The Oldest Evidence of the Rig Wete?

In the old days, a texts were written on birch bark. In northern india, hich is now gandar, kashmir, soganda and Cashmer. So i'm not sure what is the oldest, hard evidence of the rig wather is the material? Is the text dated back to 15 hundred b c? Maybe it is. Let's say it is. That is the oldt e chronology of the aren invesion myth.