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Neonatal Shoulder Width Suggests a Semirotational, Oblique Birth Mechanism in Australopithecus afarensis
Jeremy M DeSilva1, Natalie M Laudicina2, Karen R Rosenberg3
1Department of Anthropology, Dartmouth College, Hanover, New Hampshire, 03755.
Insights
Birth in early hominins like Australopithecus afarensis involved a semi-rotational oblique mechanism, unlike modern humans. This suggests a two-stage evolution of birth, linked to pelvic adaptations and neonatal shoulder breadth.
Area of Science:
- Paleoanthropology
- Human Evolution
- Reproductive Biology
Background:
- Reconstructing early hominin birth mechanics typically focuses on head-pelvis size.
- Neonatal shoulder dimensions have been largely overlooked in these reconstructions.
Purpose of the Study:
- To estimate neonatal shoulder breadth in Australopithecus afarensis.
- To reconstruct the birth mechanism in Australopithecus afarensis.
- To understand the evolutionary stages of human birth mechanics.
Main Methods:
- Established a relationship between adult primate clavicular length and neonatal biacromial breadth.
- Applied this relationship to Australopithecus afarensis clavicular length to estimate neonatal shoulder breadth.
- Combined estimated shoulder dimensions with known head dimensions to model birth mechanics.
Main Results:
- Neonatal biacromial breadth in Australopithecus afarensis was estimated from adult clavicular length.
- A semi-rotational oblique birth mechanism is proposed for Australopithecus afarensis, with a tight fit in the birth canal.
- The head-and-shoulder rotational mechanism typical of modern humans was likely absent in Australopithecus afarensis.
Conclusions:
- The evolution of rotational birth may have occurred in two stages.
- The first stage, involving broad-shouldered neonates and specific pelvic adaptations, appeared with Australopiths.
- The second stage, associated with modern rotational birth, may be linked to the increasing brain size in the genus Homo.
Abstract:
Birth mechanics in early hominins are often reconstructed based on cephalopelvic proportions, with little attention paid to neonatal shoulders. Here, we find that neonatal biacromial breadth can be estimated from adult clavicular length (R2 = 0.80) in primates. Using this relationship and clavicular length from adult Australopithecus afarensis, we estimate biacromial breadth in neonatal australopiths. Combined with neonatal head dimensions, we reconstruct birth in A. afarensis (A.L. 288-1 or Lucy) and find that the most likely mechanism of birth in this early hominin was a semi-rotational oblique birth in which the head engaged and passed through the inlet transversely, but then rotated so that the head and shoulders remained perpendicular and progressed through the midplane and outlet oblique to the main axis of the female pelvis. Any other mechanism of birth, including asynclitic birth, would have resulted in either the head or the shoulders orthogonal to the short anteroposterior dimension of the A.L. 288-1 pelvis, making birth untenable. There is a tight fit between the infant and all planes of the birth canal, perhaps suggesting a difficult labor in australopiths. However, the rotational birth mechanism of large-brained humans today was likely not characteristic of A. afarensis. Thus, the evolution of rotational birth, usually associated with encephalization, may have occurred in two stages: the first appeared with the origin of the australopiths with their platypelloid pelves adapted for bipedalism and their broad-shouldered neonates; the second which resulted in the modern mechanism of rotational birth may be associated with increasing brain size in the genus Homo. Anat Rec, 300:890-899, 2017. © 2017 Wiley Periodicals, Inc.