Brief communication: Methods of sequence heterochrony for describing modular developmental changes in human evolution

Abstract

Interest in the developmental changes leading to apomorphic features of human anatomy is longstanding. Although most research has focused on quantitative measures of size and shape, additional information may be available in the sequence of events in development, including aspects of phenotypic integration. I apply two recently proposed techniques for analyzing developmental sequences to literature data on human and chimpanzee age of limb element ossification center appearance in radiographs. The event-pair cracking method of Jeffery et al. (Syst Biol 51 [2002] 478-491) offers little additional insight on sequence differences in this data set than a simpler difference of ranks. Both reveal shifts in timing that are likely related to locomotor differences between the two species. Poe's (Evolution 58 [2004] 1852-1855) test for modularity in a sequence identifies the ankle, wrist, and hind limb as developmental modules, which may correspond to localized combinations of developmental genes. Ossification patterns of the rays of the hand and foot show little modularity. Integrating these and other methods of sequence analysis with traditional metrics of size and shape remains an underdeveloped area of inquiry.