Here's a quote by the coffee wiseman Don Schoenholt from alt.coffee, 1999:
In Harrer Ethiopia, in a dry and often arid terrain, where coffee has been realized much the same way today as in earliest times, the half dried cherries are spread out in the sun until dry, until the skins, pulp, and other material can be removed from the beans with the help of hulling by stones. With adjustments made for geography, and local traditions this is how coffee has been separated for many generations.
It looks if you put the beans in a tumbler of some sort, with some rocks, you can separate the beans and hulls.