Suppose we say that the 'concept of natural law' is the following:
There are some precepts and proscriptions which:
- are binding upon everyone
- can be naturally known, and known to be so binding; and,
- are prior to human law, in the sense that:
- any human law which commands or forbids the same thing as these is binding because of these; and
- any human law, obedience to which would require disobedience to these, is not binding.
--By 'binding' I mean 'binding in conscience'; viz. it must be followed on pain of doing what one should recognize as wrong.
--We should expect these precepts and proscriptions to be few in number and to pertain to essentials of human social life. They would not, by any means, constitute the whole of 'justice' or 'morality.'
--These precepts and proscriptions might aptly be called natural because they are naturally known; are binding on all who share human nature; and are interpretable as laws that are necessary for the social life of those who have a nature such as ours.
--We should expect that these precepts and proscriptions would, additionally, be aptly interpreted as deriving from providential divine legislators.
Then it seems clear that the 'concept of natural law', so described, antedates the Stoics by far. It is found, for instance, with all of the elements described above, in the Antigone of Sophocles:
You, you with your face bent to the ground, do you admit, or deny that you did this?
I declare it and make no denial.
You, ...tell me--not at length, but briefly--did you know that an edict had forbidden this?
I knew it. How could I not? It was public.
And even so you dared overstep that law?
 Yes, since it was not Zeus that published me that edict, and since not of that kind are the laws which Justice who dwells with the gods below established among men. Nor did I think that your decrees were of such force, that a mortal could override the unwritten  and unfailing statutes given us by the gods. For their life is not of today or yesterday, but for all time, and no man knows when they were first put forth. Not for fear of any man's pride was I about to owe a penalty to the gods for breaking these.  Die I must, that I knew well (how could I not?). That is true even without your edicts. But if I am to die before my time, I count that a gain. When anyone lives as I do, surrounded by evils, how can he not carry off gain by dying?  So for me to meet this doom is a grief of no account. But if I had endured that my mother's son should in death lie an unburied corpse, that would have grieved me. Yet for this, I am not grieved.