Where did many modern ideas about law originally come from?

Answer: theory/concept of natural state.

Explanation: Modern times (in philosophy usually associated with René Descartes, who died in 1650, i. e. times when so-called modern philosophy was initiated) mark the beginning of non-religious explanation of society. It means that social philosophers no more refer to Bible and Biblical mythology as their primary resource. Instead of that they work with the fiction of "natural state". These theories are usually called theories of natural state and within these concept one can find not only concept of human nature (as a part of modern philosophical anthropology) but also so called "natural rights" of men. It is especially Locke and Hobbes that mark that period in this respect, but also bit younger Jean Jacques Rousseau.