Over millions of years, some species go extinct, and others evolve, adapting to their ever-changing environments. Different traits are favored in different habitats and are passed on to future generations.
Extreme Climates: Huge shifts in global temperature and climate dominated the “Age of Mammals” and produced some bizarre adaptations over time.
For example, 50 million years ago the Arctic was home to warm, swampy forests that harbored climbing and wading mammals, such as tapirs, the hippo-like Coryphodon, and even early primates. But as the climate changed, a much smaller number of species evolved that were better suited to survive the bitter cold.
Extreme Isolation: When mammals are geographically isolated, they can evolve extreme traits or can come to resemble unrelated species elsewhere. For land animals, there’s no more extreme form of isolation than being completely surrounded by an ocean.
Three striking examples of island evolution are the unusual mammals of Australia, Madagascar, and South America. Their isolated mammals—including gliding possums, colorful tree-climbing lemurs, giant rodents, and other distinctive species—evolved along their own unique paths.
Adaptations that help ensure survival in a certain kind of environment tend to evolve again and again—even in species that live on different continents and are far apart on the family tree. This process is called convergent evolution.
For instance, all around the globe, we find diverse suites of mammals—including bison, horses, elephants, some rodents, and many extinct mammal groups—that have evolved hard, continuously growing, grinding teeth, which are advantageous for chewing the intense variety of grasses found worldwide.