In a world of increasing segmentation, allow me to add a couple of segments.

As I see it, the world can be divided into two distinct groups — people who were alive on June 20, 1975, and people who don’t know what it was like to live in a world without summer blockbusters.

Yes, there was a time without summer blockbusters. That time was June 19, 1975.

The next day, Steven Spielberg released a mechanical shark into an ocean of unsuspecting moviegoers and changed moviegoing habits forever. It may have changed swimming habits as well.

Of course, not everyone thinks that what the director did was such a wonderful thing. In fact, so many people have criticized the commercialization of the summer movie season that Spielberg has bent over backward to disavow any responsibility in the matter.

On more than one occasion, he has jokingly blamed the phenomenon on his good friend George Lucas, but the latter filmmaker is quick to point out that his "Star Wars" did not hit theaters until May 25, 1977.

It is a point well taken. You can’t blame event movies on a guy who released an event movie nearly two years after the first event movie was released by another guy.

Nothing has been the same since "Jaws" terrified a nation, and in doing so, gave Hollywood an idea that just won’t go away.

Remember the old days when summer began when the calendar said it began, in the third week of June? To illustrate how far we’ve come, the summer movie season begins this year today, when "Iron Man 3" opens in what could only be called a major event.

Big summer movie openings are scheduled two years in advance, with dates cast in stone, leaving lesser movies to scramble for theaters. Promotional campaigns can begin up to a year before the opening, with the intent of building anticipation. The studios do so by showcasing their summer movies during sporting events, movie festivals and multiple premieres. Robert Downey Jr., the star of "Iron Man 3," has been introducing his movie at premieres around the globe in the weeks leading up to its U.S. opening. The advertising juggernaut has hit on TV and in movie trailers. Print guys will write endless stories on the importance of summer movies.

But "Iron Man 3" does not have a lock on the summer’s biggest box office. There is plenty of competition.

Among other potential blockbusters are "Star Trek Into Darkness" (May 17), "The Hangover Part III" (May 24), "Man of Steel" (June 14), "World War Z" (June 21), "Pacific Rim" (July 12), "The Wolverine" (July 26) and "Elysium" (Aug. 9).

Each of these films has the potential to be the biggest of the summer, or the biggest disappointment.

When you look at our list of the biggest movies of the past 30 summers, it is obvious that hype can translate directly to money. But hype guarantees nothing. A movie clicks with the public, or it doesn’t.

But the list above of moneymakers does guarantee one thing: That there is no going back to a more innocent time. Summer blockbusters are here to stay. There is too much money involved to go back.