For a little while now, say 200,000 years or so, Gaia has felt that the latest mammalian climax paradigm has missed the mark. Admittedly a better craft than the dinosaurs, and much smarter and smaller than the giant sloth and its compatriots, the current mammalian lot was STILL drifting into situations of decreasing global biomass. The predator/herbivore plan was almost, but not quite, sustainable.
Time to retool. The next total revision would require an incubation environment of unprecedented regulation: the broad green leaves of the deep forests would be accented jewellike by a warm showery rain softly falling to the moss-covered loamy glades below. No time to lose: the sun wasn't getting any cooler. This time, maybe instead of drawing down the greenhouse gases and depositing them underground, maybe just tying all the organic materials into the biota would provide enough shade that the sun's heat could be controlled. Certainly worth a try.
There was certainly enough oxygen and hydrogen as raw materials; oceans full of it. Nitrogen, too, composed most of the air.
But not really enough Carbon. It would take a LOT of carbon to make all that cellulose. What was needed, was the unprecedented development of a climax mammal that would bring the huge deposits of oil and coal that had been previously sequestered deep within the earth, to the surface. The form the carbon took in the bios would not matter, the bulk of it would revert to usable forms within a few hundred years at most. But, sadly, after the mammal performed the orgasmic climax task for which it had been designed, it would have to gracefully leave the scene. The danger of course was that an animal with enough intelligence to perform such a task might also see through the mission of death it was called upon to undertake, and refuse!
[this is of course a flight of fancy; prompted by the aggravation I felt when someone told me, "The purpose of Man? Why, to make plastic, of course!"]