Just as natural history is the mother of the biological sciences, social history is the mother of the social sciences. Classical political economy was developed in a dynamic era, a period of European history where a series of long-term social developments were approaching their peak in the French Revolution and the Industrial Revolution of England. It is not at all surprising that Adam Smith and David Hume spent time with the French Physiocrats, notably Quesnay and Turgot, and with the French philosophes, especially though not exclusively Voltaire and Rousseau.
It was precisely the dynamic quality of the era that led economic and social and political theorists alike to look into the nature and causes of change, and therefore regard history as a principal source of the foundations of their knowledge. A similar process was taking place, but more slowly, among biologists who were trying to discover the nature of life and the origin of species, and the causes of long-term biological changes through time. Darwinism was in fact the end, not the beginning, of this process.