Re: Could advanced life forms evolve in reduced atmospere?

hi Tomas,

This is a very interesting question. The short answer is "we believe that single-celled microbes could exist in the conditions you describe, but not more complex organisms".

One of the most interesting findings in biology of recent years is that the range of conditions which can support life are much wider than we used to believe. We know of microorganisms that can grow from 121 °C to well below 0 °C, in 5 M NaCl solution and across the full pH range from 0-14. We have also discovered incredible versatility in microbial metabolism. There are microbes known as methanogens, for instance, which can live around thermal vents near deep oceanic trenches and not only tolerate high temperatures, high pressures and high concentrations of toxic metals, but can synthesise everything they need for growth and reproduction from only carbon dioxide, hydrogen and a few simple salts. You mention anaerobic bacterial metabolism, which I assume to mean fermentation and indeed this type of respiration gives a very low energy yield. However, many microorganisms use a molecule other than oxygen for coupled respiration (the methanogens for instance reduce CO₂ to methane instead of O₂ to water) and this can yield plenty of energy.

Astrobiologists are very excited by these findings and have speculated that we may find microbial life on Mars, Europa (a moon of Jupiter) or even Titan (a moon of Saturn). However, so far as we know, there are still some fundamental limitations to where life can exist. One is that liquid water is essential - for the diffusion of molecules into and around the cell such that chemistry can occur, so that implies an ambient temperature of around 0-100 °C. Another is some form of biogeochemical cycling. On Earth, over long periods of time (tens of millions of years), all of the nutrients required for life are cycled through the planet as the earth's crust is subsumed at subduction zones and spat out again through volcanic vents. So we would expect that for life to exist on a long-term basis, a cycling mechanism for nutrients is required.

Another limitation is that on Earth, so far as we know, respiratory metabolism using simple compounds is limited to microorganisms like the methanogens. By "advanced life" I guess you mean single-celled eukaryotes or multicellular life forms of some description, perhaps even something like ourselves. These more advanced organisms seem to be less tolerant of extreme conditions and less versatile in their metabolism than the prokaryotes (the bacteria and archaea).

Of course, it's quite possible that there are organisms on some distant planet, we may not even recognise them as alive, but that's the joy of being a science fiction writer - it allows us to imagine such things! My personal feeling is that microbial life may have existed on Mars (and just might today) and if we believe that biology is governed by the laws of physics and chemistry, it's almost certain to have originated elsewhere in the universe.

Here are some useful links for you:

ArchaeaWeb has information and links about Archaea and extremophiles

Methanogens

Hydrothermal vents, and search Google with this phrase for lots more cool stuff

Astrobiology at NASA

Astrobiology Web and Astrobiology Magazine.