The Endocast of MH1, Australopithecus sediba Kristian J. Carlson, 1,2 * Dietrich Stout, 3 Tea Jashashvili, 1,4,5 Darryl J. de Ruiter, 1,6 Paul Tafforeau, 7 Keely Carlson, 6 Lee R. Berger 1,8 The virtual endocast of MH1 (Australopithecus sediba), obtained from high-quality synchrotron scanning, reveals generally australopith-like convolutional patterns on the frontal lobes but also some foreshadowing of features of the human frontal lobes, such as posterior repositioning of the olfactory bulbs. Principal component analysis of orbitofrontal dimensions on australopith endocasts (MH1, Sts 5, and Sts 60) indicates that among these, MH1 orbitofrontal shape and organization align most closely with human endocasts. These results are consistent with gradual neural reorganization of the orbitofrontal region in the transition from Australopithecus to Homo, but given the small volume of the MH1 endocast, they are not consistent with gradual brain enlargement before the transition. T he relative importance and timing of two critical processes in the evolution of the human brain—cortical reorganization and size increase—has been debated since the dis- covery of Australopithecus (1, 2). Recent incorpo- ration and validation of computer-based techniques for reconstructing and comparing endocranial casts (endocasts, proxies of brains from fossilized crania) have substantially improved the quality of data on this issue (3, 4). Eight endocasts [MLD 1, MLD 37/38, Sts 5, Sts 19, Sts 60, Sts 71, StW 505, and Taung (5, 6)] show that Au. africanus had an average cranial capacity of 459 cm 3 (37.7 SD). Three endocasts [AL 162-28, AL 333-45, and AL 444-2 (6)] show that Au. afarensis had an average cranial capacity of 481 cm 3 (75.6 SD). The earliest representative of the “robust” australopith lineage (KNM-WT 17000), on the other hand, had a comparatively small endocast —410 cm 3 ( 5) —whereas later mem- bers of the lineage such as Paranthropus boisei [KGA 10-525, KNM-ER 406, KNM-ER 407, KNM-ER 732, KNM-ER 23000, KNM-WT 13750, KNM-WT 17400, OH 5, and Omo L338y-6; mean = 485 cm 3 , SD = 45.6 ( 6)] and P. robustus [SK 54, SK 859, and SK 1585; mean = 493 cm 3 , SD = 40.4 ( 6)] had slightly larger average cranial capacities (5). Considering these data, Falk and colleagues (5) hypothesized that australopith brain size might have begun to increase gradually, and cortical reorganization might have begun well be- fore 2.0 million years ago (Ma) and the emergence/ evolution of Homo, although confirming data are sparse between 2.0 to 2.5 Ma. The partial cranium of the holotype juvenile male from Au. sediba, Malapa Hominin 1 (MH1), is dated to 1.977 Ma (7, 8) and thus provides crucial data for evaluating the pace of brain evolution in early hominins. On the basis of epi- physeal closure patterns in the associated post- cranial elements and development of the unerupted third molars, MH1 was at a developmental stage at death equivalent to that of a human child of 12 to 13 years, with brain growth essentially com- plete. The MH1 partial cranium has a virtual reconstructed cranial capacity estimate of 420 cm 3 (7), which is higher than one estimate for the Taung specimen [adult size-corrected cranial capacity = 406 cm 3 (4)], but more than 1 SD below the Au. africanus mean. The estimated vol- ume of the MH1 endocast is 33 cm 3 higher than the smallest estimate reported for Au. afarensis (AL 288-1) but below the Au. afarensis mean by nearly 1 SD (6). Thus, the MH1 estimated cranial capacity is at the lower end of the australopith spectrum of variation. Given its younger date relative to other australopiths (8) and possibly closer phylogenetic relationship to Homo (7), the MH1 endocast is difficult to reconcile with a pro- posed gradual trend in brain enlargement leading from australopiths to Homo (5) if Au. sediba is ancestral to Homo. Retained australopith (primitive) brain size in Au. sediba is intriguing given the appearance of derived morphology elsewhere in the cranium (7) and postcranial skeleton, particularly within the pelvis (9) and hand (10). Presumed selective REPORTS 1 Institute for Human Evolution, University of the Witwatersrand, Palaeosciences Centre, Private Bag 3, Wits 2050, South Africa. 2 Department of Anthropology, Indiana University, Bloomington, IN 47405, USA. 3 Department of Anthropology, Emory Univer- sity, Atlanta, GA 30322, USA. 4 Georgian National Museum, 0105 Tbilisi, Georgia. 5 Anthropological Institute and Museum, University of Zürich, Zûrich, CH-8057, Switzerland. 6 Depart- ment of Anthropology, Texas A&M University, College Station, TX 77843, USA. 7 European Synchrotron Radiation Facility, 38043 Grenoble cedex, BP220, France. 8 School of Geosciences, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa. *To whom correspondence should be addressed. E-mail: kristian.carlson@wits.ac.za A B C D Fig. 1. Virtual endocast of MH1 in (A) superior, (B) inferior, (C) left lateral, and (D) anterior views. 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