Just like the Germans, French, Swedes, Dutch, and other Europeans are being replaced today by huge waves of foreign immigrants with much higher birthrates, the same thing happened many thousands of years ago (presumably after the last pole shift.) Though we may consider the following groups ancient ancestors of Europeans today, there was a time when new tribes like Celts, Slavs, Balts, Goths and others had not yet come on the scene to displace the earliest Indo-Europeans.