Although Human, Dr. Jones had never visited the planet Earth; she did however spend four years on Vulcan. While on Vulcan, she studied Vulcan mental discipline to turn her telepathic abilities "off" in order to maintain her sanity.
In 2268, she was given an assignment to coordinate with the Medusan Ambassador Kollos after the initial offer to Commander Spock was turned down. Her assignment's success was based upon her ability to achieve a true mind-link.
Jones' assistant on her mission was Larry Marvick, who was in love with the doctor. Dr. Jones, however, wished to avoid the struggle of Human emotion and therefore could not love Marvick the way he wanted her to.
On Stardate 5630.7, Dr. Jones, Marvick and Kollos were transported aboard the USS Enterprise. Aboard the Enterprise, Jones exhibited signs of jealousy towards Spock, because of his higher telepathic ability, whenever he was with Kollos.
Meanwhile, Marvick's jealously towards Kollos, as he felt Dr. Jones' mission with Kollos was taking her from him, grew to the point where he attempted to kill Kollos. After Marvick's failed murder attempt, and subsequent insanity and death, Jones used this as an example of why she disliked Human companionship. She viewed Human emotion as a struggle, and was willing to spend the rest of her life with the Medusans in order to avoid it.
After Spock was accidentally exposed to Kollos' true appearance, Captain Kirk forced her to put aside her jealousy so she could use her telepathic abilities to help cure Spock of his resulting insanity. (TOS: "Is There in Truth No Beauty?")