Teeth, prenatal growth rates, and the evolution of human-like pregnancy in later Homo

Significance

Humans are characterized by having very large brains relative to body size. Because gestation is critically linked to brain size, pregnancy is an important but elusive aspect of hominid evolution. We developed two methods for reconstructing prenatal growth during this earliest phase of life history using brain size and dental morphology. Our results indicate a significant increase in prenatal growth rates (PGRs) throughout the terminal Miocene and Plio-Pleistocene with the evolution of human-like PGRs in later Homo, less than 1 million years ago. These results align with fossilized pelvic and cranial anatomy to support the evolution of human-like pregnancy in the Pleistocene and open up possibilities for novel ways to explore the evolution of hominid gestation via dental variation.

Abstract

Evidence of how gestational parameters evolved is essential to understanding this fundamental stage of human life. Until now, these data seemed elusive given the skeletal bias of the fossil record. We demonstrate that dentition provides a window into the life of neonates. Teeth begin to form in utero and are intimately associated with gestational development. We measured the molar dentition for 608 catarrhine primates and collected data on prenatal growth rate (PGR) and endocranial volume (ECV) for 19 primate genera from the literature. We found that PGR and ECV are highly correlated ( R ² = 0.93, P < 0.001). Additionally, we demonstrated that molar proportions are significantly correlated with PGR ( P = 0.004) and log-transformed ECV ( P = 0.001). From these correlations, we developed two methods for reconstructing PGR in the fossil record, one using ECV and one using molar proportions. Dental proportions reconstruct hominid ECV ( R ² = 0.81, P < 0.001), a result that can be extrapolated to PGR. As teeth dominate fossil assemblages, our findings greatly expand our ability to investigate life history in the fossil record. Fossil ECVs and dental measurements from 13 hominid species both support significantly increasing PGR throughout the terminal Miocene and Plio-Pleistocene, reflecting known evolutionary changes. Together with pelvic and endocranial morphology, reconstructed PGRs indicate the need for increasing maternal energetics during pregnancy over the last 6 million years, reaching a human-like PGR (i.e., more similar to humans than to other extant apes) and ECV in later Homo less than 1 million years ago.