No one really knows why or how the first vowel shifted. Some speculate that it was because of fashionable imitation of an admired or powerful person as is alleged to be the origin of the Spanish lisp. This is in fact the correct answer. People love to imitate and pretend to be something they are not.
The Spanish were lucky. Many now extinct tribes have copied some terrible and pernicious traits. The Hen people of Ailsa Craig, Scotland had a glorious and fabulous leader in Morag. She was kind, beautiful, intelligent and benevolent, but had a habit of pecking people on the head. One person doing this would have been fine, but the entire population decided to emulate their Queen and soon became extinct from brain aneurysms.
The Tabbies of Farne Island, England looked up to Peter the Cheetah for his wisdom, sense of justice and lithe graceful movements, but soon died out for copycatting his preponderance for late night wailing and territory marking sessions.
On Samson Island, UK the Barnetmen followed a priest named Michael Bolton to their deaths for spending too much time on their preposterously elaborate hairdos.
Perhaps the most senseless waste of life was witnessed on Goat Island, New York. The most admired person here was Brentford Ulysses Brentford Xavier Brentford Nylons the Third. He was the islands top Bladder Ball player and charming, generous and gallant to a fault. But Brent had the nasty habit of getting drunk after games and making animal shapes with his scrotum. The Bat, Bunny and Rattlesnake were great crowd pleasers but the Goat was his trademark. This involved tucking his entire package between his legs so that his member and scrunched sack could be seen protruding from his backside, vaguely resembling an annoyed ram. Once again the people of Goat Island imitated the wrong trait and soon the male islanders as a source of pride and prowess would walk about with their tackle trussed up. This severely damaged sperm production and they were wiped out within a generation.
If history teaches us anything (and it doesn’t) then it is to be yourself.