The inventions that characterized the Bronze Age were metal weapons and writing. Smelting copper and tin into bronze was the primary requirement of classifying a civilization as part of the Bronze Age, and centralized government, the pottery wheel and year-round agriculture were other innovations.
The development of bronze lead to the invention of metal weapons such as swords, daggers and arrowheads; these weapons replaced earlier stone and bone weapons and tools. These defensive capabilities allowed for urban growth and the first real cities when combined with advanced forms of pottery manufacturing and more efficient agriculture. There were negative effects to these developments as well, including slavery, large-scale warfare and the early development of social classes.
Writing first appeared during the Bronze Age as well. Egypt, Sumeria, China, Crete and the Olmecs of Mesoamerica were all civilizations that developed their first writing systems during the Bronze Age. The earliest known true alphabet was invented around 2000 B.C. in Egypt's Sinai Peninsula. Chinese and Olmec writing systems were developed independently during the same era. Earlier writing systems, such Egyptian hieroglyphs, Minoan hieroglyphs and the Indus script from India, were considered proto-writing and consisted of pictographs and symbols that denoted objects rather than alphabets.