Since their inception, cities have served as gathering places for humans and have become the most densely populated areas for both living and production. Around 4000–3000 BC, with the advancement of productivity in primitive society, handicrafts separated from primitive agricultural production, gradually expanded in scale, and generated surpluses. As surpluses of products (whether agricultural or handicraft) appeared in social production, the exchange emerged. Then exchange activities formed specialised venues gradually, thus human society had ‘markets’.