What is the proof that Iago is jealous of Desdemona in Shakespeare's Othello?

Léone Teyssandier writes that a possible motive for his actions is jealousy towards Desdemona, Cassio, and Othello. Iago perceived them as more noble, generous and, in the case of Cassio, more handsome than him.[4] In particular, he sees the death of Cassio as a necessity, saying of him that "He hath a daily beauty in his life that makes me ugly".[5]

Ultimately, none of these motives is identified as primary, so it is impossible to determine conclusively which applies, if indeed any of them do in isolation, or which is most important among them.

Andy Serkis, who in 2002 portrayed Iago at the Royal Exchange Theatre in Manchester, wrote in his memoir Gollum: How We Made Movie Magic, that:

“ There are a million theories to Iago's motivations, but I believed that Iago was once a good soldier, a great man's man to have around, a bit of a laugh, who feels betrayed, gets jealous of his friend, wants to mess it up for him, enjoys causing him pain, makes a choice to channel all his creative energy into the destruction of this human being, and becomes completely addicted to the power he wields over him. I didn't want to play him as initially malevolent. He's not the Devil. He's you or me feeling jealous and not being able to control our feelings.

http://en.wikipedia.org/wiki/Iago#Motives

Iago Jealousy

The thing about Iago is this – we never really know for certain why it is that Iago wants to destroy Othello.

Since I came here from the country, I have not seen her close. Tell me, is-- is she beautiful? Thomas, if I could write with the beauty of her eyes, I was born to look in them and know myself. A-A-And her lips? Her lips? The early morning rose would whither on the branch if it could feel envy. And her voice, like lark's song? Deeper, softer. None of your twittering larks. I would banish nightingales from her garden before they interrupt her song. Oh, she sings too? - Constantly. Without doubt. And plays the lute. She has a natural ear. And her bosom. Did I mention her bosom? What of her bosom? Oh, Thomas, a pair of pippins... as round and rare as golden apples. I think milady is wise to keep your love at a distance. For what lady could live up to it close to... when her eyes and lips and voice may be no more beautiful than mine. ---------william Shakespeare---'shakespeare in love'(the new juliet)