Sequence of emergence of the permanent teeth in Macaca, Pan, Homo, and Australopithecus: Its evolutionary significance

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Abstract
There are two main questions about the sequence of emergence of the permanent teeth in humans: 1) Why is there so much variation in sequence within human populations? (2) What is the adaptive or evolutionary significance of emergence sequence? Here, the human condition is considered by comparing us to other living primates and to our evolutionary past and considered in the light of Schultz's hypothesis that sequence of tooth emergence is adapted to rate of postnatal growth (Schultz AH. In JM Tanner (ed.) Human Growth, pp 1–20, 1960). Frequencies of individual pairwise sequences (e.g., M1I1 vs. I1M1) in the emergence of the permanent dentition are described for N = 110 Macaca nemestrina, and compared to N = 157 Pan troglodytes and ≥6,000 Homo sapiens. In addition, sequences of gingival emergence are reconstructed for Australopithecus and early Homo. Trends observed across these catarrhine primates suggest that sequence and variability in sequence can be understood by a simple model of adaptation of tooth emergence to growth rate. As rate of postnatal growth slows, molars drift to later positions in sequence, either by always emerging late in sequence, or by varying in the direction of late emergence. “Augmented sequences” (sequences written with notations about variability) are important in recognizing evolutionary trends; further, they often alter perception of similarities and differences among taxa. Although samples are small, Australopithecus africanus resembles the rapidly developing genera Macaca and Pan more than it resembles Homo sapiens.

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