Tal­mu­dic Ques­tion #15

It seems to me that there are two pos­si­bil­i­ties. (An aside, was the early earth rich or poor in avail­able N? It seems likely that it was rich in it, but it's not my area)
1) Mi­crobes evolved this func­tion early in evo­lu­tion­ary time, and formed the ba­sis for other or­gan­isms to use bi­o­log­i­cally fixed N — this, along with in­or­ganic processes, pro­vided suf­fi­cient N for other life. Other or­gan­isms that evolved found bi­o­log­i­cal N to be a non-lim­it­ing re­source. (per­haps Ni­tro­ge­nase evolved in an anoxic world, so the re­stric­tion on O2 was not an is­sue?)
2) Mi­crobes evolved this late, in a com­pe­ti­tion for niche space, uti­liz­ing their adapt­abil­ity to cre­ate a space for them­selves to live. Other or­gan­isms lack the ca­pac­ity to de­velop this spe­cial­ized func­tion (en­ergy in­ten­sive, re­quires anoxia, re­moval of H2 byprod­uct), or found it more eco­nom­i­cal to co-opt mi­cro­bial N fix­a­tion.
Mi­cro­scop­i­cally, or­gan­isms ac­quire nu­tri­ents "one at a time"; sep­a­rate CNPS sources and a sep­a­rate En­ergy source are plau­si­ble. Macro­scop­i­cally, plants stand out as ac­quir­ing nu­tri­ents sep­a­rately. They've co-opted mi­cro­bial N‑fixation.
Along these lines, is the sym­bio­some a pre­lude to an legume N‑fixing en­dosym­biont (a ni­tro­plast, or some more pleas­ant name).
A bit of a ram­bling com­ment, since I'm sup­posed to be writ­ing an exam 🙂

It seems that ni­tro­gen fix­a­tion was in­vented just once in 4 bil­lion years. No new ways to do it have been found by prokary­otes or eu­kary­otes since then.
It may not be pos­si­ble to re-in­vent it in the pres­ence of oxy­gen.