Should we "play God?" We might expect Humanists, having accepted that there is no divine creator, shepherd, and purpose-giver, to respond affirmatively. However, I contend that many humanists, though pro-reason, science, and technology, and though opposed to many religion-inspired dogmas, still fear their own Promethean urge to challenge the gods.
This fear shows itself especially in the common (though not universal) humanist reaction to the possibility of the technological achievement of physical immortality or agelessness. Many humanists, even if they grant the possibility of such a monumental scientific accomplishment, shrink from this prospect. "It's unnatural." "Life without death would be meaningless." "I don't want to live longer than my allotted time." Not only physical immortality, but also the acquisition of superhuman (or posthuman) intelligence and ability they view with fear and trembling. Many episodes of the humanist Star Trek series embody these attitudes: Transcending the merely human always brings disaster, starting with the 2nd episode, "Where No Man Has Gone Before."
Such tales smell as rotten to me as those of Icarus, Frankenstein, and the Tower of Babel: Humans should just accept their limits. Don't build wings! Don't build towers that penetrate the heavens! Don't try to conquer aging and death! Cure the sick, but don't strengthen the healthy!