As late as 1625 most European houses had a tamped earthen floor. It was with the industrialization of the forest that wood floors began replacing the earth.  And now, concrete floor systems are replacing wood floors. The hardness and flatness of conventional flooring is unnatural for the biology of our feet, and actually may be the cause for some back, hip and knee injuries. Earthen floors are smooth, leathery, and cool, and are actually pleasant on the feet when they have regular imperfections, in terms of how level and smooth they are.  They can also be built to include radiant heat, so that they can be warm in cool weather.  Another important positive attribute of an earthen floor is it's thermal mass, meaning that it helps moderate indoor temperature fluctuations by absorbing and slowly releasing heat in the air.