It is generally taught that the с and з in such words as otьсь ‘father’, srьdьce ‘heart’, ovьca ‘sheep’ and kъnęз ‘prince’ of Old Church Slavonic – and, by implication, of all Slavic dialects of the 9th-10th centuries – developed from *k and *g by progressive palatalization at the very end of the prehistorical period, in the 7th-9th centuries. In this study I analyze the evidence, both external and internal, that scholars have adduced in support of this late dating. I demonstrate that the putative external evidence (place-names and loan-words) is irrelevant. This means that in studying the progressive palatalization and related problems we are free to examine the data of OCS and other pertinent Slavic linguistic evidence unconstrained by the need to cope with the alleged fact that the change was late. The internal evidence leads me to conclude that the progressive palatalization was not late: on the contrary, it was one of the first changes which clearly set a late Indo-European dialect, Pre-Slavic, apart from even its closest relative, Pre-Baltic. Thus, Proto-Slavic from the beginning was a characterized by a tendency to palatalize consonants.