Research ArticleANTHROPOLOGY

Evolution of brain lateralization: A shared hominid pattern of endocranial asymmetry is much more variable in humans than in great apes

See allHide authors and affiliations

Science Advances  14 Feb 2020:
Vol. 6, no. 7, eaax9935
DOI: 10.1126/sciadv.aax9935
  • Fig. 1 Brains, endocasts, and landmarks.

    (A) Casts of the internal bony braincase (endocasts) approximate the size and outer shape of the brain in humans (blue), chimpanzees (green), gorillas (black), and orangutans (orange). (B and C) Asymmetries of the brain, such as frontal and occipital petalias (differential projections of the left and right side indicated by arrows), are observable also on the endocast. (D) In this study, the differences (black lines) between an endocranial landmark configuration (blue spheres) and its relabeled reflection (red spheres) were used as a measure for the magnitude and spatial pattern of shape asymmetry without the need to define a midsagittal plane or any other external reference system.

  • Fig. 2 Symmetric and asymmetric variation of endocranial shape.

    (A) Principal component (PC) analysis of symmetrized endocranial shape. PC 1 versus PC 2 (59.4 and 12.8% of total shape variation, respectively). (B) Principal component analysis of endocranial shape asymmetry. PC 1 versus PC 2 (18.8 and 13.8% of total shape asymmetry, respectively). (C) Principal component analysis of endocranial shape asymmetry such as in (B), but asymmetry PC scores of each individual are shown as an arrow that represents the individual deviations from symmetry (which corresponds to the origin of the coordinate system). Humans (n = 95) are shown in blue, chimpanzees (n = 47) in green, gorillas (n = 43) in black, and orangutans (n = 43) in orange.

  • Fig. 3

    Shared directional shape asymmetry pattern. PC 1 of endocranial shape asymmetry in Fig. 2B is shown as a triangulated surface mesh of the 935 (semi)landmarks in (A) left, (B) right, (C) superior, (D) inferior, (E) occipital, and (F) frontal views. The deformation from a symmetric endocranial shape represents the spatial pattern of shape asymmetry; orange surfaces have larger areas as compared with the other side, and blue surfaces have smaller areas. See also movie S1.

  • Fig. 4 Asymmetric shape variation within taxa.

    (A) Taxon-specific mean asymmetry. The distribution of individual scores along this asymmetry pattern is shown on top (PDF, probability density function); the actual individual scores are shown below as lines. (B) Scree plots for taxon-specific principal component analyses of shape asymmetry display the portion of variance explained by PCs 1 to 10. (C) Taxon-specific principal component (PC) analyses of endocranial shape asymmetry. For each taxon, PC 1 versus PC 2 is shown. PC scores are shown as arrows, representing deviations from symmetry (which corresponds to the origin of the coordinate system). (D and E) Distribution of individuals along the first two taxon-specific PCs. Humans (n = 95) are shown in blue, chimpanzees (n = 47) in green, gorillas (n = 43) in black, and orangutans (n = 43) in orange.

  • Fig. 5 Magnitude of shape asymmetry.

    (A) Magnitude of total asymmetry (box-whisker plots by taxon). (B) Magnitude of fluctuating asymmetry (box-whisker plots by taxon). Humans (n = 95) are shown in blue, chimpanzees (n = 47) in green, gorillas (n = 43) in black, and orangutans (n = 43) in orange. Whiskers show the range (outliers as open circles), box and white line show the three quartiles, and the dumbbell represents the average.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/7/eaax9935/DC1

    Fig. S1. Endocranial landmark set.

    Fig. S2. Classic types of asymmetry and shape asymmetry.

    Fig. S3. Relationship of shape asymmetry and endocranial size.

    Fig. S4. Measurement error in repeated measurements analyses.

    Fig. S5. Effects of sample size and composition.

    Table S1. Descriptive statistics of PCA.

    Table S2. Symmetric and asymmetric variance in units of squared Procrustes distance per taxon.

    Table S3. Magnitudes of sample asymmetries decomposed into directional and fluctuating components.

    Table S4. Variance of shape asymmetry explained by log centroid size, with P values (permutation test) for the variance explained.

    Table S5. Descriptive statistics of absolute deviations from symmetry.

    Movie S1. Shared directional shape asymmetry.

    Movie S2. Human-specific asymmetry patterns.

    Movie S3. Chimpanzee-specific asymmetry patterns.

    Movie S4. Gorilla-specific asymmetry patterns.

    Movie S5. Orangutan-specific asymmetry patterns.

  • Supplementary Materials

    The PDF file includes:

    • Fig. S1. Endocranial landmark set.
    • Fig. S2. Classic types of asymmetry and shape asymmetry.
    • Fig. S3. Relationship of shape asymmetry and endocranial size.
    • Fig. S4. Measurement error in repeated measurements analyses.
    • Fig. S5. Effects of sample size and composition.
    • Table S1. Descriptive statistics of PCA.
    • Table S2. Symmetric and asymmetric variance in units of squared Procrustes distance per taxon.
    • Table S3. Magnitudes of sample asymmetries decomposed into directional and fluctuating components.
    • Table S4. Variance of shape asymmetry explained by log centroid size, with P values (permutation test) for the variance explained.
    • Table S5. Descriptive statistics of absolute deviations from symmetry.
    • Legends for movies S1 to S5

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Shared directional shape asymmetry.
    • Movie S2 (.mp4 format). Human-specific asymmetry patterns.
    • Movie S3 (.mp4 format). Chimpanzee-specific asymmetry patterns.
    • Movie S4 (.mp4 format). Gorilla-specific asymmetry patterns.
    • Movie S5 (.mp4 format). Orangutan-specific asymmetry patterns.

    Files in this Data Supplement:

Stay Connected to Science Advances

Navigate This Article