Mammals appeared over 200 million years ago. In their external features, they were similar to modern vole mice and shrews and were small in size. All mammals are warm-blooded, they feed their young with milk and breathe oxygen. All animals have seven cervical vertebrae.
Subclasses of mammals
Contrary to popular belief, the birth of live cubs is not a common sign for all animals. On the contrary, on this basis, scientists distinguish subclasses among animals: oviparous, marsupial and placental.
Oviparous mammals are the most primitive mammals living today. Their reproduction occurs in the same way as in birds and reptiles, through the laying of eggs. The platypus is a prominent representative of this subclass.
The main difference between marsupial mammals and other animals lies in the development of their embryos. First, the embryo is in the mother's body, then a tiny helpless creature is born. After birth, the cub moves to the mother's pouch, attaches itself to the nipple with milk, and continues its development there. The most famous marsupials are the kangaroo and the koala.
Placental mammals are animals that have a placenta - a special organ in which the fetus is located and develops before birth. It is this method of development of the embryo that is considered the most perfect. Placental mammals include dogs, cats, tigers, lions, dolphins, and humans.
The main features of mammals
The main characteristics of mammals that distinguish them from fish, birds and amphibians are the presence of hair and feeding the young with milk. All females of this group of animals have mammary glands on their bodies, which after childbirth are filled with nutritious fluid, which the young individual subsequently feeds on. Other distinctive features of mammals include the presence of a diaphragm, which separates the lungs and heart from the digestive tract, and the lower jaw, which consists of one bone.
But the most important trait of mammals is a highly developed brain and flexible behavior system. This means that under the same conditions, two individuals of the same species may behave differently, relying on past experience and reflection. It becomes clear why the species Homo Sapiens - Homo sapiens - appeared in the class of mammals.
The emergence of mammals
According to scientific research, the earliest mammals appeared in the era of the dinosaurs. Then they were small in size and looked like animal-like reptiles. Since archaeological scientists work with fossils, they have only the skeletons of ancient animals at their disposal, which makes it difficult to establish exactly at what stage of their development these animals were overgrown with wool and began to feed the young with milk. At the moment, it is generally accepted that the first mammals appeared about 200 million years ago.
