18 kya. Glacial periods within the present Ice Age start gradually and end rapidly. The last glacial period began around 90 thousand years ago, and reached its peak, the Last Glacial Maximum (LGM), around 20 thousand years ago. During the LGM most of Northern Europe and northern North America were under ice. Sea levels were more than 300 feet lower than today. Human beings in Europe were confined to refugia in the south, notably Spain and southern France.
In the period after the LGM, temperatures rise, the ice retreats, and humans recolonize the North. Not just Europe: people will gradually make their way into Siberia, and some of these Siberians will eventually make their way to the New World.
For a flavor of life during these times, Elizabeth Marshall Thomas is worth checking out. She is the author of two beautifully written novels set in this period in Siberia (Reindeer Moon, and The Animal Wife). As a child, Thomas was part of an expedition to the Kalahari, as recounted in her book The Harmless People. Her experiences, along with northern archeology, color her imaginative reconstruction of Ice Age hunters.