It is hypothesized that RNA may have evolved before DNA because RNA molecules were the first molecules synthesized by Miller and Urey
are more important to genetic inheritance than DNA
can catalyze a variety of biologically important reactions
are found in more modern cells than are DNA molecules

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.