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Since Darwin (1872), the origin of the laugh with an intermittent exhalation "A! Ha! Ha!" which is peculiar to human, has been a great question. The author found out that this laugh is caused by the three sets of emotion. Firstly, light surprise or discovery. The ability to estimate "light" is absolutely important, because the amount of the first exhalation "A!" caused by the stimulation is decided by the amount of "surprise" felt by the subject. The ability to estimate the amount of "surprise" to be "light", makes the partial exhalation "A!". Secondly, consciousness of this harmlessness or delight, and thirdly, the following expectation of some safe circumstances. The author proved this theory by electromyography (EMG), photoplethysmography and galvanic skin reaction (GSR). The similarity between the facial EMG distribution pattern of "the beginning of laugh" and "the light surprise" was proved by electromyography about many facial muscles, with special fine electrode which did not disturb any natural facial expression of the subjects. Plethysmography and GSR proved light sympathetic tension and following relaxation when laughing. The author also suggests relationships between human laugh and human history such as the origin of clothing, language, and use of fire, which are specific in human.