...That the U. S. government has been engaged in a series of terrifying trans-human experiments for a staggeringly long period of time; that, after all, would be nothing but the worst sort of alarmist conspiracy theorizing.
--But I've got to point that pretty much from the very start, your mail has been delivered by a "post-man" or even, in recent decades, a "post-woman." And most of them appear to be inhumanly fit, too (or perhaps merely some, per dispute in comments -- though I will suggest that "a graying hippie" with 1860s hairstyle and glasses, who "wears shorts in all weather" could very well be an example of just that sort of thing!).
Food for thought?
6 months ago