The great Russian botanist, Vavilow, about seventy years ago, identified what he called "centers of plant origin" in which the "evolution of plants was directed by the will of man." There were nearly a dozen of these centers all over the world where plants gradually took their place as foods for human beings. Of particular interest to India was the so-called Fertile Crescent of the Middle East, stretching from Israel to Iraq, which was an early center of agriculture and of plant evolution.What actually happened was this. Man would pick promising weeds growing wild which carried grains. By choosing plants with abundant or plump grains, a process called selection, the quality of the grains and the yield of the plant in the next crop were both improved. Sometimes, nature itself would take a hand in the process; a wild weed would cross by chance with a cultivated species to produce offspring of a quality superior to both.
Achaya then goes on to describe how ragi, ubiquitous in India for millennia, has an unexpected connection to a different part of the world:
Botanically, ragi is Eleusine coracana. It was born in Uganda in East Africa. How do we know this? For several botanical reasons, such as the existence of its wild ancestors, the long mention of the grain in tradition, and the fact that ragi figures in old religious ceremonies in those areas. Ragi is a tetraploid, and so is the African wild plant called E. africana, which gave rise to it. But the Indian wild plant, which is called E. indica, is a diploid which does not cross with ragi at all. Now what does this mean? Only this, that the Indian wild weed could not have given rise to the ragi plant in India. This plant must, therefore, have come to India from East Africa some time in the past. When did this happen? Ragi has been found in an Indian excavation which is dated at 1800 BC, and at several other archaeological sites in central India of a slightly later date. We must therefore infer that some unknown benefactor brought this foodgrain from Africa to India in about 2000 BC. It also seems possible that two other food-grains, jowar and bajra, also came at the same time to India, from the same area in East Africa, where they were originally evolved. You see, therefore, how no country ever stands really alone; certainly our food has come to us from unexpected places.