Histological evidence for ontogenetic differences between modern human and Neanderthal dentitions

Abstract

Humans have an unusual life history with an early weaning age, long childhood, late first reproduction, short interbirth intervals, and long lifespan. Despite 80 years of speculation, the origins of these developmental patterns in Homo sapiens remain unknown. Because they record daily growth during formation, teeth have provided important insights, revealing that australopiths and early Homo had more rapid ontogenies than recent humans. Here we apply synchrotron virtual histology to a geographically and temporally diverse sample of Middle Paleolithic juveniles, including Neanderthals, to assess tooth formation and calculate age at death from dental microstructure. We quantified the following developmental variables: cuspal enamel thickness, long-period line periodicity (number of daily increments between successive lines), long-period line number, coronal extension rate, and crown formation time in 90 teeth from 28 Neanderthals and 39 teeth from 9 fossil H. sapiens individuals. When compared with both European and African recent humans, thinner enamel, lower long-period line periodicities, and faster extension rates typically lead to lower crown formation times in Neanderthals. We find that most Neanderthal tooth crowns grew more rapidly than modern human teeth, resulting in significantly faster dental maturation. These findings demonstrate that recent human developmental standards should not be used to assess Neanderthal ontogeny. In contrast, Middle Paleolithic H. sapiens juveniles show greater similarity to recent humans. When compared to earlier hominin taxa, both Neanderthals and H. sapiens appear to have extended the duration of dental development. This period of dental immaturity is particularly prolonged in modern humans.