The earliest surviving traces of Leonardo's hand – and affection for animals – would seem to be found in the Tobias and the Angel (c. 1472–73), now in the National Gallery, London (fig. 5), a collaborative picture from the Verrocchio shop. The participation of two hands in the work's execution has long been apparent, in light of the stylistic contrast between the sculptural rigidity of the anatomy and costumes of the figures and the more freely rendered dog and fish. The wavelike patterns of the dog's unruly fur compare closely to the many sketches of the movement of water and hair that soon after streamed from Leonardo's pen. The dead-eyed helplessness and glistening scales of the carp (or tinca), gasping for breath, suggest the touch of someone who studied aquatic life closely. Famously able to survive for long periods out of water, the common river fish would have provided a live (and fresh-smelling) model for hours before becoming a meal.

An appreciative Verrocchio may have permitted Leonardo's brush to delineate as well the lively, luminescent curls of Tobias's hair and the intricate, knotted and fluttering belts and hems of the figures’ garments. Leonardo no doubt distinguished himself in the studio in such subtle application of highlights and in the other remarkable effects of illumination he could attain in the mixed media of tempera and oil paint, then relatively new to Florence. Consequently, he became the person called on to add those last, gilt-edged details, the final polish, before the works left the shop.