Reptiles gave rise to birds and mammals. In primitive birds, insulating feathers retained body heat. Consequently, these animals could adapt to cold climates. Later, some of these birds developed larger feathers on their limbs that allowed them to move through trees or helped them jump to reach an insect. From this moment the birds could evolve to the flight.

The abundant hair of the primitive limbs also made it possible to withstand low temperatures. Unlike the birds, which preserved the reptilian habit of laying eggs, in mammals gestation within the mother became possible and she had the ability to feed her offspring with secretions from their mammary glands (milk producers).

The first mammals were small creatures that probably lived in the trees and were active mainly during the night. When the dinosaurs became extinct, the mammals manifested themselves in an enormous variety of forms. Some continued to be small and eat seeds and insects, but others grew surprisingly and colonized habitats that were emptied by the extinction of dinosaurs. One group remained in the trees that gave birth to the primates.

Supposedly the first mammals had their origin at the end of the Triassic period from their therapsid ancestors. These mammals were very small animals but they were very active creatures, their diet consisted mainly of insects. The maintenance of their body temperature was constant due to their way of life which was very active, this happened in parallel with the development of the heart of four chambers and the complete separation of oxygenated blood and deoxygenated blood.

To be able to maintain the heat in its body propitious the development of hair in the mammals, also the first mammals were characterized by laying eggs like the reptiles when having their calves they fed on maternal milk that secreted their mothers by means of glands in the skin of the mother.

Just as the teeth changed and began to have a greater variety of these, to cut their food were the incisor teeth, to tear the canine teeth and to grind their food the molars. The gray matter of the brain of these mammals grew outward on the surface of the brain. This modification would have long-term consequences.

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