Aryo - ( interpreted by some as meaning `` those who plow '' ) , sometimes upheld as a self-identification of the Proto-Indo-Europeans ( see Aryan ) , is shown only in the Indo-Iranian subfamily , since it appears on written inscriptions ; however , the Proto-Indo-Europeans themselves had not yet adopted writing , so there is no way to verify that Aryo - was their self-identification . Aryo - ( interpreted by some as meaning those who plow ) , sometimes upheld as a self-identification of the Proto-Indo-Europeans ( see Aryan ) , is attested as an ethnic designation only in the Indo-Iranian subfamily , since it appears on written inscriptions ; however , the earlier Proto-Indo-Europeans had themselves never adopted writing because writing had not yet been invented , so there is no way to ever verify that Aryo - was their self-identification .