THE ROOTS of Carnival and our modern Mardi Gras go back before there was a New Orleans, before there was a United States, before the birth of Jesus Christ, all the way back to the pagan festivals of the ancients.
Although the names, dates, traditions, and areas change, the universality of what we now know as Carnivaltime goes back to the earliest civilizations.   From the Persians to the ancient Egyptians, Romans and Greeks, to western Europe and then to the Americas, the need to celebrate life and all its joys permeates the human experience.
While we cannot adequately discuss all the parts of history that contributed to Mardi Gras, consider the following a kind of beginner's manual.  Enjoy!

Your humble Professor
We begin in Persia, where the annual celebration of the completed year was marked with a festival called Sacaea.  As the old year died away, Persians greeted the new year by relaxing the rules of order.  Masters and slaves exchanged places, and a mock king was elected to rule over the masquerades that would spill out into the streets (very similar to the European tradition of the Lord of Misrule).  A good deal of Carnival comes from the Roman festivals of Lupercalia and Saturnalia.  Now, even the Romans were a bit iffy on just which gods were being honored with these festival (things got a bit sloppy with the Romans, what with all that declining and falling), but the whole of Lupercalia centered around a cave on Palatine Hill, the lupercal.  According to legend, this was the cave where the lost twins Romulus and Remus were nursed by a she-wolf and saved from starvation (above). Romulus was the founder of Rome, and Remus...well, the less said about him the better.  Originally, Lupercalia was for purification and fertility, and sacrifices of goats and a dog were made to the gods.  The priests, or lupercai, would run about in loincloths, slapping everyone with strips of goat skin because it was considered the thing to do to be fertile.  There was also lots of drinking, which is to be expected when the highlight of your festival is being slapped with goat strips.
Then, there was Saturnalia.  This festival centered around the statue at the Temple of Saturn, the god of agriculture.  The statue was hollow and filled with olive oil (a symbol of his, pardon the pun, 'roots').  The feet of the statue were bound with woolen strips that were unbound for the festival.  After the rituals, the Senators would begin the celebrating with the cry of "lo, Saturnalia!" which seemed to sum it up just fine.  They also had a Lord of Misrule, borrowed from the Persians.  Saturnalites would decorate their houses, walls, and doors with great swaths of greenery, and outside plants with festive ornaments of sun faces, stars, and the faces of the god Janus.  Gold was the colour of choice, and everything and everyone was bedecked in gilded beads, sun heads, stars, and the occasional hapless napper!  Small gifts of silver, wax tapers, and little poppets were exchanged, and families came together for private celebrations.
 
Not to be outdone, the Greeks celebrated Dionysus, the god of wine and drama.  Lots and lots of wine.  And drama.  And more wine so they could get through all the drama.  Even the grape vines got wine, sprinkled onto the pruned ends to ensure a bountiful harvest for the next celebration.  Shortly afterwards, however, the festival of Februalia would begin, a month-long period of sacrifice and atonement to the god of purification, Februus, who lived in the Underworld.  Februus was a god the Greeks "borrowed" from the conquered Sabines, and a very popular god indeed.  So popular the Romans named the second month for him.  Eventually, Februus was promoted to King of the Underworld and immediately changed his name to Pluto.  This celebration closely resembles the beginnings of modern Lent.  Much later, when the Roman and Greek traditons began to mix, Februus became Juno Februa, and was now a goddess of love and, of course, fertility.  As Juno Februa, the festival took on many of the characteristics of our Valentine's Day, and very well may qualify Februus as the first Carnival cross-dresser.