By Roger White, copyright Aug 2002
Life such as we have on Earth may be distinctive in the galaxy. One way it may be distinctive is in the sheer magnitude of the material and energy cycling through the biosphere.
Consider what life on Earth would be like without a steady supply of sunshine. Recent theories of life origin propose that life may have begun around the thermal vents that dot the mid-ocean ridges at the bottom of our seas. If that is so, then life can begin on "dark" planets or other planets that have thermal springs, but are otherwise hostile to life as we know it.
What will life consist of on such planets?
It will remain a tiny phenomenon associated with rare, exotic environments. Finding life on such a world would be like finding gold or diamonds on ours: a rare event that happens after much prospecting. So little biomass would be created on a dark planet that life forms would evolve slowly and be very primitive.
Suppose a world has sunshine and a "pleasant" environment -- potentially life sustaining over much of the surface, but "animal life" -- life that consumes organic matter -- does not evolve after plant life gets started. What will life on such a world be like?
Such a world has only half a carbon cycle. Plant life grows and converts CO2 into oxygen and organic matter, but that's the end of the process. Without "animals", nothing converts large amounts of organic plus O2 back into CO2. Most of the organic matter sinks under sediments and becomes oil or coal; most of the oxygen will find other substances to oxidize. Those life materials that can break down by inorganic means, such as exposure to UV light and fire, can recycle, but inorganic recycling won't be as efficient a process as having teeming masses of animals, bacteria and fungi searching for food and eating it.
The inefficiency will strip the atmosphere of CO2, and force life to survive near steady CO2 sources, such as mid-ocean vents. The moral: plant life without animal life will make life of any kind a rare phenomenon on the world.
Part of the miracle of life on Earth includes the huge quantities of hydrogen, carbon, nitrogen and oxygen are cycling through life processes. Everywhere on Earth, tons and tons of these elements are being used to support living processes. Searching for life on Earth is like searching for water, not gold.
Life on Earth is far from reaching 100% utilization of any cycling material. We are far from cycling every available atom of hydrogen, carbon, nitrogen and oxygen through life processes. There are still tons and tons of coal and calcium carbonate (limestone, marble) lying as inert soils. The deep sea waters and the free nitrogen in the atmosphere are not part of the biocycle. We don't convert every photon that lands on earth into a life energy unit. But even with these less-than-fully-utilized cycles, life is hardly exotic on Earth.
Widespread life that covers a world thickly may not be life's average condition on the worlds of our galaxy. On many worlds, life may be an exotic, and finding it may be more akin to finding gold on Earth than finding a tropical tidal pool bathed in sunshine.