The diversification of the common ancestor into gorillas, chimpanzees and humans can be explained through two alternative models. Both are based on the pattern of chromosomal rearrangements in extant species and entail a reappraisal of the influence of climate. The tiny genetic difference between chimpanzees and humankind is greatly amplified at the morphological level, thereby giving rise to the ‘human paradox’. The answer to the paradox lies in the economical and flexible evolutionary mechanism of mutations in regulator genes and the heterochronies they control. These are the true internal clocks of evolution. Changes in cranial morphology are quantified and utilised to analyse and explain the stages in the transition from great ape to human morphology. It is suggested that numerous heterochronies punctuate the comparative development of great apes and modern humans (hypermorphosis, hypomorphosis and post-displacements). It was once thought that bipedal gait might result from climate change, but revised assessment of the role of climate shows that it has nothing like the effect once ascribed to it, although climate does bring about prior differentiation between ecological niches. The appearance of bipedalism in australopithecines was certainly not the outcome of any gradual adaptation to an increasingly dry climate; it was rather mutations of regulator genes that instantly imposed permanent bipedal gait, thereby allowing colonisation of the savannahs.

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Natuurhistorisch Museum Rotterdam

J. Chaline. (1999). A new view of hominid evolution. Deinsea, 7(1), 67–82.