What is Life?

Life is hard to define.  Perhaps former Supreme Court Justice Potter Stewart summed it up best (though he was referring to hard-core pornography, not living organisms) when he said, "I shall not today attempt further to define [it]... and perhaps I could never succeed in intelligibly doing so. But I know it when I see it."  

Erwin Schrodinger gave a series of lectures in 1943 that were published under the title "What is Life?" 

NASA has an Astrobiology group that is charged with searching for evidence that there is life on other planets.  Key to that mission is understanding what life is.  How else would they know what to look for?

Up until today, living organisms were thought to require four elements:  oxygen, carbon, hydrogen, nitrogen, sulfur, and phosphorus.  A paper published today by the NASA Astrobiology group has challenged that belief.  NASA scientists discovered a bacterium that can live without phosphorus, using arsenic instead.

The bacterium, designated GFAJ-1, can build the molecules of life, including DNA, proteins, and the energy molecule ATP, with either phosphorus or arsenic.  This is very handy, considering the organism was found in a lake with an extraordinarily high concentration of arsenic in its water.

Up until this discovery, life forms were the stuff of science fiction novels.  I've read novels about silicon-based life forms, based on the idea that silicon, being just below carbon on the periodic table, could take the place of carbon in the building blocks of life.  This is actually the basis for how GFAJ-1 uses arsenic.  Arsenic is just below phosphorus on the periodic table of the elements.  So the bacterium just substitutes arsenic for phosphorus.

This finding has important implications for the search for life away from earth.  Clearly phosphorus is not a requirement for life.  Perhaps neither is carbon.  Maybe not even oxygen is needed.  Life can take many forms, so finding it is going to require knowing it when we see it.

Question to ponder:  if ATP is the main energy molecule in a cell, and the P stands for phosphate, does GFAJ-1 have ATA instead?