In this clade , Fabales were the first lineage to branch off ; thus , the ability to fix nitrogen may be plesiomorphic and subsequently lost in most descendants of the original nitrogen-fixing plant ; however , it may be that the basic genetic and physiological requirements were present in an incipient state in the last common ancestors of all these plants , but only evolved to full function in some of them : There are also several nitrogen-fixing symbiotic associations that involve cyanobacteria ( such as Nostoc ) :
In this clade , Fabales were the first lineage to branch off ; thus , the ability to fix nitrogen was lost in most descendants of the original nitrogen-fixing plant .