Introduction to mammalia
Mammals, the group of vertebrates (animals with backbones) to which human beings belong. Mammals are distinguished from all other animals by their mammary glands, glands that produce milk to feed the young. Mammals are also unique in having hair, although it may be scant. They are warm-blooded and air-breathing, and all but the whales and sirenians, or sea cows, have four limbs.
All mammals except the duckbill platypus and the echidna, which lay eggs, give birth to live young. Mammals care for their young for longer periods than other animals do. Mammals have a more highly developed brain than other animals, with correspondingly higher intelligence.
Some mammals are the source of products used for food, shelter, clothing, and other purposes. Some are trained to do work, and others are enjoyed as pets. By studying other mammals, people have gained insight into human behavior and learned how to prevent and cure certain diseases.
Mammals appeared more than 200,000,000 years ago; they evolved from reptile-like animals called therapsids. The earliest known mammals, called triconodonts, resembled shrews, but layed eggs. Marsupials (mammals with external pouches in which young are nourished) and placentals (mammals with wombs in which unborn young are nourished) appeared about 100 million years ago. About 65 million years ago (the beginning of the Cenozoic Era, or Age of Mammals), mammals increased in number, diversity, and size. About 10,000 years ago, a number of very large land mammals, including ground sloths and woolly mammoths, became extinct.
All mammals except the duckbill platypus and the echidna, which lay eggs, give birth to live young. Mammals care for their young for longer periods than other animals do. Mammals have a more highly developed brain than other animals, with correspondingly higher intelligence.
Some mammals are the source of products used for food, shelter, clothing, and other purposes. Some are trained to do work, and others are enjoyed as pets. By studying other mammals, people have gained insight into human behavior and learned how to prevent and cure certain diseases.
Mammals appeared more than 200,000,000 years ago; they evolved from reptile-like animals called therapsids. The earliest known mammals, called triconodonts, resembled shrews, but layed eggs. Marsupials (mammals with external pouches in which young are nourished) and placentals (mammals with wombs in which unborn young are nourished) appeared about 100 million years ago. About 65 million years ago (the beginning of the Cenozoic Era, or Age of Mammals), mammals increased in number, diversity, and size. About 10,000 years ago, a number of very large land mammals, including ground sloths and woolly mammoths, became extinct.