title: Recent origin of low trabecular bone density in modern humans.
abstract: Humans are unique, compared with our closest living relatives (chimpanzees) and early fossil hominins, in having an enlarged body size and lower limb joint surfaces in combination with a relatively gracile skeleton (i.e., lower bone mass for our body size). Some analyses have observed that in at least a few anatomical regions modern humans today appear to have relatively low trabecular density, but little is known about how that density varies throughout the human skeleton and across species or how and when the present trabecular patterns emerged over the course of human evolution. Here, we test the hypotheses that (i) recent modern humans have low trabecular density throughout the upper and lower limbs compared with other primate taxa and (ii) the reduction in trabecular density first occurred in early Homo erectus, consistent with the shift toward a modern human locomotor anatomy, or more recently in concert with diaphyseal gracilization in Holocene humans. We used peripheral quantitative CT and microtomography to measure trabecular bone of limb epiphyses (long bone articular ends) in modern humans and chimpanzees and in fossil hominins attributed to Australopithecus africanus, Paranthropus robustus/early Homo from Swartkrans, Homo neanderthalensis, and early Homo sapiens. Results show that only recent modern humans have low trabecular density throughout the limb joints. Extinct hominins, including pre-Holocene Homo sapiens, retain the high levels seen in nonhuman primates. Thus, the low trabecular density of the recent modern human skeleton evolved late in our evolutionary history, potentially resulting from increased sedentism and reliance on technological and cultural innovations.
authors.affiliation: Center for the Advanced Study of Hominid Paleobiology, Department of Anthropology, The George Washington University, Washington, DC 20052; Human Origins Program, Department of Anthropology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560; habibachirchir@gmail.com brichmond@amnh.org.
authors.forename: Habiba
authors.initials: H
authors.lastname: Chirchir
authors.affiliation: Animal Postcranial Evolution Laboratory, School of Anthropology and Conservation, University of Kent, Canterbury, Kent, CT2 7NR, United Kingdom; Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, D-04103 Leipzig, Germany;
authors.forename: Tracy L
authors.initials: TL
authors.lastname: Kivell
authors.affiliation: Center for Functional Anatomy and Evolution, Johns Hopkins University School of Medicine, Baltimore, MD 21205;
authors.forename: Christopher B
authors.initials: CB
authors.lastname: Ruff
authors.affiliation: Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, D-04103 Leipzig, Germany;
authors.forename: Jean-Jacques
authors.initials: JJ
authors.lastname: Hublin
authors.affiliation: Evolutionary Studies Institute, The University of the Witwatersrand, Braamfontein 2000 Johannesburg, South Africa; Department of Anthropology, Indiana University, Bloomington, IN 47405; and.
authors.forename: Kristian J
authors.initials: KJ
authors.lastname: Carlson
authors.affiliation: Evolutionary Studies Institute, The University of the Witwatersrand, Braamfontein 2000 Johannesburg, South Africa;
authors.forename: Bernhard
authors.initials: B
authors.lastname: Zipfel
authors.affiliation: Center for the Advanced Study of Hominid Paleobiology, Department of Anthropology, The George Washington University, Washington, DC 20052; Human Origins Program, Department of Anthropology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560; Division of Anthropology, American Museum of Natural History, New York, NY 10024 habibachirchir@gmail.com brichmond@amnh.org.
authors.forename: Brian G
authors.initials: BG
authors.lastname: Richmond
mesh_terms: D000328:Adult; D000818:Animals; D005075:Biological Evolution; D049628:Body Size; D015519:Bone Density / Q000502:physiology*; D001842:Bone and Bones / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D005121:Extremities / Q000033:anatomy & histology; D005260:Female; D005580:Fossils; D015186:Hominidae / Q000033:anatomy & histology* / Q000502:physiology*; D006801:Humans; D021621:Imaging, Three-Dimensional; D008297:Male; D059125:Neanderthals / Q000033:anatomy & histology / Q000502:physiology; D002679:Pan troglodytes / Q000033:anatomy & histology / Q000502:physiology; D011148:Pongo pygmaeus / Q000033:anatomy & histology / Q000502:physiology; D011323:Primates / Q000033:anatomy & histology / Q000502:physiology; D055114:X-Ray Microtomography
keywords: Homo sapiens; gracilization; human evolution; sedentism; trabecular bone