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Paintings found on the walls of caves in the Sahara Desert suggest that a few thousand years ago, large quantities of plants grew there and covered the desert floor. Many curious and unusual insects, birds and animals got enough food that allowed them to grow and reproduce. At that time, huge river systems and many oases supported growing communities(社团，社区) where a large number of people lived.
Research scientists believe that the land became a desert for several reasons. For centuries people there have wandered from place to place in search of food, water and grazing(吃草)land for their animals. These people have always considered the number of animals they have as a sign of wealth. It is considered more important to have 100 sick and starving animals than 15 healthy ones. The large herds(群)have been allowed to go freely and graze
