Fishes belong to a group of animals characterized by a symmetrical physical conformation, compared with a central axis; along that axis has set a rigid structure , which supports muscles, which in turn allows to swim in a sinuous manner. In fish and all their descendants, the spine is made up of a series of rings called vertebrae, resulting in the new name of the vertebral column and in the name of the group of animals who are gifted with this vertebral column, precisely the vertebrates.
At a distant age, freshwater fish have originated the group of amphibians. The first insects reached the mainland in the same period of amphibians, that is about 400 million years ago. The amphibians are specialized in the life out of the water, where they must return to spawn. Even today all amphibians seek freshwater to lay their eggs and drink, tolerating brackish water.
From amphibious then descended reptiles, which have developed the ability to lay down their eggs on land and lost the ability to breathe in water, so that the aquatic reptiles must return to the surface to breathe and should even reach the mainland to lay their eggs.
From reptiles, about 200 million years ago, have also independently arisen two large families: the birds and mammals.
The birds are distinguished by having the body covered with feathers and from having two front limbs as wings that allow almost all species to fly; mammals instead have the skin covered with hair and are predominantly soil or tree animals; they feed their little ones with milk secreted from breast and hence their name.
All vertebrates have the following characteristics: a spine with a skull to one end, a brain contained and protected by the skull and ribs as support for muscles and protection of internal organs; also almost all have a jaw with teeth, four limbs and a tail. From the anatomical point of view the human being is therefore a typical vertebrate and in particular a mammal.
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