A continental origin

Simplified recolonisation of Europe by hunter-gatherers according to Dr. Oppenheimer.

The Ice Age ended with the beginning of the Younger Dryas Age (12000 - 8000 BC) [1].

But the Ice Age actually ended in two steps: the first warming happened around 12000 BC, the second one, one can call this one 'the real warming', around 8000 BC and is called the Holocene. We still do live in the Holocene period. The Younger Dryas was still cold and dry. Bordeaux had then the same climate as Stockholm today. As a consequence the ice melted in two stages.

Previously, the north of Europe was simply too cold to be inhabited by the Cro-Magnon man, a human species of tropical origin. Actually, this Cro-Magnon man (we!) could cope with the cold, but it was the absence of wild game on the ice plains that was impossible to overcome.  We can assume that there was some human presence in Britain 15000 years ago, but the numbers must have been negligible and seasonal. With the start of the Younger-Dryas came the first stage of ice melting and much of Europe became far more accessible for humans, although the climate was still cold. The second, final stage would happen when the Younger Dryas (9000 - 8200 BC) ended and the Holocene period began (since 8000 BC).

According to Dr. Oppenheimer, it was at the beginning of the Younger Dryas period that Europe was colonized from Ice Age refuges. He called people in the northern Spain refuge Ruisko. Ruisko colonized mainly the regions next to the Atlantic coast. The refuges in Croatia (called Ivan) and in southern Russia (called Rostov) colonized most of the Continent itself. All genes became eventually unevenly mixed. E.g. Britain is more Ruisko, Germany is more Ivan and Rostov.

The first wave of humans who came from Ice Age refuges Ivan and Rostov spoke a language which was the earliest form of PIE [2]. To give it a name: we call it Maglemosian. The location of the origin of the PIE language on the map is deceiving: the language was spoken by a tribe that lived on the coasts of the Black Sea. Dr. Oppenheimer found typical Ivan-genes in Serbia and typical Rostov-genes in southern Russia, but that does not mean that these genes were originally from there. What we know is that the genes remained more concentrated there.

Language evolution

Maglemosian was carried by people who genetically resembled people from the Ivan or Rostov refuges respectively in Serbia and in south Russia.  In the west and northwest a different language was spoken: Azelian, which might be a parent of Basque.  Both Azelian and Maglemosian were the most important languages in western Europe.  Not in numbers of native speakers, but in surface. The languages evolved and unified regionally because of the annual migrations and the winter gatherings for maybe 4000 years.

At the beginning of the Holocene, the last big warming up of about 8000 BC, the opposite happened: migrations stopped and the languages diversified, more strong dialects appeared. Three major groups had emerged in western Europe: (1) Azilian in west Britain, France and Spain (Atlantic coast), (2) Maglemosian in Germany and Scandinavia, (3) Various non-PIE languages settled probably in northern Italy and southeast France at first. Only Maglemosian had connections with PIE.

Azilian is a name given by archaeologists to an industry of the Epipaleolithic in northern Spain and southern France. It probably dates to the period of the Allerød Oscillation around 10,000 years ago (10,000 BC uncalibrated) and followed the Magdalenian culture. Archaeologists think the Azilian represents the tail end of the Magdalenian as the warming climate brought about changes in human behaviour in the area. The effects of melting ice sheets would have diminished the food supply and probably impoverished the previously well-fed Magdalenian manufacturers. As a result, Azilian tools and art were cruder and less expansive than their Ice Age predecessors - or simply different. (Wikipedia)
Maglemosian (ca. 9500 BC–6000 BC) is the name given to a culture of the early Epipaleolithic period in Northern Europe. In Scandinavia, the culture is succeeded by the Kongemose culture.

Seasonal migrations

Our story takes place circa 11 000 years ago (Younger Dryas - see picture above). It is winter and very cold. Look at the map hereunder. There is an ice cap over most of Scandinavia. Europe is covered with a forest-steppe (in pink), which is a mixture of patches of trees (a few birch and pine) and a lot of grassland. The more to the north, the less trees there are (tundra-steppe). The yellow area represents a dry steppe, mostly void of trees. Purple represents woodland. The sea level is much lower than today. Britain is a part of the continent. The first deciduous trees appear only in the far south of France and in Spain. 

Big animals need a lot of grass. A steppe is covered with grass. The conditions are ideal for big migrating herds of European wisent (=bison), deer, etc. This meant enough food for the humans, and a subsequent growth of their population.

The Maglemosians had their winter quarter in (modern) Bavaria, the south of Germany,  where the wild herds sheltered. These herds were blocked there on their way south because of the ice cap over the Alps and the Rhine in the west. Estimated human population: 50 000-60 000. They spoke an ancestor of PIE called Maglemosian.  From time to time, new proto-PIE speaking people migrated from the shores of the Black Sea to south Germany.

The Azilians stayed in (modern) southwest France for similar reasons. Their origins must be searched in the north of Spain and south of France. Estimated population: 80 000-90 000 souls. In the north, close to the ice-cap, were some 5000-6000 proto-Scandinavians. Their lifestyle resembled that of the modern Inuit (Eskimos).

Spring came late to the barren land. In May the herds began to travel north. Herds travel much faster than humans can follow (on foot). The humans prepared themselves to go to their summer quarters, also in the north. They were organized in small clans, on average some 25-35 people; 4 up to 7 adult men, some elderly people and the rest were women, children and babies. Each clan had a well-known summer territory, inherited for many generations.

A fixed summer territory had many advantages.
(1) Mankind is very territorial. The reason is simple: it's all about food. The size of such a territory had to be just big enough (= enough game). It was determined by experience. Too big meant too much competition with other clans.
(2) It avoided yearly disputes and the occasional casualties.
(3) Each clan knew exactly where to go, was able to prepare for the voyage, knew what dangers lay ahead, how to overcome obstacles, etc.
(4) It allowed more investment in the local huts or shelters. In some places mammoth tusks were collected to build those huts. Mammoth tusks are very heavy. Sometimes they had to be carried by at least 2 men over kilometres. Not something you would do for just one summer. Wood was scarce in the steppe and needed for fuel. Such huts have been found.
(5) Efficient hunting depends upon a good knowledge of the territory. This knowledge was acquired over the years. 

Some clans would have to travel for one month. As the good season lasts only 5 months, including one month to go and one month to come back, thus remain 3 months to stay. Not much. Going further north would narrow the season too much. The summer is normally a time of plenty, and fattening up for the next winter is what you would do. So, some time is needed. In spring the Azelians spread all over France from the south. Some clans reached their northern limit : Belgium, at the maximum reasonable distance from South-France, some 750 km. That meant an average of 25 km a day on foot during a month. Arduous, especially for elder or very young people. The proto-Germans did the same and some ended up near the border of Denmark, a very similar distance. Of course, there were no roads. However, there was another way to reach the North. By boat.
Technology around 10 000 BC was comparable with that of Canadian Indians just before the white man came. People could make boats, large canoes probably made of hide, would be available. Those boat types are still made by the Eskimos, and are surprisingly strong in icy conditions. The Welsh word for such boats is currach. The English version of the word is coracle. Today, coracles are (very) small and often completely round while currachs are elongated and much bigger, but they are both really the same word. With those currachs the Azelian peoples could travel north by sea. This way of voyaging is much more comfortable and quicker than walking. They followed the coast to the north, and colonized the complete west of the British Isles.

The Maglemosian people had a similar technology, and used the Rhine, but also the Weser and Elbe. They followed the rivers to the north and colonized the riverbanks. That's how Alsace became German. 

Where the Rhine merged with other rivers in what would become the North Sea, Maglemosian people found the place already occupied by Azelian speaking tribes.