Laughter is a physiological response to humor or a particular stimulus, usually associated with feelings of joy or amusement. It involves complex mechanisms within the brain and body.
Scientifically speaking, laughter is a complex response that involves many parts of the body and brain. The process generally starts in the brain where the humor or stimulus is recognized, and then signals are sent to different parts of the body. The respiratory system is most notably involved, as laughing involves taking in air, holding the breath, and then expelling it in short bursts or 'laughs'. The face, especially the muscles around the mouth and eyes, and sometimes the whole body (in the case of a 'belly laugh') can also be involved. The endorphins released during laughter create a sense of happiness and can even relieve pain temporarily.
As for the soul or spirit, those are metaphysical concepts that science has yet to quantify or measure. Some people might say that laughter is an expression of the soul or spirit, a manifestation of joy or amusement on a deeper, immaterial level. However, these interpretations are subjective and depend on one's beliefs about what constitutes the 'soul' or 'spirit'.
Regarding other species, humans aren't the only ones who exhibit laughter-like behaviors. Many animals, especially mammals, have similar responses. For example, rats have been observed making a unique, high-pitched sound when they play or are tickled, which scientists equate to laughter. Primates like chimpanzees and gorillas also have a laughter-like response to play and tickling. However, human laughter is unique in the sense that it is often a response to complex cognitive processes (like understanding a joke), something that is less common in other species.