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.
Europeans experienced multiple waves of immigration into Europe, changing the mix substantially each time. Neanderthals were dominant up until approximate 52,000 years ago, when Cro-Magnon man began to take over. Europeans had pale eyes but darker skin up until about 8,000 years ago, when huge waves of “Caucasians” started pushing west from the Middle East and Asia, bringing genes for lighter skin. (Aryans also came from the north into ancient Persia and India.)
Of the earliest Europeans, only the Basques remain in the mountains surrounded by Spain, France, and the Atlantic. Their genetics and language prove a distinct lineage somewhat unrelated to modern Europeans, and many believe they are modern descendants of the survivors of Atlantis.