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 human results from the occasional expression of these latencies in Homo sapiens; they do not recapitulate ancestral conditions.
In their classical study on cranial suture closing. Todd and Lyon (1924-25) expressed their belief that suture closure proceeds according to some periodic pattern. Spectral analysis applied to their data corroborates the opinion of Todd and Lyon. Closure of the cranial sutures in humans is pervaded by a circaseptennian periodicity. Pulses of accelerated closure occur at intervals of about 7 years, and are separated by periods of relative quiescence.
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