It was a process that unfolded over thousands of years, with the landscape and climate changing in ways that were gradual by geological standards but rapid by ecological ones.

The early Holocene — the period roughly 11,000 to 8,000 years ago — was characterized across much of North America by conditions somewhat warmer and drier than today.

Climate scientists call this the Hypsithermal or the Holocene Climatic Optimum — a period when temperatures in many mid-latitude regions of the Northern Hemisphere reached or exceeded modern values, driven by changes in the Earth’s orbital geometry that affected the distribution of solar radiation through the year.

In the Rocky Mountain region, this warm, dry period had significant effects on the ecology.