Possibly as early as 40,000 years ago, a series of mountain glaciers moved downslope carving out many valleys.During full glaciation, an icecap as thick as 3,000 feet covered most of what is now Yellowstone. These glaciers retreated 14,000 years ago. In a short period of time, plants and animals colonized the landscape and humans arrived in this area about 12,000 years ago. Following the melting of the glaciers or deglaciation, the climate was still colder and moister than present conditions. High altitude subalpine and alpine vegetation communities dominated the area, with mixed conifer forests at lower elevation basins. Trees appeared 11,500 years ago, beginning with Englemann spruce, followed by lodgepole pine, Douglas fir, whitebark pine, and limber pine. Between 9,500 and 5,000 years ago, a period of increased warmth and aridity occurred resulting in an increase of Douglas fir and other drought resistant species. About 80% of the park has soil derived by rhyolite lava flows and the lodgepole pine is the predominant tree on these nutrient-poor soils. Fire frequency may also have increased during this time.By about 5,000 years ago, the vegetation was similar to that of today.
We know more details about the environment the closer time gets to modern days. The Mideval Climatic Anomalie was a world-wide drought between 1,000 and 750 years B.P. Following this dry period, the Little Ice Age was another world-wide climatic event between A.D. 1450-1850. The Little Ice Age can be identified in the tree rings of Douglas fir trees as they grew very slowly during this cold period. Archeologists have found very few sites in the park that date to the Little Ice Age time. The park has been warming since the end of the Little Ice Age. Understanding changes in the past climatic is key to understanding how early people lived in the park.