Phanerozoic data show that during continental dispersal the total length of the global plate boundary is directly related to the size distribution and the surface area of continental fragments. We use this observation to test theories about the length of ancient plate boundaries and how the increased mantle heat production in the Precambrian was manifested by the crust. Using Monte Carlo simulations of plate boundaries on a sphere, we calculate estimates of global plate-boundary length. For the present-day continents and plates, the mean plate-boundary lengths are within 1% of each other; 3.40 x 105 km and 3.43 x 105 km. From the size distribution and normalized area of Archean cratons, estimates of the length of plate boundary at 2.4 Ga range from ≤1.7 to 2.7 times the present length; the preferred mean value is 2.2 times the present length. This result favors a model of sea-floor tectonics in which global plate velocities and sea-floor spreading rates were about 16% slower than at present, and in which the oceanic crust was more than 20 km thick.