A number of common English surnames are the same as common English given names, with the addition of an "S." Examples are Peters, Daniels, Michaels, Matthews, Roberts, Phillips, Isaacs, Williams, etc.
What is the root cause of this phenomenon? Is it related to the formation of surnames from patronyms? Most of that same list also has common variations ending in "-son" (Peterson, Danielson, Robertson, etc.)
Note: I did a Google search around this question, but the results all had to do with pluralizing a surname, or making it possessive.