Some critics of the ‘Earlier Mona Lisa’ have doubted that it is by the hand of Leonardo, because it was executed on canvas. It is true that Leonardo’s “preferred medium” was wood. However:

1. ‘The Benois Madonna’, an authenticated Leonardo work at the Hermitage Museum, in Saint-Petersburg, Russia, was executed on canvas.
2. The execution of the ‘Earlier Mona Lisa’ occurred at a particular time in the history of art when the kinds of “supports” were in transition, and the use of canvas was starting to become popular. In 1501, Leonardo had just returned from Mantua (home of Mantegna who was promoting canvas as a support) and Venice (where canvas was then used by some prominent artists, such as Vittore Carpaccio).
3. There is evidence going back to his apprenticeship days in Verrocchio’s studio (mid-1470s), that Leonardo had painted on canvas before (see his drapery studies now in the Louvre Museum).
4. Leonardo gave specific instructions about how to paint on canvas in his ‘Trattato’.

These make it clear that it cannot be disproved to be a Leonardo because it is on canvas.

As a side note on this point, in 2010, Professor Martin Kemp published a book (‘La Bella Principessa’ – The Story of the New Masterpiece By Leonardo Da Vinci’) about a small work of art that he attributed to Leonardo da Vinci. The image is of a young girl, in profile, executed on vellum. It is the only known work by Leonardo on vellum and Professor Kemp argues that: “It testifies to his spectacular exploration and development of novel media, tackling each commission as a fresh technical and aesthetic challenge.” If this can justify Leonardo’s use of vellum, then it can probably also justify his use of canvas.