In the 18th century, the most ancient known Indo-European languages were those of the ancient Indo-Iranians. The word "Aryan" was therefore adopted to refer not only to the Indo-Iranian peoples, but also to native Indo-European speakers as a whole, including the Romans, Greeks, and the Germanic peoples. It was soon recognised that Balts, Celts, and Slavs also belonged to the same group. It was argued that all of these languages originated from a common root – now known as Proto-Indo-European – spoken by an ancient people who were thought of as ancestors of the European, Iranian, and Indo-Aryan peoples.