It is the case that many mammals shed tears, especially in response to pain or irritation of the eye, and that tears protect the eye by keeping it moist, but it is a truism, observed by many, including Charles Darwin, that humans are the only living species that cries for emotional reasons. Therefore, there must have been a point in the evolution of Homo sapiens when tears became a way to express the mental state of the crier. The awareness of the emotions of others (via mirror neurons)—which appeared at some point after the rise of consciousness, and almost certainly consequent to the development of linguistic skills—was signified by the use of deictic words (I, here, and now) that intone individuality, and perhaps dreaming. This allowed for the development of empathy, which is the embodiment of those feelings. It could be that the death of a member of a closely knit social group and the appearance of that person in a dream led to the enhancement of communal activities, such as attempts to find or visit another world, which, in turn, led to storytelling, religious rituals, and ideas of life after death.
the development of the facial muscles allowed for much greater expression in Homo sapiens than in other primates, and the eyes underwent astonishing alterations. Look into the eyes of any other living primate and you will see that the sclera (the area around the iris) is dark. In the human eye, the sclera is white, so changes in the size of the iris that accompany shifting emotions are visible. The ability to feel the sadness of others was a critical part of the development of Homo sapiens, and is directly related to neurobiologic changes that occurred in the central nervous system during the evolutionary process. Recent research in the field of neuroscience has revealed that certain brain circuits are activated, rapidly and unconsciously, when we see another in emotional distress.emotional crying, as an embodiment of empathy, played an essential role in human evolution and the development of culture. Tears became more than a biologic necessity to lubricate the eye and developed into a cipher of intense emotion. They became a social signal with strong bonding properties, useful when our ancestors began to contemplate life in the context of loss and death.
read more in a great article by Michael Trimble: http://www.medscape.com/viewarticle/870200_2?nlid=110242_1842