Mammals form the most evolved and best known group of chordates. This class includes moles, bats, rodents, cats, monkeys, whales, horses, deer and many others, the man himself among them. All (with rare exceptions) are hair-covered and have a constant internal temperature.
Offspring are the most developed in the animal kingdom and reach their climax with the human species. They are also extremely adaptable, modifying their behavior according to the conditions of the environment. Some groups, especially primates, form very complex societies.
A unique feature of mammals is their ability to play. Young mammals learn almost everything they need to know for their adult life through play, where youngsters experience hunting, fighting and mating techniques among themselves and with adults. These games often establish a hierarchy that will continue into adulthood, avoiding potentially dangerous conflicts for individuals.
Mammalian ancestors were a group of reptiles called terapsids. These animals were small active carnivores and lived in the Triassic period (225 M.a. ago).
In addition to important skull differences, the therapsids have developed a lighter, more flexible skeleton, with limbs aligned beneath the body, making them more agile and faster.
Drawing of what would have been a terapsids
The transition from reptile to mammal ended about 195 M.a., coinciding with the rise of dinosaurs, threatening the newly formed mammals of extinction. However, their ability to control internal temperature may explain why mammals survived the global cooling of the late Mesozoic.