A chronomic tree of life: ontogenetic and phylogenetic ‘memories’ of primordial cycles — keys to ethics

Lamellar bone is an incremental tissue reconciling enamel rhythms, body size, and organismal life history

Mammalian enamel formation is periodic, including fluctuations attributable to the daily biological clock as well as longer-period oscillations that enigmatically correlate with body mass. Because the scaling of bone mass to body mass is an axiom of vertebrate hard tissue biology, we consider that long-period enamel formation rhythms may reflect corresponding and heretofore unrecognized rhythms in bone growth. The principal aim of this study is to seek a rhythm in bone growth demonstrably related to enamel oscillatory development. Our analytical approach is based in morphology, using a variety of hard tissue microscopy techniques. We first ascertain the relationship among long-period enamel rhythms, the striae of Retzius, and body mass using a large sample of mammalian taxa. In addition, we test whether osteocyte lacuna density (a surrogate for rates of cell proliferation) in bone is correlated with mammalian body mass. Finally, using fluorescently labeled developing bone tissues, we investigate whether the bone lamella, a fundamental microanatomical unit of bone, relates to rhythmic enamel growth increments. Our results confirm a positive correlation between long-period enamel rhythms and body mass and a negative correlation between osteocyte density and body mass. We also confirm that lamellar bone is an incremental tissue, one lamella formed in the species-specific time dependency of striae of Retzius formation. We conclude by contextualizing our morphological research with a current understanding of autonomic regulatory control of the skeleton and body mass, suggesting a central contribution to the coordination of organismal life history and body mass.

Prokaryotic and eukaryotic unicellular chronomics

An impeccable time series, published in 1930, consisting of hourly observations on colony advance in a fluid culture of E. coli, was analyzed by a periodogram and power spectrum in 1961. While the original senior author had emphasized specifically periodicity with no estimate of period length, he welcomed further analyses. After consulting his technician, he knew of no environmental periodicity related to human schedules other than an hourly photography. A periodogram analysis in 1961 showed a 20.75-h period. It was emphasized that "... the circadian period disclosed is not of exactly 24-h length." Confirmations notwithstanding, a committee ruled out microbial circadian rhythms based on grounds that could have led to a different conclusion, namely first, the inability of some committee members to see (presumably by eyeballing) the rhythms in their own data, and second, what hardly follows, that there were "too many analyses" in the published papers. Our point in dealing with microbes and humans is that analyses are indispensable for quantification and for discovering a biologically novel spectrum of cyclicities, matching physical ones. The scope of circadian organization estimated in 1961 has become broader, including about 7-day, about half-yearly, about-yearly and ex-yearly and decadal periodisms, among others. Microbial circadians have become a field of their own with eyeballing, yet time-microscopy can quantify characteristics with their uncertainties and can assess broad chronomes (time structures) with features beyond circadians. As yet only suggestive differences between eukaryotes and prokaryotes further broaden the perspective and may lead to life's sites of origin and to new temporal aspects of life's development as a chronomic tree by eventual rhythm dating in ontogeny and phylogeny.