The concept of natural rights blossomed in European thought in the 12th century and was fully developed into a coherent theoretical construct by the end of the 14th century. Thomas Aquinas was an early developer of natural law.
Natural law was the basis for the political philosophies of Thomas Hobbes and John Locke, among many others. Any political philosophy premised upon the sociality of mankind has its roots in natural law.
Common law – the accretion of law through precedential court decisions – was heavily influenced by natural law. This is particularly true in the instance of English common law, from which American law took its cue.
1/3rd of the world’s population – ~2.3 billion – live under common law, or a mixture of it and civil law.