Gradually, man came to know how to use metals. Metals were more durable than stone and could be worked for making variety of tools, implements and weapons. The first metal to be discovered was copper. For a long time stone implements continued to be used along with copper. This period when man used both stone and copper tools is known as the Chalcolithic Period. Later on, man learnt the art of mixing copper with tin or zinc to produce the alloy called Bronze. Bronze is harder and more ductile than copper and is, therefore, more suitable for the manufacture of tools and weapins. Because of the importance of bronze in the growth of the first civilisations, these civilisations are known as the Bronze Age Civilisations. By about 2500 BC, four bronze age civilisations emerged. These were: (1) the Harappan civilisation in northern and western parts of India and Pakistan: (2) the Mesopotamian civilisation on the banks of river Euphrates and Tigris in modern Iraq: (3) the Chinese civilisation in the valley of Hwang Ho and Yangtze rivers: and (4) the Ehyptian civilisation on the banks of Nile river in Egypt. The Harappan civilisation The discovery of Indus valley civilisation in the 20th century was an important event in the history of India. Upto this time it wa believed that the history of India began with the advent of Aryans. However, with the discovery of Indus valley civilisation it became clear that the history of India began from 2500 BC instead of 1000 BC. The Idus valley civilisation is best known as the Harappan culture or the Harappan civilisation. It is called the Harappan culture or the Harappan civilisation, because the Harappan site was the first to be discovered in 1921 at the modern site of Harappa situated in the province of West Punjab in Pakistan. Mohenjp-daro was discovered in 1922. Both lie in the Indus valley region in the north-west.