A riddle

Leonardo often used the first and/or last page of a notebook to write down miscellaneous thoughts such as lists of things to do. On the last page of the Codex Madrid I (folio 191v) he leaves us a riddle:

I am he who was born before the father
I killed one third of humanity
and then returned to the womb of my mother

To solve the riddle it may help to look at the Italian:

I' son colui che nacqui
inanzi al padre,
la terza parte delli omini ucisi,
po' tornai nel ventre alla
mia madre.

So the middle line has been translated "I killed one third of the human race." While he doesn't leave the answer, scholars think they've figured it out. Before you look at the answer (below) here's a clue:

Source: Woodcut from Anonimo fiorentino c.1500 (Incisioni Fiorentine del Quattrocento a cura di Lamberto Donati, Istituto Italiano d'Arti Grafiche editore Bergamo, 1944, pl.58).

The answer is Cain, whose father Adam was not technically born. When he killed his brother Abel he killed one third of all men. Then he died and returned to the womb of his mother, the earth.