Certainly ancient Greece and Rome, medieval England, and the minds of Washington, Franklin, Jefferson, and others were vital contributions to the ideas of the Declaration of Independence and the Constitution.
But one source, hiding in plain sight, so to speak, is frequently overlooked. This is the contribution of native Americans, particularly the Iroquois, to the mix. The Iroquois constitution, called the Great Law of Peace, or Gayanashagowa, contains many echoes of our Constitution, and in a number of respects, is more advanced in thought than the Constitution that resulted from the Convention of 1787.
This is not something I made up. If you read the original documents from the time, from people like Benjamin Franklin and George Washington, you will easily see that they deeply acknowledged their debt to the Iroquois and other native Americans. It's no accident that the protestors at the Boston Tea Party chose to disguise themselves as Indians. They did this out of respect for the democratic and free nature of Indian society - something they were trying to establish in the face of what they considered British tyranny.
Who were the Iroquois? Here's how the Wikipedia article on the Iroquois describes it:
The Iroquois Confederacy (Haudenosaunee, also known as the League of Peace and Power, Five Nations, or Six Nations, mostly Six nations now a days) is a group of First Nations/Native Americans. It was made up of six tribes: the Mohawks, the Oneidas, the Onondagas, the Cayugas, and the Senecas. A sixth tribe, the Tuscarora, joined after the original five nations were formed. They are also sometimes called the people of the Long house. They are often referred to as Iroquois, a term that some members of the group consider derogatory.
The Union of Nations was established prior to major European contact, complete with a constitution known as the Gayanashagowa (or "Great Law of Peace") with the help of a memory device in the form of special beads called wampum that have inherent spiritual value...(by Ira Krakow)