The first industrialised country was the UK, followed by Belgium. Later industrialised countries were Germany, the US, France and other Western European countries. According to Jeffrey Sachs, the current split between the developed and developing world is mostly an event of the 20th century.
The G7 has 10% of the world's population, but produces nearly 40% of the world's GDP. They also control many of the economic decisions that affect the world. The United Nations Security Council has five permanent members, four of those are in the G8 (G7 plus Russia).