8. VULNERABILIDAD DEL SISTEMA ACTUAL
8.1. NORMA MIL STD 414 (ANSI/ASQC Z1.9)
Histological studies of tooth crown and root formation have found much to support relatively brief periods of maturation, initially for the australopithecines, and now for early fossil Homo (Dean 1987, Beynon & Dean 1988, Dean 1989, Beynon & Dean 1991, Dean 1995, Dean 2000, Dean et al. 2001). The periodicity between adjacent striae of Retzius, the conspicuous enamel growth lines regularly formed by active fronts of ameloblasts, was probably the same seven to nine day interval in fossil hominin teeth that it is in the teeth of modern humans and great apes (Dean & Beynon 1991, Dean & Reid 2001). Armed with this key of striae periodicity, counting the Retzius lines or their surface manifestations, the perikymata, of a tooth is equivalent to counting the days its owner lived or took to grow its teeth. From this it could be determined that permanent first molar emergence probably happened between the ages of three and four years in australopithecines (Dean & Beynon 1991, Smith 1991). Mineralization times suggest that the permanent third molars probably did not begin to erupt until australopithecines were older than seven- and-a-half or eight years of age (Bromage 1987). Paranthropus were probably precocious young that became nutritionally independent of their mothers early in life (Bromage 1990): australopithecines likely shared a similar infancy. These works and others have teased absolute schedules and lengths of permanent tooth development apart from the collection of ‘human-like’ and ‘ape-like’ sequences and patterns of tooth mineralization and emergence. They have lucidly interpreted the maturational significance of different rates and durations of dental development and demonstrated that schedules of fossil hominin growth were not only distinct from our own but also from those of living great apes and each other.
Homo erectus, H. habilis and H. rudolfensis enamel growth trajectories were very similar to those of living great apes but exclusive to the enamel growth trajectory of modern humans (Dean et al. 2001). This suggests that
early Homo retained a primitive chronology of enamel growth. However, beyond this it is not possible to infer the presence or duration of specific developmental phases such as adolescence in early Homo maturation (Dean et al. 2001). Although tooth development sequences are nearly identical between modern humans and Homo erectus, it is improbable that both taxa shared comparable schedules of dental development (Dean et al. 2001). The much- acclaimed Turkana boy' KNM ER WT 15000, a juvenile H. erectus, appeared to have grown its comparatively modern skeletal morphology and body proportions within a primitive (read brief) maturational period! Smith (1993) proposed that the modern human pattern of growth suppression (extended childhood) had not yet evolved in Homo erectus so that its ‘growth spurt’ was unchecked and began markedly earlier in this hominan compared with modern humans. Tardieu (1998) disagreed with the length of Smith’s (1993) estimate, but agreed that the growth spurt in Homo erectus was of markedly longer duration than it is in modern humans, and that this spurt would have started during what remains a modern human period of childhood. Similarly, Moggi-Cecchi (2001) proposed the intriguing scenario that the bipedal frame shared of Homo sapiens evolved independently of the prolonged childhood that is perhaps the hallmark of modern human growth. What is very clear is that the modern human pattern of maturation that is predominantly a product of our protracted childhood evolved quite late in our evolutionary history (Dean 2000, Thompson & Nelson 2000, Dean etal. 2001, Moggi-Cecchi 2001).
Dean and colleagues’ (2001) compelling study of fossil hominin maturation estimated that a modern human maturation evolved by at least hundred thousand years or so. In his review of the conclusions drawn by Dean et al. (2001), Moggi-Cecchi (2001) summarised, “the slow rate of enamel growth that is typical of modern humans, and is associated with an extended growth period, is first seen with the large-brained Neanderthals’’ (2001:597). Neanderthals matured over a significantly longer period of years than either australopithecine or early fossil Homo taxa. The dento-skeletal growth period of the large-brained Neanderthals was similar in length to that of modern humans, but not identical (Dean et al. 1986). Several workers have reported evidence of advanced mineralization of at least some permanent tooth crowns such as the lateral and central incisors Neanderthals relative to modern human populations
(Dean et al. 1986, Stringer et al. 1990, Thompson & Nelson 2000). Other findings have suggested accelerated eruption schedules in Neanderthals (Legoux 1970, Wolpoff 1979, Dean et al. 1986). However, Thompson and Nelson (2000) highlighted that if crown mineralization was indeed accelerated in Neanderthals relative to modern humans this acceleration was not linear. Still, many aspects of Neanderthal tooth development pattern and morphology were distinct from those of Australopithecus, Paranthropus, Homo erectus and Homo sapiens (Zilberman & Smith 1992, Thompson & Nelson 2000). Zilberman & Smith (1992) concluded that Neanderthals had thinner enamel than modern humans that was probably secreted at either slower rates or shorter periods of ameloblast activity. Their teeth also had significantly larger pulp chambers relative to the thickness of the crown compared to any other hominin (Zilberman & Smith 1992). However, very recent histological work by Dean et al. (2001) on the crowns of several fossil hominins demonstrated that Neanderthal tooth enamel formed in a way that falls within modern human variation for an equivalent mineralization process.
Perhaps it should have been intuitive that fossil hominin patterns and schedules of maturation would be as unique to these creatures as are patterns and schedules of modern human maturation. The species-centric slant that defines palaeoanthropology also confounds it. Bromage (1987:268) aptly checked the debate about fossil hominin maturation with the splendid question, “...if early hominids were growing like humans, then why were they not human in such characters as brain and body size?”. Others have commented that if early fossil hominins matured as modern humans do then either we evolved in unprecedented ontogenetic leaps and bounds or that patterns of physiological growth in fossil hominins defied those of all other primates for which maturational data is known (Smith & Tompkins 1995, Smith et al. 1995). Today, the spirit of Bromage's query is as relevant as ever, and its inference equally topical.