A male apprentice, long-time companion and possible lover of Leonardo da Vinci may have been the main influence and a model for the Mona Lisa.
But Silvano Vinceti said the portrait also represented a synthesis of da Vinci's scientific, artistic and philosophical beliefs.
Because the artist worked on it at various intervals for many years, he was subjected to different influences and sources of inspiration, and the canvas was full of hidden symbolic meanings.
"The Mona Lisa must be read at various levels, not just as a portrait," Mr Vinceti said.
The male model is one of many theories to have circulated over the decades about the Mona Lisa and the meaning for her famously enigmatic smile. Others have said the painting was a disguised self-portrait, but most art scholars insist it is the depiction of a Florentine merchant's wife.
The world-famous portrait is on display at the Louvre Museum in Paris.
The apprentice Gian Giacomo Caprotti, known as Salai, worked with da Vinci for more than 20 years starting in 1490.
Mr Vinceti described their relationship as "ambiguous" and most art historians agree he was the artist's lover.
Several da Vinci works, including St John the Baptist and a lesser-known drawing called Angel Incarnate, were based on Salai, Mr Vinceti says. These paintings show a slender, effeminate young man with long auburn curls.
Mr Vinceti said similarities with the Mona Lisa were striking.
"Salai was a favourite model for Leonardo," he said. "Leonardo certainly inserted characteristics of Salai in the last version of the Mona Lisa."
However, expert Pietro Marani called the theory "groundless".