Dental growth in early Homo15 Mar 2005
Dean and colleagues (2001) present a study of perikymata counts of anterior teeth (incisors and canines) in early humans and australopithecines, compared to extant apes and humans. The basic microanatomy of teeth and their process of development is briefly described in this write-up about Afropithecus.
The operating assumption that makes this study interesting is that "brain size, age at first reproduction, lifespan and other life-history traits correlate tightly with dental development" (628). Of course, this is true not only of dental development but all these other traits with each other and with others (most notably, body mass), since these kinds of "correlations" are really interspecies allometries of one kind or another. But the interesting thing is the apparent deviation of dental development rates in humans from other hominoid species. We take longer to grow our dental enamel, and this difference is likely to reflect a difference in the rate of growth or maturation:
Modern human enamel develops along a slower trajectory because the earliest-formed enamel, closest to the enamel dentine junction, is secreted in smaller increments for a longer time period. None of the trajectories of enamel growth in apes, australopiths of fossils attributed to Homo habilis, Homo rudolfensis or Homo erectus falls within that of the sample from modern humans (628-629).
Despite considerable variation in external tooth morphology, early fossil hominins share a common and fundamental enamel growth trajectory with the African ape clade that is derived in modern humans. (629).
It is now generally held that a prolonged life-history schedule reflects a reduction in the mortality rate of adults, triggered perhaps by behavioural, dietary or other changes. An increase in brain size (and cognitive ability) is associated with this reduction, but does not necessarily drive it, even in the human lineage. The size of key brain components associated with learning and cognition correlates with the timing of dental development in primates as the cost in time needed to grow and learn to use a larger brain increases. In this context a slower trajectory of enamel growth in permanent teeth, one of the basic determinants of tooth formation time, can be regarded as a life-history attribute associated with the extended, or prolonged, growth period of modern humans. The first evidence for a shift in enamel growth rates in the hominin fossil record seems to be with the origin of the larger-brained Neanderthals (at least by 100 kyr ago; see Tabun specimen C1 in Fig. 1) and modern humans (629-630).
Dean and colleagues also posit that the dental development data for specimens attributed to Homo habilis and/or Homo rudolfensis are consistent with an assignment to Australopithecus. Certainly the data do not contradict such an assignment, since there is no apparent difference between australopithecines, early humans, or any of the habilines in the study. Of course, the data also do not contradict the assignment of KNM-WT 15000 to Australopithecus -- they are just uninformative about taxonomic assignment. The key here is that the enamel formation times of the teeth show that early humans were essentially australopithecine-like in their dental development; and that both kinds of hominids were basically apelike.
The authors promote an interesting linkage between dental development times and brain size expansions:
If correct, these estimates of molar emergence times have shifted a little, in step with brain size, from those known for African great apes and australopiths. Nevertheless, it now seems increasingly likely that a period of development truly like that of modern humans arose after the appearance of H. erectus, when both brain size and body size were well within the ranges known for modern humans (630).
This is not to say that dental development times determine brain sizes or vice versa, but that the processes that promote delayed maturation connect both of these features. I would suggest that this connection might involve a genetic correlation between dental maturation and brain size that altered both characters with selection on overall maturation rate. If a prolongation of juvenile growth was a product of selection acting on social learning, then both slower growth and larger brains may have emerged in concert with each other.
The interesting part of this potential linkage is that growth may not have reached its modern rate 2 million years ago, but instead may have continued to change as brain sizes increased during the Middle and Late Pleistocene. That scenario is consistent with the dental development patterns in this study. In that perspective, this study can be chalked up in a list of ways early Homo was not very much like modern humans in its adaptive pattern.
Dean C, Leakey MG, Reid D, Schrenk F, Schwartz GT, Stringer C, Walker A. 2001. Growth processes in teeth distinguish modern humans from Homo erectus and earlier hominins. Nature 414:628631. Nature online