Originally, dinner referred to the first meal of a two-meal day, a heavy meal occurring about noon.
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In Europe the fashionable hour for dinner began to be incrementally postponed during the 18th century, to two and three in the afternoon, until at the time of the First French Empire an English traveler to Paris remarked upon the "abominable habit of dining as late as seven in the evening".
India also has hot meals during lunchtime (nice story at http://37signals.com/svn/posts/2882-the-incredible-delivery-...)
I think it made sense to have a hot meal during lunchtime in places where the sun may set early at night because cooking in the relative dark would be more dangerous than cooking during daylight (swaying around a burning stick inside a wooden house to search for ingredients, spare wood, etc, is not a good idea)
English-speaking cultures use the word "dinner" for the main meal of the day whenever it happens. Cultures that eat breakfast, dinner, and supper have a large midday meal and a small evening meal. Cultures that eat breakfast, lunch, and dinner are the other way around. It just so happens that the English-speaking cultures that make more movies and TV shows (namely London and the coasts) are breakfast-lunch-dinner cultures, so people assume this is normal everywhere.