What are mammals? This class of the most developed animals includes completely different animals: a huge elephant and a small mouse-vole, a predatory wolf and a meek sheep. They live in all elements: on land, in the sea (dolphins and whales), in the air (bats). But what makes them different from other animals? Warm blood? The same trait is inherent in birds. The ability to give birth to live babies? Well, firstly, not all mammals are viviparous. Some of them lay their eggs, such as a platypus, while in others the calves at birth are more like underdeveloped embryos and are forced to “ripen” in a special incubator bag on the mother’s belly, like a kangaroo. And secondly, viviparous species are also found in some reptiles, fish, and even insects.
The question of who the mammals are and how they differ from fish can be answered simply: by the presence of lungs. All animals breathe air. Therefore, the shark does not need to rise to the surface of the water to take a breath, unlike a dolphin that looks like it, which can simply drown out without life-giving oxygen. But, again, many primitive amphibians and all birds have lungs.
Among all the enormous differences of animals, only two parameters can be distinguished that determine who mammals are. This is the presence of hair and feeding the young with milk. All females of this group of animals have mammary glands on their bodies, which are filled after delivery with nutrient fluid. This property gave the name to all mammals. Among the other signs that characterize animals, you can add the presence of the lower jaw, consisting of one bone, and a diaphragm that separates the lungs and heart from the digestive tract.
But the most important sign in determining who these mammals are is their incredibly developed brain and a flexible system of behavior. This means that two individuals of the same species, falling into the same life circumstances, can behave differently. They know how to use their experience, observe and reason. It is not surprising that it was in the class of mammals that the species Homo sapiens - Intelligent Man appeared. Well, besides him, about 5,000 species of other animals live on Earth, which the taxonomists grouped into 1,100 genera, 140 families and 18 orders and 3 groups.
The extremities of mammals are no less interesting. For all their apparent diversity (claws of predators, fingers of humans and monkeys, hooves), they all end with a horn process - a nail. In animals living in water, these processes atrophy. This feature of adapting to environmental conditions contributed to the fact that mammals spread throughout the planet and occupied all life niches - on land, in the air, in the sea, underground. Thanks to the ability to maintain body temperature with warm blood, they can live in the arctic (polar bears) and tropical (jerboas, camels) deserts.
The question arises: when did the first animals appear? There is no doubt that the oldest mammals found the dinosaur era. Then, they were mainly of small stature and showed signs of "animal-like reptiles." Since scientists working with fossils have only skeletons at their disposal, it is difficult to determine at what stage these animals developed wool and the ability to feed their babies with milk. If we proceed from the criterion of a unicostral lower jaw, then we can safely say that another 215 million years ago in the territory of South Africa, Europe and China there lived a small (15 cm) animal, similar to a modern shrew.