Complex sentence: In a prologue, the Journalist states that--though few people in the late 19th century had even considered the possible existence of extraterrestrial life--planet Earth had long been enviously observed by advanced beings.
Split into simpler sentences: In a prologue the Journalist notes that, in the late 19th century, few people had even considered the possible existence of extraterrestrial life. And yet, the Earth had in fact long been enviously observed by advanced beings.