Heterochronic patterns in primate evolution: evidence from endochondral ossification.

Heterochrony (evolutionary modifications in developmental timing and/or rates) is widely recognized as an important agent of morphological change. The adaptive significance of heterochronic changes might lie either in the advantages of the derived morphologies (organ size and shape) or the derived growth parameters themselves (rate and duration of growth). We have tested these hypotheses by comparing the growth rate, the duration of growth and the relative length of the adult tibia in Primates in a phylogenetic context. We report an evolutionary decrease in growth rates (paedochronocline) and an increase in the duration of growth (perachronocline), lying in the cline from the last common ancestor of Primates, passing through the last common ancestor of Haplorhini, that of Catarrhini, to the last common ancestor of the Hominidae. However, the variation in the relative length of the adult tibia does not show any phylogenetic pattern. The derived growth parameters in themselves (slower rate, longer duration) would be of adaptive significance and they would have been selected because a prolonged learning period prior to maturity conferred advantage. The proximate (developmental) causation of differences in bone growth rate were also investigated and it was found that cell production rate in the growth plates rather than the chondrocyte size, underlies the variation in bone growth rate.