Research ArticleEVOLUTIONARY BIOLOGY

Evo-devo models of tooth development and the origin of hominoid molar diversity

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Science Advances  11 Apr 2018:
Vol. 4, no. 4, eaar2334
DOI: 10.1126/sciadv.aar2334
  • Fig. 1 Anatomical and developmental configuration of molar cusps.

    Occlusal view of the EDJ of an (A) upper and (B) lower molar with cusp nomenclature. (A) Paranthropus robustus (SK831a, ULM3) and (B) Australopithecus africanus (STW560b, LLM3). An occlusal view of outer enamel surface (OES) is shown in red as an inset (not to scale). pro, protocone; pa, paracone; me, metacone; hyp, hypocone; C5, cusp 5; CC, Carabelli’s cusp; prod, protoconid; med, metaconid; end, entoconid; hyd, hypoconid; hyld, hypoconulid/cusp 5; C6, cusp 6 (tuberculum sextum); C6D, double cusp 6; C7, cusp 7. The OES in (A) exhibits both cusps 5 and 6 (cusp 6 not present at EDJ). (C) Schematic of potential developmental pathways during tooth crown morphogenesis. Location of EKs (yellow spheres) superimposed onto an OES and exaggerated for visualization purposes. Note that in developing teeth, cusp spacing is regulated by EKs, affecting the growth of the intercuspal regions and distances (denoted by blue polygons) at which new EKs and cusps form. Growth of tooth after the cusp patterning can further modify the cusp distances.

  • Fig. 2 Accessory cusp expression of hominoid upper molars.

    Box plots of relative intercusp distance by degree of expression for the hypocone (A) in H. sapiens and for cusp 5 (B), cusp 6 (C), and Carabelli’s cusp (D) in hominoids (generic analysis for Carabelli’s cusp in fig. S1). Line of fit and 95% confidence intervals are shown in light blue for visualization of trends only. The PCM predicts a negative relationship between trait expression and mean intercusp distance for the hypocone and Carabelli’s cusp and a positive relationship between trait expression and expanded intercusp distance for cusps 5 and 6. For box plots, cusp absence is in gray, cusp “suspected” is in yellow, and cusp presence is in green. Cusp analyzed in each box plot is circled in red, and landmarks used to calculate intercusp distance are in yellow. Genera are not depicted if the accessory cusp was invariably absent/present in our sample. The asterisk indicates outliers. UM, upper molar.

  • Fig. 3 Accessory cusp expression of hominoid lower molars.

    Box plots of relative intercusp distance by degree of expression for cusp 5 (A), cusp 6 (B), cusp 7 (C), and cusp 6 double (D) in hominoids. Line of fit and 95% confidence intervals are shown in light blue for visualization of trends only. The PCM predicts a positive relationship between trait expression and intercusp distance for cusps 5, 6 (single and double), and 7. For box plots, cusp absence is in blue, cusp suspected is in yellow, and cusp presence is in red. Cusp analyzed in each box plot is circled in red, and landmarks used to calculate intercusp distance are in yellow. Genera are not depicted if the accessory cusp was invariably absent/present in our sample. The asterisk indicates outliers. LM, lower molar.

  • Table 1 Kendall’s rank correlation coefficient between accessory cusp expression and relative intercusp distance.

    Sample size for each pairwise comparison in parentheses. Significant values (at P < 0.05) bolded. Blank cells indicate that the feature was invariably absent/present or that samples did not have enough grades of expression for statistical analysis. AUS, Australopithecus; PAR, Paranthropus; HOM, Homo; HSr, recent H. Sapiens; PAN, Pan; GOR, Gorilla; PON, Pongo.

    TaxonHYPUMC5CCLMC5LMC6LMC6DLMC7
    AUS0.21 (35)−0.31* (24)0.39* (48)0.29 (13)0.26 (19)
    PAR0.22* (39)−0.09 (24)0.37* (46)0.38* (38)
    HOM−0.24* (94)−0.03 (113)−0.01 (79)0.55* (144)0.36* (127)0.17 (29)0.14 (39)
    HSr−0.23* (48)0.07 (49)0.14 (35)0.56* (104)0.39* (67)
    PAN−0.02 (49)0.09 (35)0.28* (69)−0.04 (46)
    GOR0.51* (16)−0.16 (11)
    PON−0.26 (16)0.13 (14)0.33* (29)

    *Adherence to PCM predictions (only significant results considered).

    • Table 2 Sample composition for hominoid upper and lower molars used in this study.
      TaxonUpper molarsLower molars
      UM1UM2UM3UMTotalLM1LM2LM3LMTotal
      A. anamensis235347
      A. afarensis211153429
      A. africanus1011712914211651
      P. aethiopicus22
      P. boisei11134116
      P. robustus161283620141347
      Homo sp./habilis s.l.4265319
      H. erectus s.l.2351214
      Middle Pleistocene hominins1121113
      H. neanderthalensis191931421824749
      H. sapiens (Pleistocene)4512123109325
      H. sapiens (recent)1420213493764243128
      P. troglodytes162164328341072
      P. paniscus516101121
      Gorilla178161101122
      Pongo773171215633

    Supplementary Materials

    • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/4/eaar2334/DC1

      Supplementary Materials and Methods

      fig. S1. Relationship between mean relative intercusp distance and Carabelli’s cusp per genus.

      fig. S2. Relationship between mean relative intercusp distance and Carabelli’s cusp per molar type in Homo.

      fig. S3. Right lower molar with example of homologous landmarks (yellow dots) placed at the cusp tips from which Euclidean distances were calculated.

      fig. S4. Ordinary least squares regression of upper (top row) and lower (bottom row) molar size comparisons estimated from crown outline, centroid size, and 3D surface area.

      table S1. Ordered logistic regression of cusp expression and relative intercusp distance.

      table S2. Fossil hominin upper molars used in this study including accession number, locality/site, and source.

      table S3. Fossil hominin lower molars used in this study including accession number, locality/site, and source.

      table S4. System used in this study for scoring accessory cusps.

      table S5. Tooth size comparisons estimated from crown outline, centroid size, and 3D surface area.

      References (3450)

    • Supplementary Materials

      This PDF file includes:

      • Supplementary Materials and Methods
      • fig. S1. Relationship between mean relative intercusp distance and Carabelli’s cusp per genus.
      • fig. S2. Relationship between mean relative intercusp distance and Carabelli’s cusp per molar type in Homo.
      • fig. S3. Right lower molar with example of homologous landmarks (yellow dots) placed at the cusp tips from which Euclidean distances were calculated.
      • fig. S4. Ordinary least squares regression of upper (top row) and lower (bottom row) molar size comparisons estimated from crown outline, centroid size, and 3D surface area.
      • table S1. Ordered logistic regression of cusp expression and relative intercusp distance.
      • table S2. Fossil hominin upper molars used in this study including accession number, locality/site, and source.
      • table S3. Fossil hominin lower molars used in this study including accession number, locality/site, and source.
      • table S4. System used in this study for scoring accessory cusps.
      • table S5. Tooth size comparisons estimated from crown outline, centroid size, and 3D surface area.
      • References (34–50)

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