The earliest Indians, the Harappans, probably ate mainly wheat and rice and chickpeas and lentils, and occasionally cows, pigs, sheep, and goats, and chicken. Rice and chicken seem to have come from Thailand, and wheat and chickpeas, lentils and sheep from West Asia. Some of the wheat was made into stews or soups, and some into flat breads called chapatis. Indian people also ate sugar cane, which grew naturally in India.
But by around 300 BC, under the Mauryans, a lot of Hindus felt that animal sacrifices added to your karma and kept you from getting free of the wheel of reincarnation. Animal sacrifices became less popular, and although people didn't give up eating meat entirely, they ate much less of it. A lot of people became vegetarians.
In the Gupta period, around 650 AD, Hindus began to worship a Mother Goddess. Cows were sacred to her, and so Hindus stopped eating beef pretty much completely. About the same time, Indian scientists invented a way to make sugar cane juice into sugar cubes, so more people began to eat more sugar and sugary desserts.
A fourteen-year-old Indian boy making bread
And then around 1100 AD, with the Islamic conquests in northern India, most people in India stopped eating pork as well, because it is forbidden by the Koran. People could still eat sheep or goats or chicken, but most of the people in India became vegetarians, and only ate meat very rarely or not at all.
The vegetarian food that Indians ate was mainly wheat flatbreads or a kind of flatbread made out of chickpeas, with a spicy vegetarian sauce with lentils, and yogurt. Or people ate rice, with yogurt and vegetables. A lot of spicy peppers grew in India.
To find out more about Indian food, check out these books from your local library or from Amazon:
Cooking the Indian Way, by Vijay Madavan (2002). Written for middle schoolers, with an emphasis on low-fat and healthy meals.
Land of Milk and Honey: Travels in the History of Indian Food, by Chitrita Banerji (2002). Not a cookbook, but a discussion of food in India, for grown-ups.
Eyewitness India, by Manini Chatterjee (2002). Written for kids.
Ancient India, by Virginia Schomp (2005). Written for middle schoolers. Very good for reports.