The history of developed countries reveals a general pattern of development in their economic structure.
First, Primary sectors predominandy contributed to GDP and held most of the employment. Then when agricultural activities increased, there was need for industrialisation and gradually industrial sectors dominated the economy. .
Much of the workers shifted to industrial sector, but Primary productivity did not hamper as industrial sector produced much sophisticated instruments and inputs that increased the productivity and filled the gap of loss of workforce. After a hundred years, service sector groomed up and most of the workers shifted to service sector and now, service sector contributes maximum to the share of economy.
The service sector has now become the most important sector in terms of total production and employmeht generation without disturbing the production and productivity of other two sectors.
So, it is seen that central tendency of economic structure had been shifting from Primary to Secondary and finally to Tertiary sector in developed countries. Similarly, almost all the developing countries are following the same path but may be in a different pace.