The defensive mimic theory offers an explanation for the striking similarities between crying and laughing.
It states that crying and laughing, as well as other emotional human expressions may originally stem from our defensive reflexes, which were and still are essential to our survival by protecting the body’s surface ((contracting the eyes, pulling the cheeks and upper lips up, lifting your shoulders, bending your knees, the head goes to the chest etc.) which all are characteristics that occur in both laughing and crying, especially as the expression intensifies)
More precisely, they might stem from the act of exaggeratedly mimicking defensive reflexes in stressful situations, especially in situations of conflict.
Throughout processes of evolution, crying and laughing may have proven to be useful social signals, effective for regulating ones and others emotions and thus regulating group dynamics by reducing aggression and evoking comfort.
Its important to note that while mimicking defensive reflexes may be the starting stone of crying and laughing, they now have evolved, through evolution into something much more complex.