There is some confusion over what constitutes an ancient grain. Although the Whole Grains Council defines ancient grains loosely as grains that are largely unchanged over the last several hundred years, no clear definition exists. The term seems to be a marketing construct with little basis in history or science.
Many of the commodities which are categorised as ancient grains are, in fact, no older than grains such as wheat or oats.
If we look at what has changed and what has not changed, the picture becomes somewhat clearer. Despite having roots going back into antiquity, modern common varieties of wheat, rice and corn are not ancient grains. These modern grains are products of thousands of years of selective breeding for rapid growth and easier milling.
In contrast, the pseudo-grain, amaranth is a good example of what is considered an ancient grain. It was a staple of the Aztec civilisation prior to colonisation. Following the Spanish conquest, cultivation of the crop was outlawed. Having largely maintained its genetic base, amaranth was recovered from wild varieties in the 20th century; it is now cultivated commercially and considered an ancient grain.