The cosmic abundance of atoms tends to fall steadily with 

 atomic number. None of the key atoms in the biopolymers of early 

 life is heavier than sulfur, atomic number 16. Living forms do make 

 some use of a good many elements in addition to the major constit- 

 uents of their biopolymers. Every shelly or bony creature uses cal- 

 cium, while some forms have skeletal frameworks of silica; these are 

 specialized structures, but they all use elements rather common in 

 the surface minerals of Earth. Among the most abundant of minor 

 atoms are the volatile elements of the mineral world which are easily 

 outgassed, dissolved by water, and weathered out of the rocks to salt 

 the sea and make up the fundamental electrolyte solutions within all 

 living cells. These are magnesium, sodium, potassium, calcium, and 

 chlorine (chlorine is in fact rarer than the others listed). Next most 

 important is iron, a rather heavy atom (atomic number 26), which, 

 because of its intrinsic nuclear stability, is unusually abundant in the 

 cosmos for its weight. The iron atom plays a central role in life today 

 within a number of indispensable metal-containing organic struc- 

 tures, the blood-red pigments of course, but others as well. Similar 

 roles can be played by less common metal atoms, e.g., copper, 

 cobalt, zinc, manganese, even molybdenum, and vanadium. These are 

 by no means common elements, but their exploitation by life seems 

 to be an opportunism of natural selection, making use even of a trace 

 of some rare atom to take advantage of its special properties. The 

 secondary elements in life, in addition to hydrogen (H), carbon (C), 

 oxygen (O), nitrogen (N), phosphorus (P), and sulfur (S), amount at 

 most to one or two atoms in a hundred; no living species is known to 

 require any element heavier than iodine (atomic number 53). The 

 vital iron atom is the most abundant heavier atom in the human 

 body, yet there is in the body only about one iron atom for every 

 15,000 of carbon! In all of biology only humans make any use of the 

 rare heavy metals, like lead, gold, and uranium; that use is not 

 biological, but cultural. 



WATER 



The two most abundant compound-forming atoms in the cos- 

 mos are hydrogen and oxygen. Their most familiar compound, water, 

 is widespread. But it is grains of solid ice and very dilute water vapor 



