An apocryphal reason for the pauses in
conversation that seem to every seven to twenty minutes or so. The
story goes that early humans evolved to periodically be quiet so
that they could listen for the approach of mammoths. Those
people who kept babbling incessantly would be trampled by
mammoths.
Occasionally people have been known to shout
Mammoth! during those conversation pauses.
Pauses of this sort in conversations seem to occur
when all conversation threads come to an end at the same
time. Even conversations containing only highly gregarious
people seem to enter these lulls.