DOI: 10.3886/e208647v1
title: Sacum Data

description: The
ultimate sacral vertebra and its attendant sacral hiatus have anatomical and
evolutionary significance, as these structures serve as landmarks in clinical
practice and comparative studies. The
sacral hiatus is pivotal for identifying the terminus of the spinal cord in
radiological imaging, particularly in obstetrics. Its position, typically at
the S4 vertebra in most humans, is influenced by genetic factors, which play a
role in vertebral arch formation, but a systematic comparative analysis of a
diverse, global sample of humans and non-human apes is lacking. 

 

Using
digital imaging techniques and geometric morphometrics, this study examines
ultimate sacral morphology across a global sample of both preindustrial (i.e.,
“traditional living”) and postindustrial Homo sapiens (n = 82) and six
species of apes (n = 146). Results
indicate that humans typically exhibit the sacral hiatus at the penultimate
(S4) level, whereas apes show greater variability, often lacking a sacral
hiatus or having it located at the ultimate sacral level. Geometric
morphometric analyses, including principal component analysis (PCA), reveal
distinct shape differences between human and ape ultimate sacra. Humans
generally display wider sacral foramina and more laterally positioned sacral
cornua compared to apes, which typically have narrower foramina and more
medially placed cornua. These morphological differences likely relate to
biomechanical adaptations associated with bipedalism in humans, where a higher
position of the sacral hiatus may offer enhanced mechanical advantages for
trunk stabilization and movement. The
sacral hiatus, positioned higher in humans, may be an adaptation linked to the
unique demands of bipedal locomotion, suggesting that changes in sacral
morphology could be indicative of broader evolutionary shifts in locomotor
behavior from ape-like ancestors to modern humans.
description: These data express morphological and linear variation in the ultimate sacral
vertebra and sacral hiatus in a global sample of both preindustrial (i.e., “traditional
living”) and postindustrial Homo sapiens (n = 82) and across six species
of non-human apes (n = 146).
description: Adult individuals from nonindustrial (traditional) and industrial human populations; Adults from six species of non-human apes.
description: other; Extant
comparative specimens are from fully fused sacra from the Department of
Anthropology, New York University; Dart collection, University of the
Witwatersrand; American Museum of Natural History; Cleveland Museum of Natural
History; Florisbad Quarternary Research Station, National Museum, Bloemfontein;
Natural History Museum, London; Musée de l’Homme; University of Tennessee
Knoxville; Texas State University; University of Tokyo; and University of
California, Los Angeles. Data were collected from adult individuals for all
taxa (determined by fusion of the vertebral epiphyses).  Both postindustrial and preindustrial
individuals are represented in the global H. sapiens sample (n = 82). <br><br>The
preindustrial human skeletal remains were uncovered in archaeological sites
during the late 19th and early 20th centuries. These individuals were often
categorized by geographic region, and in some cases by population, though such
classifications are approximate and may not precisely represent ancestry. To
our knowledge, none of the individuals studied are currently being pursued for
repatriation by their descendant communities. The second author has signed
ethical agreements with most museums, ensuring that 3D data or images of the
remains are not shared without the museum's explicit consent. The authors will
continue to follow any new policies established by these institutions. We
respectfully acknowledge the significant scientific contributions of all individuals
or communities who have, whether knowingly or unknowingly, become part of the
skeletal collections studied. Similarly, we honor and recognize the descendants
of these individuals for their contributions with the utmost respect and
humility.

 

 

The
nonhominin hominoid sample (henceforth referred to as apes) is comprised of Pan
troglodytes (n = 69), Pan paniscus (n = 8), Gorilla
gorilla (n = 30), Hylobates lar (n = 28), Pongo pygmaeus
(n = 10), and Symphalangus syndactylus (n = 1).  Because the sample
sizes for Pan paniscus, Pongo,
and Symphalangus are small, they may provide unstable
estimates of effect size and we regard these as pilot data.  None of the specimens in the extant
comparative sample exhibited obvious pathological, developmental, traumatic, or
degenerative malformations. The sex ratio across taxa was nearly equal for each
taxon, although sample sizes for some analyses may differ where aspects of
sacral morphology were ambiguous.  

 

Linear
measurements were digitally collected via ImageJ (Version 14.9; (Schneider et al., 2012). To avoid perspective
distortion, photographs were taken with a lens-to-object distance in accordance
with the recommendations of Spencer and Spencer (1995).  A small number of 3D scans were aligned in
orthographic view in MeshLab (Version 2022.02; Cignoni
et al., 2008) and converted into 2D images for standardizing the
analyses.  A full list of linear and
angular variables and their descriptions is given in Table 1.  Student's t-tests (α = 0.05) were performed
on linear variables to assess statistical differences between groups, and
between individual fossils and the extant comparative sample using JMP®
(Version 16.0, SAS Institute Inc., Cary N.C.). 

 

Additional
analyses were conducted via two-dimensional geometric morphometric (GM)
analysis, an approach that can further elucidate morphological patterns (Hlusko, 2002, Martinon-Torres et al., 2006, Rosas et
al., 2008).  Nine landmarks were digitized along the dorsal side of the
sacrum using TPSdig2 digitization software (Version
2.31; Rohlf, 2017).  Generalized Procrustes superimposition of
landmark configurations was computed, generating a common coordinate system,
and set of shape variables (Bookstein et al.,
1985, Dryden and Mardia, 1998, Rohlf, 1999, Rohlf and Slice, 1990).  After landmarks were standardized via
generalized Procrustes analysis, Procrustes shape coordinates were analyzed via
principal component analyses (PCA) performed in PAST statistical software
(Version 4.13; Hammer, Harper, & Ryan, 2001). Relative warp analysis was
performed on the Procrustes shape coordinates to provide low-dimensional
representations of shape space (Bookstein, 1991).  Principal component analysis (PCA)
scatterplots using the variance–covariance matrix and relative warps were
generated for comparison of the first three principal components.