This is an animal cell, one of the essential building blocks of life. All animals are made of one or millions of cells. Inside is a nucleus. It holds all the information an animal needs to be born, to grow and live successfully and to reproduce itself.
All animals have evolved from life forms which had very few of these life-building cells. Some of these primitive species still live in the oceans today. Over millions of years, some animals develop simple segmented bodies. Some creatures slowly evolve protective cases. These animals are known collectively as mollusks. Some mollusks have learned to live successfully without a shell. By contrast, crustaceans, insects have developed external skeletons to aid mobility and to house a complex internal life support system.
These seemingly very different creatures have one thing in common. They are all invertebrates, that is, they have no spine. Only five percent of the world’s animals have evolved a back bone. Fish have spines of bone or cartilage. All fish from shark to Mino breathe by drawing water through the mouth and over the gills. Oxygen is absorbed by blood vessels in the gills. Carbon dioxide is expelled.
Over millions of years, some fish developed the ability to breathe the oxygen in the air above the sea. Amphibians are able to live both on land and in water.
Because the reptiles have finally conquered dry land, the leathery shells of their eggs could safeguard the embryo without needing the protection of water.
When some reptiles evolve warm blood and their scales become feathers, they conquered the air as well. The only other warm-blooded creatures on earth are the mammals. These animals nurture their young internally and suckle the offspring with milk. They are the most complex animals on the planet. They’re highly developed creatures which seem to have nothing in common with the simple sponge or the butterfly. But look again. The common characteristics begin to appear. For one thing, all animals are mobile whether they move slowly or quickly. They all need oxygen to breathe. All creatures must reproduce themselves in order to survive. We all need to sustain ourselves with food from our immediate surroundings whether we eat the plants around us or other animals. But most importantly the animal world is interdependent. The survival of any one species is dependent in some way on the success of other animals in the environment, and that is just as true for the Cattle bird as it is for people.