India is rightly called the Land of Spices. From black pepper to sunth (dry ginger powder) there were a wide variety of spices for inducing heat in food. Indian traditional cooking, also called Paak Shaashtra that derived from Ayurveda (a traditional Indian medicinal system), mainly rely on balancing of tastes for developing better digestion.
It is based on 5 ras (flavours): sweet, bitter, acidic, salty & heat.
Ayurveda originated in about 2000 - 5000 BC (some consider it older), depicting use of chillies as painkillers and for relieving stress. Various chillies which are only cultivated in India are Bhoot Jholakia, Naag Jholakia, Boria Marcha, Bhavnagri Marcha, Surti Marcha, Lavingyaa Marchaa have found place in Ayurveda & Indian Cuisine from the oldest available reference. It is very unlikely that Vasco Dagama or Spaniards would have introduced chillies to India. Al-Biruni (a very famous Muslim scholar who lived in 937 AD to 1048 AD) recorded trade of chillies (no particular chilli species is quoted) by Indians in Egypt & Europe in his book Tarikh-Al-Hind (History of India).
Though Indian Cuisine is rich in variety of flavours with spices, facts say that it's unlikely to that Indian cuisine existed without chillies.