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It's natural for people to consider mammals the pinnacle of evolution--after all, humans are mammals, and so were our ancestors. (In fact, mammals are among the least diverse animal groups—there are only about 5,000 species overall!) Mammals are characterized by their hair or fur (which all species possess during some stage of their life cycles), the milk with which they suckle their young, and their warm-blooded metabolisms, which, as with birds, allows them to inhabit a wide range of habitats, ranging from deserts to oceans to arctic tundra. .

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Birds evolved from dinosaurs—not once, but probably multiple times—during the Mesozoic Era, and today they are by far the most prolific flying vertebrates, numbering about 10,000 species spread across 30 separate orders. Birds are characterized by their coats of feathers, their warm-blooded metabolisms, their memorable songs (at least in certain species), and their ability to adapt to a wide range of habitats—witness the ostriches of the Australian plains and the penguins of the Antarctic coastline.

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Reptiles, like amphibians, make up a fairly small proportion of terrestrial animals—but in the form of dinosaurs, they ruled the earth for over 150 million years. There are four basic types of reptiles: crocodiles and alligators, turtles and tortoises, snakes, and lizards. Reptiles are characterized by their cold-blooded metabolisms—they fuel themselves up by exposure to the sun—their scaly skin, and their leathery eggs, which, unlike amphibians, they can lay some distance away from bodies of water.

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When the first amphibians evolved from their tetrapod ancestors, 400 million years ago, they quickly became the dominant vertebrates on earth. However, their reign wasn't destined to last; the frogs, toads, salamanders, and caecilians that make up this group have long since been out-competed by reptiles, birds, and mammals. Amphibians are characterized by their semi-aquatic lifestyles (they have to stay near bodies of water, both to maintain the moisture of their skin and to lay their eggs), and today they are among the most endangered animals on earth.

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The first true vertebrates on earth, fish evolved from invertebrate ancestors about 500 million years ago, and have dominated the world's oceans, lakes, and rivers ever since. There are three main types of fish: bony fish (which includes such familiar species as tuna and salmon); cartilaginous fish (which includes sharks, rays, and skates); and jawless fish (a small family made up entirely of hagfish and lampreys). Fish breathe using gills and are equipped with "lateral lines" that detect water currents and even electricity.

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The first animals to evolve, as far back as a billion years ago, invertebrates are characterized by their lack of backbones and internal skeletons, as well as their relatively simple anatomy and behavior, at least as compared to most vertebrates. Today, invertebrates account for a whopping 97 percent of all animal species; this widely varied group includes insects, worms, arthropods, sponges, mollusks, octopuses, and countless other families.

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Strauss, Bob. "The 6 Basic Animal Groups." ThoughtCo, Oct. 22, 2018, thoughtco.com/the-six-basic-animal-groups-4096604.