Patterns of permanent incisor, canine and molar development in modern humans, great apes and early fossil hominins

Developing teeth pass through successive stages of formation. Patterns of dental development result when in several developing dentitions two or more teeth at different stages of formation consistently coincide with each other. There is also a pattern to the sequence of gingival emergence, or eruption, where teeth of different tooth types consistently erupt close in time to each other. Differences in the sequence or pattern of dental formation stages and of eruption have been described between modern humans and great apes and have been used to distinguish various early fossil hominin taxa as being like either living great apes or modern humans. In this study we revisited this issue using imaging and radiographic data derived from a larger number of individuals than has previously been available.

Studies of skeletal and dental growth in great apes have a long history and have sought to provide a comparative framework for both modern human growth and for studies of human evolution (Bingham, 1929, Brandes, 1928, Fooden and Izor, 1983, Gavan and Swindler, 1966, Gavan, 1953, Kraemer et al., 1982, Krogman, 1930; Schultz, 1935; Tanner, 1978; Willoughby, 1978; Zihlman et al., 2007; Zuckerman, 1928). Studies of the timing of gingival emergence and of the sequence of permanent tooth emergence in great apes also have a long history. These studies have identified differences in the sequence, or pattern, of tooth eruption between great apes and modern humans and in many cases have provided chronological data for gingival emergence (Clements & Zuckerman, 1953; Conroy et al., 1991; Kuykendall et al., 1992; Machanda et al., 2015; Nissen & Riesen, 1945; Nissen & Riesen, 1964; Simpson et al., 1990; Simpson et al., 1992; Smith, 1986; Smith, 1994a; Smith, 1994b; Smith et al., 1994; Smith et al., 2010; Smith & Boesch, 2011; Smith et al., 2013; Zihlman et al., 2004).

From these studies there is consensus about two things. First, compared with modern humans, the permanent incisor teeth of great apes emerge relatively later in the growth period, closer to second permanent molars than to first permanent molars as in modern humans. By way of example, the sequence of incisor emergence occurring before first permanent molar emergence, that occurs commonly in modern humans (Sato & Parsons, 1990) is unknown in great apes (Kuykendall et al., 1992). Second, the permanent canines in great apes also emerge relatively late in development compared with modern humans, closer to the third permanent molars than to the second permanent molars as they do in modern humans. Other so-called sequence polymorphisms, including those that involve the premolars and second permanent molars, have also been documented both within and between hominoid taxa but these seem to be less consistent and far more variable (Clements & Zuckerman, 1953; Cofran & Walker, 2017; Conroy & Kuykendall, 1995; Garn & Lewis, 1963; Kuykendall & Conroy, 1996; Simpson et al., 1990, Simpson et al., 1992).

Radiographic studies of tooth development in great apes have provided further information about the sequence of crown and root formation stages both prior to and after gingival emergence (Anemone et al., 1991, Boughner et al., 2012, Boughner et al., 2015, Dean and Wood, 1981, Winkler et al., 1991) and when the chronological age of individuals has been known it has been possible to put a time scale to radiographically defined stages of tooth development in great apes (Anemone et al., 1996, Kralick et al., 2017, Kuykendall, 1996). Besides these radiographic studies, histological studies have provided yet more evidence for the chronology of crown and root formation stages in developing great ape dentitions (Beynon et al., 1991, Dean, 2010, Kelley and Schwartz, 2010, Reid et al., 1998, Schwartz and Dean, 2001, Schwartz et al., 2006, Smith, 2016, Smith et al., 2007).

An unresolved issue arising from radiographic and histological comparisons of modern human and great ape dentitions is to what degree there is overlap in the development of M1 and M2 crown formation and of M2 and M3 crown formation and whether this relates simply to differential tooth size, space in the jaws and/or jaw growth (Boughner and Dean, 2004, Simpson et al., 1992, Tompkins, 1996aaa, Tompkins, 1996bbb). Prolongation of dental development in modern humans compared with that in great apes has been attributed in part to a delay in the timing of molar initiation, however, variation in molar overlap has never been quantified adequately in modern humans and great apes in a comparable way.

Studies that have attempted to compare the sequence or pattern of dental development and eruption in fossil hominins with those observed in modern humans and great apes have overall been problematic. Clearly, gingival emergence can never be observed in fossils or in museum specimens and mixing observations about radiographic stages of development with assumptions about gingival emergence has generally been regarded as less than satisfactory. Observations about the stages of dental development or tooth eruption status made using the same definitions and on large enough samples of modern humans and great apes have not always been readily available for comparison (Beynon and Dean, 1991, Broom and Robinson, 1951, Conroy, 1988, Dean and Liversidge, 2015, Dean, 1985, Dean, 1987, Dean et al., 1993, Garn and Lewis, 1963, Garn et al., 1957, Grine, 1987, Kelley and Schwartz, 2012, Kuykendall and Conroy, 1996, Mann et al., 1990, Moggi-Cecchi et al., 1998, Smith, 1986, Smith, 1994aaa, Smith, 1994bb, Wallace, 1977).

The aim of this study was to combine radiographic data for a large sample of modern humans with comparable data collected from existing radiographs of once free-living great ape specimens of Pan, Gorilla and Pongo. We aimed to resolve several issues that have compromised previous studies. First, we aimed to define stages of tooth eruption that can be scored on the same radiographs as tooth formation stages. Second, we defined stages of tooth development that are comparable across the morphologically distinct teeth of all three great ape taxa and modern humans. Third, by employing radiographs of great ape dentitions taken in three planes at right angles to each other we aimed to minimise error in defining stages of tooth formation that cannot always be clearly imaged in one plane alone. With this comparable database we then aimed to investigate ways to best quantify the variation in incisor and canine delay and of molar overlap in great apes compared with modern humans. Finally, we asked if any early fossil hominins, that are already well-described in the literature, fell beyond the range of variation observed in our modern human sample.

留言 (0)

沒有登入
gif