Humanity may devolve or evolve. Behavior becomes post-, whether ex- or super-, human.
(i) In SM Stirling's Emberverse, wild men attack outsiders, squealing a syllable that had once been the word, "Eat."
(ii) Morlocks eat Eloi.
(iii) In Olaf Stapledon's Last And First Men, human beings adapted to live on Neptune become quadrupedal and revert to animality. Periodically, a pack gathers in a circle to hear one of its members howl.
(iv) Poul Anderson's Time Patrol is founded by the post-human Danellians. One appears as a blazing form, another as a kindly monk. What is the reality behind the appearances?