Table 1.
Table 2.
Fossil material represented in Figure 3.
Figure 1.
Tooth tissues and their growth increments.
Incremental markings within the enamel cap (E) and dentine (D) are depicted on the model tooth (left). The red and blue boxes show details of measurements made along the EDJ and CDJ (from one circle up to the next) using examples of micrographs taken at higher power (right) in which fine daily markings 2–3 µm apart can be seen in enamel and dentine. A thin layer of cementum (CEM) covers the root surface that is not shown in the model.
Figure 2.
Tooth enamel (E) and dentine (D) both contain widely spaced, obliquely orientated, long-period growth lines. The distance between successive points where growth lines intersect the EDJ or root surface (bold open circles) can be measured in longitudinal thin sections of teeth (Figure 1). Counts of much smaller daily incremental markings between long-period growth lines provide a time scale for successive segments of tooth length (Figure 1). The growth rate is shown by the velocity curve obtained by differentiating a cubic smoothing spline curve through the points. Teeth show a characteristic pattern of growth in length. Growth rates are initially high but then slow towards the end of enamel formation before rising again to a peak in the root dentine. Thereafter, rates reduce again as tooth root formation approaches completion. Approximately twice as many data points appear on the scatterplot as are represented in the simplified model tooth depicted on the left (bold open circles).
Table 3.
Ages at stages of root completion in mandibular teeth (years) compared.
Figure 3.
Growth curves for fossil teeth and unerupted human teeth.
Reconstructed growth curves, focused here on the root spurt for each tooth, run from bottom left to top right in each plot. Length in μm (left y axis) is plotted against age in years (x axis). Its derivative curve (dashed line) representing growth velocity (in μm/year, right y axis) is superimposed. Age at peak velocity (apv) is shown by the vertical dotted line and the plot title. The fossil molar teeth span the past 18 Mya from one of the first hominoids (Proconsul heseloni) to the Late Miocene ape (Hispanopithecus laietanus) and early hominins (Australopithecus anamensis and Homo erectus) and each shows a common pattern of root growth with a clear peak in rate as even do human teeth (bottom row) that never erupted into function.
Figure 4.
Tooth growth curves for Pan troglodytes and modern Homo sapiens.
Reconstructed growth curves for tooth length. Length in μm (left y axis) is plotted against age in years (x axis) for each tooth type. Average ages for the start of tooth mineralisation were used for each tooth type [6].
Figure 5.
Synchronised growth curves and age at peak velocity.
SuperImposition by Translation And Rotation (SITAR) is a shape invariant growth curve model [83] that summarises the individual root growth curves as a single mean curve by transforming each curve so that they are all superimposed. The mean growth curve (bold black) representing the transformed individual curves of each tooth type appears with its derivative (dashed black line) representing root growth velocity (in μm/year, right y axis). Age at peak velocity (apv) is shown by the vertical red line and the plot title.
Table 4.
Summary of gingival emergence times in captive Pan troglodytes.
Table 5.
Gingival emergence times and root apv in Homo and Pan.
Figure 6.
Average and peak rates of root extension.
Values, on the y axis, for maximum root extension rates of 100 teeth at Stage G or beyond include those for Pan, Homo unerupted teeth and fossils in Figures 3 and 4. These are plotted against the average of all estimates made for rates of root extension in the same tooth (but exclude those estimates made within the root spurt itself).
Figure 7.
The timing of tooth development in Pan and Homo sapiens.
The sequence of initiation of tooth mineralisation through to tooth root completion is compared for incisors, canines and molars in Pan and Homo. Time is on the y axis and teeth begin to develop between birth (top line) and ∼12 yr in Pan or ∼18 yr in Homo (bottom line). Teeth are vertical, as they appear in the lower jaw, and increase in height through time. Tooth size and shape are not to scale. M1 in Pan and Homo begin to mineralise slightly before birth, but M2 and M3 in Homo initiate at later ages than in Pan. Consequently, M3 root completion occurs ∼6 yr later in Homo than in Pan. Molar eruption ages in Homo (dashed red lines) are drawn out disproportionately to later ages such that M1, M2 and M3 erupt into functional occlusion with increasingly longer roots formed. As a result, M3 eruption in Homo occurs ∼9 yr later than in Pan. In Pan for all teeth the timing of average age at peak root velocity (green lines) corresponds closely with age of eruption into function, but in Homo this has not kept pace with the later eruption ages for canines or molars from which it has become dissociated. Ages for tooth initiation and root completion are mean values from the literature [6], [79], [85].