Nisäkäslajien

Nisäkäslajien (Mammalia) are a class of animals characterized by several key features, including the presence of mammary glands, which are used for the production of milk to nourish their young. This class is further divided into several subclasses, including Prototheria, Metatheria, and Eutheria. Prototheria, also known as monotremes, are the most primitive group of mammals, including species like the platypus and echidnas. Metatheria, or marsupials, are characterized by their pouches, which carry and protect their developing young. Eutheria, or placental mammals, are the most diverse group, including humans, primates, and many other familiar mammals.

Mammals are endothermic, meaning they generate their own body heat, and they have a high metabolic rate. They also possess a complex nervous system, including a well-developed brain, and many species have a keen sense of smell. The class Mammalia is one of the most successful and diverse groups of animals on Earth, with over 6,000 known species.

The evolution of mammals began during the late Paleozoic era, with the first known mammal-like reptiles appearing around 200 million years ago. Over time, mammals diversified and adapted to a wide range of environments, from the oceans to the skies. Today, mammals play a crucial role in many ecosystems, serving as predators, prey, and decomposers. They are also an important part of human culture and history, with many species being domesticated and used for food, labor, and companionship.