(ii) Cells with internal metabolism and reproductive ability.
(iii) Photosynthesis, cells producing sugar and other foods by using sunlight to energize carbon dioxide and water, also generating free oxygen as a byproduct.
(iv) Multi-celled organisms.
Apparently, before (iii), life might have spread but then starved several times! (I never knew that.) Although much remains unknown or speculative, enough has been learned to show that life could and did emerge from simpler substances by natural processes on Earth and can to be expected to do so elsewhere.
Anderson goes on to say (Is There Life On Other Worlds?, New York, 1963, p. 58) that there are three "...really basic requirements of biopoesis...", i. e., of the origin of life. I had already come to understand that the three basic requirements were complex molecules, energy and time. Thus, energized complex molecules randomly interact and change until, given time, one becomes self-replicating. Taking the time for granted, Anderson presents as his three requirements:
abundant molecules capable of complexity - on Earth, methane, ammonia, phosphates etc;
energy - on Earth, ultraviolet light, heat, lightning, maybe also radioactivity and acoustic phenomena;
a solvent for concentration and reaction - on Earth, water.
(Another requirement is a temperature at which the solvent is liquid.)
Discovered since 1963: an outer satellite (Europa?) has heat, an energy source, inside and ice outside. Thus, between heat and ice (the dialectical opposites of the Norse creation myth), there might be liquid water containing life independent of the Sun, to be discovered by a space probe.
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