Human‐like enamel growth in Homo naledi

Abstract

ObjectivesA modern pattern (rate and duration) of dental development occurs relatively recently during human evolution. Given the temporal overlap of Homo naledi with the first appearance of fossil Homo sapiens in Africa, this small‐bodied and small‐brained hominin presents an opportunity to elucidate the evolution of enamel growth in the hominin clade. Here we conduct the first histological study of two permanent mandibular canines and one permanent maxillary first molar, representing three individuals attributed to H. naledi. We reconstruct the rate and duration of enamel growth and compare these findings to those reported for other fossil hominins and recent humans.Materials and MethodsThin sections of each tooth were produced using standard histological methods. Daily and longer period incremental markings were measured to reconstruct enamel secretion and extension rates, Retzius periodicity, canine crown and molar cusp formation time.ResultsDaily enamel secretion rates overlapped with those from recent hominins. Canine crown formation time is similar to that observed in recent Europeans but is longer than canine formation times reported for most other hominins including Australopithecus and H. neanderthalensis. The extended period of canine formation appears to be due to a relatively tall enamel crown and a sustained slow rate of enamel extension in the cervical portion of the crown. A Retzius periodicity of 11 days for the canines, and nine days for the molar, in H. naledi parallel results found in recent humans. An 11‐day periodicity has not been reported for Late Pleistocene Homo (H. erectus, H. neanderthalensis) and is rarely found in Australopithecus and Paranthropus species.DiscussionEnamel growth of H. naledi is most similar to recent humans though comparative data are limited for most fossil hominin species. The high Retzius periodicity values do not follow expectations for a small‐brained hominin.

Linear enamel hypoplasia inHomonaledireappraised in light of new Retzius periodicities

“…However, this is misleading since it is well known that, due to the way in which enamel layers are deposited, longer lasting LEH become naturally deeper as well (Guatelli-Steinberg, 2003). To avoid this problem, "daily enamel deficit" (last column in Table 7) is calculated (Skinner, 2023) by dividing depth by the number of days over which an LEH formed (PK count in occlusal wall  Retzius periodicity: 9 or 11 [Mahoney et al, 2024]). Indeed, as shown in Table 7, the daily enamel deficit is significantly greater among short duration LEH.…”

“…LEH1 through LEH3 account for 74% of all LEH in our data. In other words, most anterior teeth capture only about 1 year of developmental stress despite some crowns taking as long as 5 years to form (Mahoney et al, 2024) presumably because developmental stress only commences relatively late in crown formation (Table 9).…”

“…The advent of newly-determined RP for H. naledi (Mahoney et al, 2024) enables a reappraisal of the timing and duration of developmental defects of enamel in this taxon and provides a glimpse into the timing of early childhood experiences. Retzius periodicity has now been observed among three teeth from H. naledi: two canines with a RP of 11 days and a molar with a RP of 9 days (Mahoney et al, 2024). The canines are not antimeres.…”

“…Regrettably, at the time of our examination and taking of molds, the labial surface of this tooth was partially obscured by glue, precluding direct counting of perikymata. These RP values are relatively high for Homo (see discussion in [Mahoney et al, 2024]), resulting in significantly increased estimates of duration and recurrence of developmental stress in this taxon, necessitating reconsideration of potential etiologies. In addition, a new catalogue of dental remains ('canonical tooth identifications') from the Rising Star assemblage has recently appeared (Delezene et al, 2023), which cautiously attributes some of the teeth to individuals designated as "associations."…”