T. rex and the Dodo Bird
Survival of the Fittest(?) Smartest?... not really
Mother nature is rather capricious in her almost noncholant manner with which she "chooses" particular species for extinction.
The "choosing" is so thoughtless in its randomness, that one has to question whether mother nature ( or whoever or whatever controls her) even cares. T. Rex and all the other dinossaur specie lorded it over the entire earth for millions of years, but in one clean swoop ( an asteroid hitting earth?) they disappeared leaving not much of a trace of their existence except for random fossils scattered all over the place. Raphus Cuccullatus (the lowly DoDo bird) did not lord it over their island habitat of Mauritius, but seemed to be doing just fine, until Homo Sapiens arrived and with them, dogs, swine, and calculus.... then its earthly existence was no more
So what would it take for any specie to survive the whims and wrath of an uncaring environment? Brains,one might conjecture, but humans with all their daunting intelligence could find themselves holding the short sick of the extinction process, notwithstanding their vainglorious perception that they are the "annointed ones", because they were formed in the exact image of a Godly creator.
For any specie to continue making it within the realistic bounds of space-time continuum, it has to be well suited for communal and cooperative existence. Humans are a cooperative specie (and so they say), and they have shown that they can go by within the barest minimum of communal living( a concept called communism) , but the practical application of the elegant equation E=mc2 continues to hang precariously above their collective heads like the mythical sword of Damocles. Anytime, they could find themselves evaporated in an instant via multiple atomic fireballs.
Other specie may be more suited to survive the nuclear holocaust that man could impose on all earthly life. Insects for example, specifically the social insects. If, as is posited by scientist, nuclear winter would result from such multiple atomic fireballs, the social insects i.e. ants and bees whose absolute numerical superiority alone could assuage their survival beyond such a world-wide. deep freeze. Add to this their superior, however, minimalist bodily design, the ants and the bees and similar socially preoccupied, cooperatively predisposed, and communally prescribed insects could rule the earth, long after man has left the scene, with a few scattered fossilized remains, the only evidence of their earthly presence. Not that I suppose the ants and the bees would place any importance to them.