Canine sexual dimorphism in Ardipithecus ramidus was nearly human-like

Bibliographic Collection: 
APE
Publication Type: Journal Article
Authors: Suwa, Gen; Sasaki, Tomohiko; Semaw, Sileshi; Rogers, Michael J; Simpson, Scott W; Kunimatsu, Yutaka; Nakatsukasa, Masato; Kono, Reiko T; Zhang, Yingqi; Beyene, Yonas; Asfaw, Berhane; White, Tim D
Year of Publication: 2021
Journal: Proc Natl Acad Sci U S A
Volume: 118
Issue: 49
Date Published: 2021 Dec 07
Publication Language: eng
ISSN: 1091-6490
Keywords: Animals, Bayes Theorem, Biological Evolution, Cuspid, Female, Fossils, Hominidae, Humans, Male, Models, Theoretical, Phylogeny, Sex Characteristics
Abstract:

Body and canine size dimorphism in fossils inform sociobehavioral hypotheses on human evolution and have been of interest since Darwin’s famous reflections on the subject. Here, we assemble a large dataset of fossil canines of the human clade, including all available Ardipithecus ramidus fossils recovered from the Middle Awash and Gona research areas in Ethiopia, and systematically examine canine dimorphism through evolutionary time. In particular, we apply a Bayesian probabilistic method that reduces bias when estimating weak and moderate levels of dimorphism. Our results show that Ar. ramidus canine dimorphism was significantly weaker than in the bonobo, the least dimorphic and behaviorally least aggressive among extant great apes. Average male-to-female size ratios of the canine in Ar. ramidus are estimated as 1.06 and 1.13 in the upper and lower canines, respectively, within modern human population ranges of variation. The slightly greater magnitude of canine size dimorphism in the lower than in the upper canines of Ar. ramidus appears to be shared with early Australopithecus, suggesting that male canine reduction was initially more advanced in the behaviorally important upper canine. The available fossil evidence suggests a drastic size reduction of the male canine prior to Ar. ramidus and the earliest known members of the human clade, with little change in canine dimorphism levels thereafter. This evolutionary pattern indicates a profound behavioral shift associated with comparatively weak levels of male aggression early in human evolution, a pattern that was subsequently shared by Australopithecus and Homo.

DOI: 10.1073/pnas.2116630118
Alternate Journal: Proc Natl Acad Sci U S A