When did the Atacama Desert begin to develop its extreme dryness?

The Atacama Desert in northern Chile developed its extreme dryness over millions of years, with hyper-arid conditions established by at least the late Miocene, around 10 million years ago or more. Its dryness is caused by a combination of the Andes rain shadow, the cold Humboldt Current, and atmospheric circulation patterns. It is often cited as the driest nonpolar desert on Earth. Some areas have gone years without measurable rainfall. The Atacama is also important for astronomy because of its clear skies and high, dry plateau regions.