Louis Bolk revisited: I. Is the human lung a retarded organ?

The human lung shows permanent retention of certain features (such as the absence of an infracardiac lobe, or the subdivision of the lobes in lobules) found in most mammal fetuses or youngsters. This phenomenon seems to be in line with the fetalization theory of Louis Bolk, who considered man as a retarded and unspecialized mammal. Moreover, the retention of certain fetal or unspecialized traits in the human lung favors a body configuration suited to an upright stance. This suggests that the generalized mammalian body plan is that of a bipedal being.