Leibniz's Protogaea, written between 1691 and 1693, and published posthumously in 1749, witnesses the interest that this great scientist and philosopher had, as many scholars of his time, for the history of the Earth and the indentification of "Fossil objects" as organic remains. A manuscript written by him in French and recently discovered in Leibniz' archives at Hannover surprisingly gives evidence to the fact that Leibniz had previously rejected the idea of the organic origin of fossils and of the long duration of the history of the Globe. Referring both to the Holy Scriptures and to the Galilean principles of mechanics. Leibniz, probably under the influence of Kircher and of traditional Renaissance beliefs, argues in this text that "fossil objects" are "sports of nature", produced by the Earth itself. This note, very likely to have been written during his youth, puts into light an interesting shift in Leibniz's thought, and may explain his energy in fighting these ideas in his Protogaea. Leibniz's "conversion"--which probably occurred in 1678--to Steno's ideas on the organic origin of fossils and on the history of the Earth, is contemporary with an important paradigm shift in the European natural sciences at the turn of the century.