In The Quest for Life on the Guardians of the Millennium Peace in Space Web site, we saw how Earth is a pleasant environment that allows life to flourish, diversify and evolve into creatures as complex as ourselves.   For us as humans to evolve and survive, we need quite specific conditions;  but in the case of microbial bacteria, for instance, are the limits the same?

No, the limitations discussed in The Quest for Life are associated more with how far a simple organism can evolve into a more complex form - a so-called higher life form - than with the extremes of tolerance that life at its simplest, can sustain.   Simple life can be relatively robust when confronted with the various and extreme environments the Universe has to offer - conditions such as these:

• Radiation - highly damaging streams of particles that break up molecules
• Low energy environments - not many resources to sustain metabolism
• Vacuum - no atmosphere for respiration
• High Pressure - deep below ground, under the sea or under a dense, thick atmosphere
• Cold - below 273 Kelvin (32 F: water's freezing point)
• Heat - above 373 Kelvin (212 F: water's boiling point at one atmosphere pressure)
• Long time scales in adverse conditions - during prolonged ice ages
• Unpleasant chemical environments - toxic volcanic gases
• Dryness - lack of exposure to liquid water, associated with heat or cold
• Gravity - excessive force from a more massive planet than Earth

The limits for simple life are governed primarily by the chemistry concerned, and whether or not it can take place under given conditions. As we see, simple microbial life can survive in the cold, or heat, vacuum, and harsh radiation of space (go to extremophiles).  It can cope with caustic and acidic chemicals, and arid conditions too.  Many biologists have now concluded that all life needs in order to cling on to some sort of existence is chemical "drivers"  for metabolism - and these can be as simple as a supply of hydrogen -  and liquid water.  If this is so, then the range of life habitats in the Universe may be much more diverse and numerous than we had assumed just a few years ago.

However, while simple life may be able to adapt to survive in such extreme environments, such conditions impose severe limitations on the evolution of such lifeforms into more complex beings.  Living organisms exploit their environment.  They change or develop to take advantage of it, through the process we call evolution.  Environments put various limits on such development - temperature and chemistry being examples.  These limitations can pose severe problems for organisms to overcome.  Sometimes this takes a long time - millions or even billions of years.  Other times the limitations could be so severe that organisms may never overcome the problem.

None the less, it should not be assumed that there is any "force of nature" driving simple living organisms towards higher complexity and the development of what we refer to as advanced life forms.  There is no evidence in nature for this assumption.  Complexity seems to occur because it can rather than because it needs to.

Question to think about:  How does this fit in with Darwinian Evolution?