Besides opening a debate on the criteria for historical greatness, the BBC's Great Britons has revealed something about our need for, and obsession with, "heroic" figures. Leonardo da Vinci's victory in one of the spate of "man of the millennium" polls three years ago reminds us that the Italian Renaissance remains a privileged site for the sort of cultural heroics that the BBC series celebrates. The 15th and 16th centuries have often been viewed as a period of glorious individuality in which a series of artistic supermen - Leonardo, Michelangelo, Raphael - emerged in unfailing succession. More plausible explanations of this undeniably creative chapter in the history of art point, however, to somewhat different roles for its creative geniuses.

The whole notion of individual genius and lone enterprise in the Italian Renaissance is belied by both the co-operative nature of workshop production and the technical complexities of the finished products themselves. As Jerry Brotton illustrates in his recent book The Renaissance Bazaar (Oxford University Press), a work such as the Linaiuoli Altarpiece, painted by Fra Angelico in 1432, was really a group effort rather than the creation of a single man, the result of the painter's collaboration with carpenters, stonecarvers and another well-known artist, the goldsmith Lorenzo Ghiberti, himself the head of a team of roughly 20 workers. It is a modern conceit, Brotton points out, to ascribe the work to Fra Angelico alone.

This pooling of resources can be seen even in the case of one of the most enduring Renaissance myths of individual genius, Michelangelo's fresco on the ceiling of the Sistine Chapel in Rome. Goethe once wrote that without seeing the vault of the Sistine Chapel, it was impossible to understand what one man is capable of achieving. However, when Michelangelo was commissioned to paint the vault of the chapel in the spring of 1508, the first thing he did was to set about recruiting a team of assistants. As many as a dozen men ultimately took active part in executing the work - a small cast of characters whose contributions to the enormous fresco have been eclipsed by the towering reputation of Michelangelo.

An intimidatingly difficult and laborious technique, fresco-painting was of necessity a co-operative venture, calling for many hours of preparation in the studio and on the scaffold before the first brushstroke of paint could be applied to wall or vault. Popularised in Italy in the 13th century, the technique required the painter to conceive of his work as a whole but then execute it in patchwork fashion, in a series of small fields of wet plaster - known as giornate, or "day's work" - laid at the start of each working day. Because of its myriad difficulties, one of Michelangelo's contemporaries, Giovanni Paolo Lomazzo, described fresco-making as a manly pursuit, in comparison with which tempera painting was the domain of "effeminate young men".

In 1508, the 33-year-old Michelangelo was relatively inexperienced in fresco, having made his name almost exclusively as a sculptor, with works such as the Pieta and the David. As an adolescent in the 1480s, he had served as an apprentice with one of Florence's greatest masters of fresco, Domenico Ghirlandaio, at a time when Ghirlandaio was executing his magnum opus, the Scenes from the Lives of the Virgin and of St John the Baptist, for the Tornabuoni Chapel in Santa Maria Novella, Florence. This fresco, the largest of the 15th century, covered 5,900 square feet; it took Ghirlandaio and his small army of assistants and apprentices more than four years to complete it. Given that the vault of the Sistine Chapel was exactly double the size of the Tornabuoni, and that by 1508 he had not worked in fresco for almost 20 years, Michelangelo was clearly in great need of a team of able assistants.

The task of recruiting these helpers was entrusted by Michelangelo to his oldest and most trusted friend, another former pupil from Ghirlandaio's workshop, a 39-year-old Florentine painter named Francesco Granacci. The faithful Granacci was an undistinguished painter himself (his carefree way of living was little suited to the discipline of fresco), but he found in the workshops of Florence four painters willing to travel to Rome to assist in the Sistine Chapel: Bastiano da Sangallo, Giuliano Bugiardini, Agnolo di Donnino and Jacopo del Tedesco. Not exactly household names even in Florence, these men were none the less competent frescoists who had learnt their trade in the studios either of Ghirlandaio or of another Florentine master, Cosimo Rosselli. A number were even veterans of the Tornabuoni Chapel, where, it is to be supposed, they had worked side by side on the scaffold with the young Michelangelo. And all of them, crucially, had more recent experience with fresco than Michelangelo.

Other helpers for the project were also assembled, including a carpenter, Piero Basso, and a Florentine sculptor and architect named Piero Rosselli, both of whom assisted with the construction of the scaffold. In addition, at least three other painters eventually joined the team, among them Jacopo Torni, known as Indaco, a good friend of Michelangelo, but a notorious gossip whose senseless chatter frequently drove the sculptor to distraction. Many of these men lived with Michelangelo in his workshop near the Piazza San Pietro, where his tetchy manner and slovenly habits seem to have been matters of some concern to his guests.

Apart from grinding colours, fixing cartoons to the vault, and in general offering their advice and expertise, the assistants painted numerous passages on the vault. Just as Sir Joshua Reynolds would delegate the backgrounds of his portraits to his youthful apprentices, so Michelangelo assigned large tracts of the fresco - especially the many repetitive architectural details - to his crew of Florentine assistants.

Few of these men were present for the whole period of four years that it took to design and paint the vault. One of the painters, Tedesco, departed after only a few months' work, following a disagreement with Michelangelo. Several of the others left the project after about a year - possibly due to Michelangelo's dissatisfaction with their work - to be replaced by a team of new recruits. Although art historians cannot be certain who painted precisely what part of the vault, the handiwork of the assistants is none the less evident throughout, though this is sometimes in a dramatically inferior style to that of Michelangelo as he found his feet in the difficult medium and began to paint with an inspired brio.

Despite their well-documented presence in the Sistine Chapel, Michelangelo's assistants have often been overpainted in the history, not least by Michelangelo himself in the version of events he later gave his two biographers, Giorgio Vasari and Ascanio Condivi. Writers of the Renaissance, such as Vasari, needed a myth to sustain a belief in the greatness of modern man, of his abilities to equal and even surpass the artists of the ancient world from whom those of the Renaissance took their cue. Michelangelo's work in the Sistine Chapel appeared to offer this evidence in spades, and so, obviously, the idea that the fresco was actually a bickering group effort held less appeal. And we, with our own fantasies about greatness, have frequently been only too happy to play along.

Michelangelo and the Pope's Ceiling by Ross King is published by Chatto & Windus (£17.99)