Aristotle famously said that human beings are political animals. Adam Smith said that human beings are trading animals. I believe both claims are right, in a quite literal sense: human being are evolved to live in groups and to trade with one another. Being able to navigate issues that arise when living in groups and being able to execute trades with others are evolutionary adaptations in same way that the even more basic instincts of interest in food and sex are evolutionary adaptations. The books above pursue these two themes of human beings as social animals and human beings as trading animals.
There are many who doubt the existence of "natural law." Natural law may be quite incomplete and fail to answer many key questions we face, but evolutionary adaptations for living in groups and for trading provide at least some limited corpus of "natural law" written on our genes. (Though when I say "written on our genes" I mean written on the genes of the majority of us who are not sociopaths.)
To those that say, there were never any men in the state of nature, I will not only oppose the authority of the judicious Hooker, Eccl. Pol. lib. i. sect. 10. where he says, “The laws which have been hitherto mentioned, i. e. the laws of nature, do bind men absolutely, even as they are men, although they have never any settled fellowship, never any solemn agreement amongst themselves what to do, or not to do: but forasmuch as we are not by ourselves sufficient to furnish ourselves with competent store of things, needful for such a life as our nature doth desire, a life fit for the dignity of man; therefore to supply those defects and imperfections which are in us, as living single and solely by ourselves, we are naturally induced to seek communion and fellowship with others: this was the cause of men’s uniting themselves at first in politic societies.” But I moreover affirm, that all men are naturally in that state, and remain so, till by their own consents they make themselves members of some politic society; and I doubt not in the sequel of this discourse, to make it very clear.
One thing Hooker seems to take for granted, but is in fact quite remarkable, is the capacity of human beings to make solemn agreements and have those agreements mean something. The ability to make and keep promises—even if not 100 percent of the time—is an amazing ability that does a lot to help make us who we are. This ability to make and keep promises is crucial both for human beings as political animals and for human beings as trading animals.