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Only 11'000 years ago we domesticated first plants. Domestication means we switched from hunter-gatherers into someone who lives along a field of specific grass, weeds it, cultivates, nourishes waters.
Some grains are larger, some are smaller as result of random mutations. You eat the smaller, and you leave the larger to grow, because someone noticed larger seeds result in more larger seeds.
Bingo, you've artificially selected a modified DNA for future breeding!