Abstract
Cranial capacity, a proxy for the volume of the brain and associated cranial contents, is an important yardstick used to compare early hominin species because increasing brain size is a key characteristic of our lineage. In 1925, Raymond Dart claimed that a natural endocast found at the Buxton Limeworks near Taung, South Africa (which he named Australopithecus africanus), showed signs of neural reorganisation, but its juvenile status complicated comparison to other hominoid species. In an attempt to put its brain size and reorganisation into a comparative context, subsequent researchers have tried to estimate Taung’s adult cranial capacity by comparison to coarse-grained hominoid growth data. In this study, we simulated brain growth in A. africanus using asymptotic growth models in known-age mountain gorillas, chimpanzees and modern humans, and show that, at just under 4 years old, Taung’s brain had already finished or nearly finished growing according to hominoid developmental schedules. Percentage-growth remaining estimates are lower here than in previous studies using cross-sectional ontogenetic samples of unknown chronological age. Our new adult estimates (between 404 cm3 and 430 cm3 overall and 405–406 cm3 for chimpanzee models) are smaller than previous estimates with a ‘starting’ cranial capacity of 404 cm3, supporting the hypothesis that Taung’s adult brain size would have fallen toward the lower end of the A. africanus range of variation and strengthening the case that Taung was female.
 Significance:
 
 This is one of several recent studies to show that brain growth is completed in African apes and humans earlier than previously appreciated.
 New adult cranial capacity estimates for Taung are lower than previous estimates, supporting the hypothesis that Taung was female.
 Cessation of brain growth in hominoids at earlier ages than previously reported suggests that adult cranial capacities for hominin juvenile specimens have been overestimated.
 
 Open data set:
 http://dx.doi.org/10.17632/wyfvwd4s22
Highlights
The type specimen of Australopithecus africanus, Taung, is a juvenile skull consisting of a partial face with fragmentary pieces of the basicranium attached, a mandible, and a natural hemi-endocast.[1]
Taung has been the subject of intensive research focus because of its potential to resolve questions about hominin brain size and reorganisation[2,3,4,5,6,7] and A. africanus craniofacial growth[8,9,10,11], dental maturation[12,13,14,15,16,17,18] and brain ontogeny[19,20,21]
Researchers working with different age estimates and differing ideas about A. africanus growth trajectories have sometimes modelled Taung with a large percentagegrowth remaining, producing adult cranial capacity estimates >600 cm,3 2,3,19,24-26 which are larger than any known adult A. africanus specimens
Summary
The type specimen of Australopithecus africanus, Taung, is a juvenile skull consisting of a partial face with fragmentary pieces of the basicranium attached, a mandible, and a natural hemi-endocast.[1]. At 404 cm[3], Taung’s cranial capacity is already at the lower end of the range of A. africanus variation (Table 2), even though the Taung juvenile died after gingival eruption of the first molars but before the they had moved into functional occlusion, and so still had several years remaining to reach adulthood.[12,13,14,15,16,17,18,19] Taung’s importance to studies of hominin brain evolution and the scarcity of relatively complete crania and endocasts of adult A. africanus specimens have tempted researchers to estimate Taung’s adult cranial capacity. Researchers working with different age estimates and differing ideas about A. africanus growth trajectories have sometimes modelled Taung with a large percentagegrowth remaining, producing adult cranial capacity estimates >600 cm ,3 2,3,19,24-26 which are larger than any known adult A. africanus specimens. Size of the adult brain, which is approximated to some extent by cranial capacity, is an important parameter for understanding adaptive shifts in brain size and neural reorganisation early in human evolutionary history
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