Human Life History Evolution Explains Dissociation between the Timing of Tooth Eruption and Peak Rates of Root Growth
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).