Final answer:
During the Pleistocene and the Quaternary Period, Earth's climate underwent significant changes, including ice ages and lower sea levels. This resulted in the formation of land bridges and migration of species.
Step-by-step explanation:
In time periods past, such as the Pleistocene (2.6 million years ago), Earth's climate was significantly different from what it is today. The Quaternary Period, which began 1.8 million years ago and continues to the present, was marked by multiple ice ages that caused the formation of large ice sheets and lowered sea levels.
This enabled the creation of land bridges between continents, allowing for the migration of land animals. Some species adapted to the cold climate by evolving larger size and thicker fur, while others moved closer to the equator or faced extinction.