https://www.pnas.org/doi/10.1073/pnas.2200689119
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 (R2 = 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 (R2 = 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.
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