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. (shrink)

An atavism is the ..reappearance of a lost character (morphology or behaviour) typical of remote ancestors and not seen in the parents or recent ancestors of the organisms displaying the atavistic character (Hall, 1984). In humans, hypertrichosis (extensive body hair), the presence of a tail and supernumerary nipples are often quoted as examples (Hall, 1995). However, Louis Bolk (1866–1930) explained these phenomena in another way. He considered human morphology as an unspecialized expression of the mammalian developmental pattern. The latter also (...) encompasses potentialities for unilateral or propulsive development pathways (specializations) that usually remain latent in humans, but can become expressed in other species. According to Bolk, the appearance of so-called atavisms in humans results from the occasional expression of these latencies in Homo sapiens; they do not recapitulate ancestral conditions. (shrink)