Who were the Shudras how they came to be the fourth varna in the Indo-Aryan society

Who Were the Shudras? is a history book by Indian social reformer and polymath B. R. Ambedkar. The book discusses the origin of the Shudra Varna.In the book Ambedkar, citing Rigveda, Mahabharata and other ancient vedic scriptures, estimates that the Shudras were originally Aryans.Ambedkar writes in the preface of the book,""Two questions are raised in this book: (1) Who were the Shudras? and (2) How they came to be the fourth Varna of the Indo-Aryan society? My answers to them are summarised below.The Shudras were one of the Aryan communities of the solar race""There was a time when the Aryan society recognised only three Varnas, namely. Brahmins, Kshatriyas and Vaishyas.The Shudras did not form a separate Varna. They ranked as part of the Kshatriya Varna in the Indo-Aryan society.There was a continuous feud between the Shudra kings and the Brahmins in which the Brahmins were subjected to many tyrannies and indignities.As a result of the negligence towards the Shudras generated by their tyrannies and oppressions, the Brahmins refused to perform the Upanayana of the Shudras.Owing to the denial of Upanayana, the Shudras who were Kshatriyas became socially degraded, fell below the rank of the Vaishyas and thus came to form the fourth Varna.""Ambedkar also discusses Aryan race theory and rejects Indo-Aryan migration theory in the book.