Short answers, pulled from the story.
Rhythm is the invisible pulse that governs the heartbeat of human existence and extends far beyond the confines of music to dictate the very rhythm of life itself. The Greek word rhythmos means any regular recurring motion or symmetry and captures the essence of a force that operates from microseconds to millennia. This universal pattern is a fundamental neurological process that allows humans to anticipate and organize time.
The sense of rhythm was not a cultural invention but a survival mechanism forged in the fires of early hominid evolution. Joseph Jordania suggests that natural selection developed this ability to create a specific neurological state known as the battle trance. In this state, individuals could unite into a collective identity, putting the interests of the group above their own safety and survival.
One of the most profound difficulties in defining rhythm lies in the dependence of its perception on tempo and conversely the dependence of tempo perception on rhythm. This rhythm-tempo interaction is context-dependent as demonstrated by the leading rhythm of Moussorgsky's Promenade from Pictures at an Exhibition. The loop of interdependence between rhythm and tempo is overcome due to the simplicity criterion which optimally distributes the complexity of perception between rhythm and tempo.
In the Griot tradition of Africa everything related to music has been passed on orally with Babatunde Olatunji developing a simple series of spoken sounds to teach the rhythms of the hand-drum. In Indian classical music the Tala of a composition is the rhythmic pattern over which the whole piece is structured with Tabla players learning to speak complex rhythm patterns before attempting to play them. These traditions highlight the diversity of rhythmic expression from the call-and-response forms of Africa to the intricate Tala systems of India.
In the 20th century composers like Igor Stravinsky Béla Bartók Philip Glass and Steve Reich wrote more rhythmically complex music using odd meters and techniques such as phasing and additive rhythm. Henry Cowell wrote music involving multiple simultaneous periodic rhythms and collaborated with Leon Theremin to invent the rhythmicon the first electronic rhythm machine. Conlon Nancarrow wrote for the player piano pushing the boundaries of rhythmic possibility.
In linguistics rhythm or isochrony is one of the three aspects of prosody along with stress and intonation forming the rhythmic backbone of human communication. Languages can be categorized according to whether they are syllable-timed mora-timed or stress-timed with speakers of syllable-timed languages such as Spanish and Cantonese putting roughly equal time on each syllable. Speakers of stressed-timed languages such as English and Mandarin Chinese put roughly equal time lags between stressed syllables.