When was potato discovered to be edible?

Answer 1

The tough pre-Columbian farmers first discovered and cultivated the potato some 7,000 years ago. They were impressed by its ruggedness, storage quality and its nutritional value. Western man did not come in contact with the potato until as late as 1537 when the Conquistadors tramped through Peru. And it was even later, about 1570, that the first potato made its way across the Atlantic to make a start on the continent of Europe.

Taken from http://www.indepthinfo.com/potato/history.shtml

Answer 2

In prehistory. Hunter-gatherers dug up a lot of roots and plants, and discovered a number of tubers that were edible. The potato was only one of many.