The conclusion is based on the residual remains on food infused on pottery, human teeth, and cow teeth from archaeological digs in the Indus River Valley dated to between 2500 and 2200 B.C.E. These are the oldest discovered examples of ginger and tumeric in the region.
They found additional supporting evidence of ginger and turmeric use on ancient cow teeth unearthed in Harappa, one of the largest Indus cities, located in Pakistan west of the border with India. Why [?] . . . in the region today, people often place leftovers outside their homes for wandering cows to munch on. There are numerous ancient Indus images of cattle on terra-cotta seals, suggesting that during Indus times, people may have regarded cows as sacred, as Hindus do today. The Harappan ruins also contain evidence of domesticated chickens, which were likely cooked in those tandoori-style ovens and eaten. . . . tandoori-style ovens [are] often found at Indus sites. . .
Many archaeologists once thought that Indus peoples were restricted to a few grains like wheat and barley. But Cambridge University archaeologist Jennifer Bates, part of a joint Indian-U.K. team, has been examining the relative abundance of various crops at two village sites near today’s Masudpur, also west of Delhi. She found that villagers cultivated a wide array of crops, including rice, lentils, and mung beans. Finding significant quantities of rice was a particular surprise, since the grain was long thought to have arrived only at the end of the Indus civilization. In fact, inhabitants of one village appear to have preferred rice to wheat and barley (though millet was their favorite crop).The evidence that Harappan culture may have been a rice eating one almost from the beginning of its golden age, that it was a culture in which cows were revered, and that its favorite recipes survived the Indo-Aryan invasion, helps to unravel the mystery of what components of Vedic culture were part of the Harappan substrate and which were instead part of the Indo-Aryan superstrate.
In particular, this evidence disfavors a hypothesis that the Harappan culture had so completely collapsed and was in such disarray upon the arrival of the Indo-Aryans, that there was no substrate influence on the income conquerors culture. This not only lets us know something about the Harappans. It also, by substraction, tells us what elements of early Vedic culture that it might be tempting to attribute to Proto-Indo-Europeans was in fact a Harappan substrate influence.
This also provides culturally persistent benchmarks that can be searched for in other Indo-European cultures in an effort to discern any possible distinctively pre-Indo-Aryan Harappan influences. If all of the known cultural indicators of Harappan culture are absent from other Indo-European cultures, this is highly relevant evidence disfavoring an "Out of India" theory of Indo-European origins.