative nature of human crying that led him to investigate the phenomenon.
'Humans cry for many reasons,' he told Scientific American. 'But crying for emotional reasons and crying in response to aesthetic experiences are unique to us.
'The former is most associated with loss and bereavement, and the art forms that are most associated with tears are music, literature and poetry.
'There are very few people who cry looking at paintings, sculptures or lovely buildings. But we also have tears of joy the associated feelings of which last a shorter time than crying in the other circumstances.'
He added: 'We should not be afraid of our emotions, especially those related to compassion, since our ability to feel empathy and with that to cry tears, is the foundation of a morality and culture which is exclusively human.'
'Attachment emotionally to others, with the development of sophisticated facial gestures associated with suffering, and with loss and bereavement ensued,' he said.