Bovine flatulence — the intimate intestinal activities of cows, reindeer, elephants, and elk — is detectable over interplanetary distances, while the bulk of the activities of mankind are invisible. We would not ordinarily consider the flatulence of cattle as a dominant manifestation of life on Earth, but there it is.
The Cosmic Connection: An Extraterrestrial Perspective, Chapter 22 (p. 150), Dell Publishing, Inc. 1975