In order to prevent the incursions of the dreadful Thread, the Pernese, with the ingenuity of their forgotten Terran forebears, developed a highly specialized variety of life-form indigenous to their adopted planet. To all Human beings what were affected were animals with an unusual ability to teleport such that when they pressed to maintain pern on Earth, such humans as had a high empathy rating and some innate telepathic ability were trained to use and preserve this unusual animal whose ability to teleport was of great value in the fierce struggle to keep Pern bare of Threads.

What is

certain about the second sentence in the introduction of such humans as had a high empathy rating and some innate telepathic ability?

Is there a way to make a good video?