(Related to this question about the smallest land mass to support a civilization, possibly dropped in fully-formed by magic or advanced technology. This question is asked in response to comments there asking that the questions be separated.)
Assume a planet of Earth size and general composition. Presumably if the planet was entirely covered in water, then mammalian/primate life as we know it could not have evolved, and human civilization as we recognize it could not have developed. I imagine that there is some lower-bound for the dry land mass that would be required to support that development. What is it?
I'm looking for an alternate historical sequence that could lead to human society to the technological level of ancient cultures with organized cities with populations of tens-of-thousands (e.g.: Babylonian, Egyptian, Roman; pick whichever has the smallest dry-land requirement, if it matters greatly). Assume an expanse of land with optimal natural resources (whatever that would be; please specify if possible).
Edit: Assume specifically that we're looking for the evolution of a humanoid, intelligent, and landbound lifeform.