Short answers, pulled from the story.
Rhythm comes from the Greek word rhythmos, meaning any regular recurring motion or symmetry. It generally describes a movement marked by the regulated succession of strong and weak elements, or of opposite or different conditions.
Joseph Jordania suggested that the sense of rhythm developed in the early stages of hominid evolution through natural selection. He argued it was central to achieving the battle trance, a neurological state crucial to the defense system of early hominids.
Neurologist Oliver Sacks stated that some parrots can know rhythm, but chimpanzees and other animals show no similar appreciation of it. He wrote that there is not a single report of an animal trained to tap, peck, or move in synchrony with an auditory beat.
A continuous pulse cannot be perceived as a musical beat if it is faster than 8 to 10 per second or slower than one every 1.5 to 2 seconds. Musical tempo is generally specified between 40 and 240 beats per minute, which roughly matches the human heart rate.
Metrical or divisive rhythm calculates each time value as a multiple or fraction of the beat with accents recurring regularly to form measures. Measured or additive rhythm uses a time unit but its accents do not recur regularly, while free rhythm, as in Christian chant, has neither and flows more like prose.
In the Griot tradition of Africa, Babatunde Olatunji developed six spoken vocal sounds, Goon, Doon, Go, Do, Pa, Ta, to teach hand-drum rhythms. In Indian music, tabla players learn to speak complex rhythm patterns before playing them, and the Tala is the rhythmic pattern over which a whole piece is structured.
Igor Stravinsky, Bela Bartok, Philip Glass, and Steve Reich used odd meters, phasing, and additive rhythm. Olivier Messiaen and his pupils disrupted regular beats toward New Complexity, while Henry Cowell collaborated with Leon Theremin to invent the rhythmicon, the first electronic rhythm machine.