For a long time I have sen that many last names can also be first names, like Howard, Stewart, Madison, or Hunter, and was wondering how that came to be.
In Spain, many first names can be found as last names (Juan, Esteban, Domingo), but not the other way around; you will not find someone whose first name is Sánchez, Carnicero, or Verde .
1500s!
William Camden's Remaines of a Greater Worke, Concerning Britaine was published in 1605. In it, in his essay on names, he says that using family names as personal names, baptismal names, is a relatively new minority practice, which many decry as un-Christian and disrespectful. He feels, though, that it is done with respect and affection, using either names connected to the family of the child or the name of a godparent. When he gives examples, though, it is of grown men, so it has been going on since probably 1550 or so as a trend, rather than an occasional eccentricity.
He also records other practices for us, like the general lack of middle names among the English, however much those on the Continent may pile them up, and the very rare use of double-naming as he can only think of a handful of Englishmen using a practice not common even among the French at that time.
However, you may note that these are all men. The use of family names for women I have not found until much later. It does not start as a trend until, I estimate, in the late 1800s. Not being hard line on this, as I have not researched this recently (been doing Welsh and Arabic), but that's the impression I've picked up from dated name gathering over the years. It also seems a stronger trend among Americans than the English.