Part of the problem is that the Renaissance does not begin everywhere all at once. These phases represent trends in art and are tied to specific artists who tried something "new" and whose work influenced the style of others. The Proto-Renaissance in Italian art was dominated by two figures: Cimabue and Duccio of Siena (his Madonna and Child, now in the London National Gallery, is shown above). Along with two contemporaries, Guido of Siena and Coppo di Marcovaldo, they seem to have been influenced by the unknown the so-called Master of St Bernardino. They specialized in stylized religious paintings in which the angle of the head and position of the hands, for instance, were determined by traditional icon paintings in the Byzantine style.
Proto-Renaissance painting was dominated by religious art. During this Proto period the Black Death inspired a change in theme to the need to approach death in a state of penitence; images of death and the torments of Hell began to dominate church art. More than one painting is named "Triumph of Death" from this era.
Much of this was happening in Siena, ruled by a republic since 1125. I'd like to talk about its history next time.