Squirrels are common throughout North American temperate and boreal forests. Photo by Maya Walters.
|Because of their bone structure, mammals fossilize easily, which means their evolution is better understood than that of other animals. When mammals first started to become a common and dominant type of animal, the continents of Europe, Asia, Africa, and North America were all linked by land bridges, creating a "World Continent". The mammals that evolved on this World Continent were therefore fairly closely related. South America and Australia, however, were not joined to this World Continent, so three distinct populations of mammals eventually developed: the Australian mammals, the South American mammals, and the mammals of the World Continent.|
|Many similar habitats and ecological niches existed on all of the three continents. This led to many unrelated species evolving in similar ways--examples of convergent evolution. When North and South America were connected by Central American landforms, animals were able to expand their ranges onto a whole new continent. Many animals weren't successful in moving from one continent to the other, but others completely took over ecological niches that had previously belonged to mammals that had evolved on that continent. Early monkey-like primates gradually migrated to South America, where they evolved into the spider monkeys, howler monkeys, and the other prehensile-tailed primates which now inhabit the tropical forest canopies.||There are fewer than 5,000 species of mammals but the diversity of shapes and behaviors is astonishing. Most of these species have been studied more than any other class of animals.|
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