Bolkian and Bokian retardation in Homo sapiens.

Although a low genetic barrier is said to separate humans from apes, Homo sapiens is characterized by striking developmental and anatomical particularities. On the one hand, humans have a very extended life history (retardation). On the other hand, human anatomy shows many instances of both neoteny and hypermorphosis. In 1918, Bolk proposed his 'retardation theory' that links both aspects of the human condition. We show in this paper that his theory becomes surprisingly powerful when Bolk's retardation principle is applied to generalized developmental gradients (such as the cephalocaudal gradient that dominates overall ontogeny). In this way, the main particularities of the human body (extended life history; long limbs, and very long legs; very low brachial and crural indices; highly developed pollex and hallux; reduced prognathism; advanced telencephalization etc.) can be reduced to a single developmental matrix. In 1926, Bok noted that, in a series of equivalent organs (such as the finger row), the ontogenetic latecomer tends to be more neotenic than a corresponding forerunner. We show that Bok's observation is a logical corollary of Bolk's retardation theory and that it is important for understanding some pecularities of human anatomy such as the pronounced differentiation between hands and feet.