A、When they learnt how to keep animals in herds. B、When they had to go about from place to place for food. C、Before they learnt

When men first lived on the earth a long time ago, they had no proper place to live in. In order to get away from the heat of the sun, to keep themselves warm and dry, they found caves. Caves really gave them some kind of help. Some of these early men built their shelters up in the tree to escape wild animals. They had a hard life and had to go about from place to place for food. Much later on, their lives began to change. They learnt how to keep animals in herds. As they had to move their herds of animals from place to place for grassland, they had to build shelters which were easily carried about.
    When they finally learnt the art of farming, they had to clean forests and started planting crops. After that, they had to think about building more permanent shelters. Old houses were built at first with wood and they even use grass as roofs. This went on for a long time. They kept making their houses better. Today, houses of all kinds are built. They are strong, permanent and can last a very long time.