A comprehensive survey of Retzius periodicities in fossil hominins and great apes

Abstract

Recent studies have provided great insight into hominin life history evolution by utilitizing incremental lines found in dental tissues to reconstruct and compare the growth records of extant and extinct humans versus other ape taxa. Among the hominins, studies that have examined Retzius periodicity (RP) variation have come to contradictory conclusions in some instances. In order to clarify RP variation among hominins and better place this variation in its broader evolutionary context, we conduct the most comprehensive analysis of published RP values for hominins and great apes to date. We gathered all available data from the literature on RP data from extant humans, great apes, and fossil hominins, and assessed their variation using parametric and nonparametric analyses of variance. We also performed phylogenetic generalized least-squares (PGLS) regressions of RP data for these taxa as well as a larger set of hominoids for which RP data have been published against data for body mass, encephalization, and mean semicircular canal radius (SCR, a proxy for metabolic rate). Our results show that modern humans have a mean RP significantly differing from that of other hominins. Pongo also is significantly different from nearly all other taxa in all analyses. Our results also demonstrate that RP variation among hominins scales with respect to body mass, encephalization, and SCR similarly to other hominids, but that modern humans and Pongo stand out in this regard. Operating within the hypothesis that RP reflects autonomic biorhythms that regulate multiple life history variables, our results reinforce the idea that Homo sapiens has evolved a life history distinct from other hominins, even from other members of Homo, and suggest that many of these life history differences may be driven by hypothalamic output from the brain.

Human‐like enamel growth in Homo naledi

“…The mean RP of recent humans is higher than that of other hominins (Hogg et al, 2020) though RP values of individual teeth for some species (e.g., A. anamenis, P. robustus, A. africanus; Smith et al, 2015) and the Middle Pleistocene Xujiayao juvenile (Xing et al, 2019) lie within the upper range of values reported for recent humans.…”

“…Others have identified a strong relationship between the timing of enamel layers (Retzius periodicity) and encephalization when compared across fossil hominins (Hogg et al, 2020). Based upon these studies, the dental development of H. naledi should be similar to earlier hominins.…”

“…Mean RP scales positively and significantly with relative encephalization (endocranial volume / body mass) when just hominins are considered (Hogg et al, 2020). Relative encephalization for H. naledi is 13.2 calculated from an endocranial volume of 535.0 mL and body mass of 40.5 kg (mean values calculated from Garvin et al, 2017;Hawks et al, 2017;Holloway et al, 2018).…”

“…It is hypothesized that the cycle regulates cell growth when compared between mammalian species (Bromage et al, 2009), and it has been observed that RP relates to body mass and maturation rate within human adolescents (Mahoney et al, 2022). These findings have been important for developing ideas about the evolution and pace of primate life history (Bromage et al, 2012), whereby the higher mean RP of recent humans relative to other hominins may relate to a unique life history profile (e.g., Hogg et al, 2020).…”

Dental biorhythm is associated with adolescent weight gain

“…Researchers during the 1990s 40,41 suggested variation in RP might relate to species-specific average body mass. Interspecific studies (meaning studies comparing different species) confirmed these observations revealing that, with exceptions 42,43 , RPbiorhythm was higher ('slower') in larger-bodied living species, including anthropoids 30,31,[44][45][46] . In these studies, biological pathways connecting RP and interspecific variation in body size were proposed.…”