My analysis of consumption, population, and group sizes leads to the following rule of thumb. As the daily time taken by the average person in a community to interact with others (consume resources) increases by a factor of three, the community’s size and per capita consumption increase by the same factor, longevity (time taken for the community to deplete all of its resources) decreases by 90 percent, life expectancy increases by half a generation (a decade), and the required speed increases by a factor of nine.
Using one full hour per day, the life expectancy is one generation (20 years), the community has 40 million people, and the speed is one foot per minute. In 0 A.D., longevity for one hour of interaction was about 3 million years; it is now 2 million years.
Beyond a full day of consumption, with a population of one billion people, automation must be used to enable the required interactions for consumption and life expectancy to increase (with such “productivity” squeezing out more than 24 hours in a day). Put another way, the world depends on technology to entirely support more than five billion people.