On the evolution of the morphology and resilience of molar cusps in fossil hominid teeth.

Teeth play an important role in evolutionary studies due to their good preservation and direct link to diet. The present work makes use of a previously generated database on molar teeth of fossil hominids which consists of cuspal enamel thickness dc, dentin horn angle φ and section width D, all measured on a given histological tooth section. These data are here interpreted with the aid of "fracture stress" QF = PF/D2 and geological age t, where PF is the occlusal force needed to cause cusp failure as determined from dc and φ. QF is virtually a constant in non-hominins ("apes") while monotonically increasing toward present time in hominins. These two trends intersect at t = ts = 4.5 (0.11) mya, a value similar to other divergence estimates. QF was fitted with a function f(t) which is proportional to (dc/D)2. The monotonic variation of QF and in turn dc/D with t contrasts the more complex behavior generally characterizing other physical entities of fossil hominids. The increase in dc/D in hominins promotes tooth resilience and in turn life span. Finally, it is suggested that PF provides an upper bound to the maximum bite force produced by the jaw structure.