Life is a characteristic distinguishing objects having signaling and self-sustaining processes from those that do not, either because such functions have ceased (death), or because they lack such functions and are classified as inanimate. Biology is science concerned with the study of life. Though life is confirmed only on the Earth, many think that extraterrestrial life is not only plausible, but probable or inevitable. Other planets and moons in the Solar System have been examined for evidence of having once supported simple life, and projects such as SETI have attempted to detect radio transmissions from possible alien civilizations. According to the panspermia hypothesis, microscopic life exists throughout the Universe, and is distributed by meteoroids, asteroids and planetoids. Early theories Materialism Herds of zebra and impala gathering on the Maasai Mara plain Democritus (460 BC) thought that the essential characteristic of life is having a soul (psyche).
• Life's Secrets
• Interest & Intrigue