The demography of man on earth is treated by a physical model for that forward transport for the subspecies in time. By examining the distribution of man and his Neanderthaler precursor, it is inferred, as a likely hypothesis, that the initial “Phase I” expansion of man on earth (40,000–15,000 ybp) was at a constant density of about 0.04 persons per sq.km with little or no remixing. We suggest that the nature of the physical process was a diffusion, a “random walk” process with a diffusive velocity of one roaming range (30 km) per generation, i.e., 1.5 km per year. The physics and physiology of the breeding process permits us to estimate the total earth population, the birth rate b, the death rate d, and the net difference (that is the Malthusian constant K, dP/dt = KP, where K = b-d, P = population, and dP/dt is the rate of change in population).