Some small squirrel-like animals were able to survive the Cretaceous/Tertiary catastrophe. There were some other animals that did the same. These animals were to give rise to the life forms of the next era. Some of the new life forms were warm-blooded and fur-bearing mammals, which eventually gave rise to the human species.
Contrary to some peoples beliefs of mammals evolving after dinosaurs became extinct, they actually first started to evolve within 10 million years of the dinosaurs about 200 million years ago in the late Triassic. There were a lot of insectivores similar to hedgehogs and shrews along with marsupials. which are mammals who raise their young in a pouch. The mammals began their diversification and became the dominant land animals only after all of the dinosaurs were extinct. Within 10 million years after that all kinds of mammals were living in different habitats on the land, in the air, and in the sea.
While the plants and animals were evolving, the worlds' geography changed alot. All of the continents came together to form one continent that was called Pangaea during the Paleozoic era. During the Mesozoic era the continent gradually came apart to form the present-day continents.In the Cenozoic Era there was a lot of radiation in other groups of animals. Some of those other groups of animals were birds, amphibians, and fish which gradually led up to the peak of biological diversity which occurred in the recent past. The separation of Pangaea into different continents let biological evolution take different paths in other parts of the world. The formation of oceanic islands made by volcanic activity made a lot more areas where evolution would be able to experiment with other forms of life.
During the Cenozoic Era lowering of the temperatures and the establishment of different climatic zones of the earth had started to happen. The cooling trend was known as the Great Ice Age of the last 3 million years. During that time, Eurasia and North America were covered with large continental glaciers. These glaciers advanced during the four ice ages, then they retreated during the three interglacials. It is believed that we are now living in the fourth interglacial stage. The sea level got a lot lower during the glacial periods because a lot of the water was turned into ice. One good thing that came out of this was that land bridges like the Bering land bridge which connected Asia with North America became available for animal migrations. Mammals reached their evolutionary peak during Pleistocene times. They had produced a very large variety of different species. Some of these species were very large.
Pjbryant @ uci. edu