Short answers, pulled from the story.
The word jealousy stems from the French jalousie, formed from jaloux and further from Low Latin zelosus. This Latin term traces back to the Greek word zēlos, which sometimes means jealousy but more often signifies emulation, ardour, or zeal.
Evolutionary psychology suggests that jealousy evolved to maximize the success of our genes by fostering certainty about the paternity of one's own offspring. It serves as a biologically based emotion selected to avoid sexual betrayal and ensure access to the best reproductive partners.
Research by Sybil Hart at Texas Tech University indicates children are capable of feeling and displaying jealousy at as young as six months. Infants showed signs of distress when their mothers focused their attention on a lifelike doll.
A study was done to cross examine jealousy among four different cultures: Ireland, Thailand, India and the United States. The survey found that Thais are less likely to express jealousy than the other three cultures because men in these cultures are rewarded for showing jealousy due to the fact that some women interpret it as love.
Out of the 230 instances identified in the 51 top-grossing romantic comedies from 2002 to 2014, 58% were reactive jealousy while 31% showed possessive jealousy. The last 11% displayed anxious jealousy seen least in all cases.