The bill of rights came from the English Bill of rights when America was still under Britain's control. When we left Britain's control, we needed a foundation for our government to run off of. So, the founders went to work writing the Constitution. Federalists argued that the Constitution did not need a BoR. Anti-federalists refused to sign the const. without a BoR. The Bill of Rights was added and the Constitution was ratified.
In 1787 delegates from the thirteen states convened in Philadelphia to replace the unwieldy Articles of Confederation with a Constitution to endow the federal government with sufficient power to hold the country together. This short document, which was a result of both high idealism and low political horsetrading, has survived a Civil War and the massive expansion of the country to become the oldest constitution in the world. The Constitution itself, however, merely describes the functions of the federal government. It was left to ten Amendments, known as the Bill of Rights, to protect individual freedoms.