Imagine there's no Beatles. It's impossible, even if you try. Their music is too well known and too deeply loved. When Paul McCartney sang "Hey Jude" at the opening of the London Olympics two weeks ago, people around the world sang along — they all knew the melody and the words. What other band in history ever inspired such universal and ardent response?

This year is being celebrated as their 50th anniversary. According to Beatles lore, 50 years ago in August, drummer Ringo Starr played for the first time with the other three members, John Lennon, who was tragically shot and killed in 1980, Paul McCartney and George Harrison, who died of cancer in 2001. The rest, as is said about so many events with less global impact, is history.

The Beatles are perhaps the single most popular musical group, and by most accounts, The Beatles remain the best-selling band, ever. Though they played together for less than a decade, they changed the direction of popular music with some 300 moving, memorable songs that transformed how we think and feel about music. Music company executives might see them as the first globally marketed pop group, but it would be more accurate to say Beatlemania swept the world on its own.