Research ArticleNEUROSCIENCE

Australopithecus afarensis endocasts suggest ape-like brain organization and prolonged brain growth

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Science Advances  01 Apr 2020:
Vol. 6, no. 14, eaaz4729
DOI: 10.1126/sciadv.aaz4729
  • Fig. 1 Virtual reconstruction of Australopithecus afarensis infants from Dikika and Hadar.

    DIK-1-1 (A to G) and A.L. 333-105 (H to O) as preserved and reconstructed. (A) Frontal view. (B) Superior view. (C) Manual segmentation of the endocranial matrix reveals exceptional preservation of the endocranial cavity. (D to G) 3D models of the DIK-1-1 skull before (D and F) and after (E and G) virtual reconstruction in frontal and left lateral view. (H to K) Scan of the original A.L. 333-105 fossil specimen. (L) Manual segmentation. (M to O) Virtual reconstruction. The reconstructed outer shells of the braincases are shown as semitransparent surfaces. Scale bar, 1 cm.

  • Fig. 2 Virtual reconstructions of A. afarensis adults.

    (A) Reconstruction of A.L. 822-1 in superior view. One of the 122 thin-plate spline (TPS)–based reconstructions of the endocast is shown in blue. (B) A.L. 444-2; a TPS estimation of the endocranial surface is shown in red. (C) A.L. 333-45; endocast in green. This endocranial reconstruction was created by scaling the endocranial surface of 444-2 based on landmarks and semilandmarks on the available morphology. (D) A.L. 288-1 (Lucy); endocast in purple (TPS estimation). (E) A.L. 417-1; the endocast of A.L. 288-1 is shown as a semitransparent surface. (F) A.L. 162-28; the endocast of A.L. 288-1 is shown as a semitransparent purple surface for size comparison. Scale bar, 1 cm.

  • Fig. 3 Endocranial morphology of DIK-1-1.

    Virtual endocast in superior (A and B) and posterior view (D and E). Comparison of the endocranial surface with a juvenile chimpanzee brain (C and F) [3D model built from magnetic resonance images (MRIs)] illustrates the overall ape-like brain organization of the DIK-1-1 endocast, including an anteriorly placed lunate sulcus (L). Gyri are color-coded; sulci are labeled as in (C) and (F). Meningeal vessel impressions are shown in red. C, sulcus centralis; fs, frontalis superior; fm, frontalis medius; fi, frontalis inferior; fo, fronto-orbitalis; h, horizontal ramus of pci; ip, s. intraparietalis; pci, praecentralis inferior; pcs, praecentralis superior; ps, parietalis superior; pti, postcentralis inferior; ptm, postcentralis medius; pts, postcentralis superior; L, s. lunatus; ts, temporalis superior; ts-a, ramus temporalis superior; tm, temporalis medius; occi, occipitalis inferior; lc, s. calcarinus lateralis; u, s. calcarinus ramus superior; cereb, cerebellum; ld, lambdoidal suture. Scale bar, 1 cm.

  • Fig. 4 Endocranial morphology of A.L. 162-28.

    (A) Comparison of the partial cranium A.L. 162-28 with the reconstructed skull A.L. 822-1 (B). (C) Anterior view of A.L. 162-28. The micro-CT data reveal a previously undetected impression of a lunate sulcus (L; red) on the left and right side. What had previously been identified as the intraparietal sulcus (ip) is an impression of the lateral calcarine sulcus (lc) on the occipital lobe. The feature previously incorrectly identified as a possible human-like lunate sulcus impression is related to the remnants of the fused lambdoidal suture (ld; yellow) and the occipital inferior sulcus (occi; green). (D) Posterior view of the endocranial surface of A.L. 162-28. (E) Posterior view of a chimpanzee brain based on an in vivo MRI scan (“Amanda” from the Yerkes National Primate Research Center). (F) Chimpanzee endocast based on a postmortem CT scan (P. troglodytes verus from the Taï forest). We superimposed a grayscale gradient based on the local curvature to visually enhance the sulcal impressions.

  • Fig. 5 Age at death and endocranial growth curves.

    (A) Matching of DIK-1-1’s permanent right lower first molar and LUC for age at death determination using synchrotron virtual dental histology. The age at death is the summation of the initiation age (9 days), cuspal enamel formation time (228 days), and lateral enamel (624 days) in the canine: 861 days (2.4 years). (B) Absolute EV in the first 10 years of life in modern humans, chimpanzees, and A. afarensis. 1, A.L. 333-105; 2, DIK-1-1; 3, A. afarensis adult mean and range; 4, A. afarensis adult mean and range without A.L. 444-2; F, female; M, male. Average growth patterns are visualized using Gompertz curves for males and females (open circles and dotted line) separately; 95% single prediction bands based on the pooled sample. (C) Relative growth curves based on the ratio between EV and the adult mean EV (rEV) for modern humans and chimpanzees, split by sex. (D and E) As in (C), but the infants DIK-1-1 and A.L. 333-105 are compared against individual A. afarensis adults. The rEVs of the A. afarensis infants indicate protracted brain growth, in that it takes A. afarensis individuals longer than chimpanzees to reach their adult EVs.

  • Table 1 A. afarensis EVs.

    The means and SDs are based on the distribution of reconstructions (i.e., on the hundreds of individual estimates).

    SpecimenAgeEV mean (ml)SDMinMaxMethod
    DIK-1-12.4 years2751273277TPS estimation
    A.L. 333-1052.2–2.4 years313.53310317TPS estimation
    A.L. 822-1Adult3824374392TPS estimation
    A.L. 288-1Adult3889365417TPS estimation + scaling of A.L. 822-1
    A.L. 333-45Adult4881486492Scaling of A.L. 444-2
    A.L. 444-2Adult5221519526TPS estimation
    Adult A. afarensis44560365526

Supplementary Materials

  • Supplementary Materials

    Australopithecus afarensis endocasts suggest ape-like brain organization and prolonged brain growth

    Philipp Gunz, Simon Neubauer, Dean Falk, Paul Tafforeau, Adeline Le Cabec, Tanya M. Smith, William H. Kimbel, Fred Spoor, Zeresenay Alemseged

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