The evolution of an extended childhood had implications for human society and culture. New analyses of dental development in fossil hominins suggest that our lengthy growth processes arose quite late in evolution.
Humans are unique among primates. Some of our unusual characteristics — our relatively large brains and the way we move, for example — are readily apparent. Others are less so, such as our long lifespan, delayed reproduction and prolonged childhood1. These 'life-history' features do not leave direct traces in the fossil record, so it is incredibly difficult to figure out when and how they evolved. In an effort to find out, palaeoanthropologists have turned to teeth. In primates, there is a correlation between some life-history variables and features of dental development2. Moreover, the eruption of certain teeth acts as a marker of growth; for example, the appearance of the third permanent molar — the wisdom tooth — marks the end of the juvenile period. So studies of tooth development in fossil hominins should provide information about growth processes and, by extension, the origin of our prolonged childhood. On page 628 of this issue, Dean and colleagues3 analyse dental development in a wide range of extinct hominin species, with intriguing results.
Hominins include modern humans, as well as fossil species that are more closely related to humans than to chimpanzees — in other words, the genus Homo and the australopiths (Ardipithecus, Australopithecus, Paranthropus and Kenyanthropus)4. One approach to studying dental development requires fossils of young hominins5. These can be studied either directly or with computed tomography scans6, revealing the pattern of tooth development. Results from this method have been used to suggest that Homo habilis (which dates to some 2.3 million to 1.8 million years ago) had a pattern of tooth development similar to that of the more primitive Australopithecus, whereas H. erectus/H. ergaster (1.9 million to 0.8 million years ago) shares similarities with modern humans7.
A second kind of analysis provides information on the rate, rather than pattern, of tooth development. This approach is based on the fact that the structure of a tooth preserves a record of development, mainly in the enamel (found on the crowns of the teeth) and the dentine (the internal hard tissue in a tooth)8,9. The inner enamel in particular shows incremental markings (cross-striations), seen in naturally fractured or experimentally sectioned teeth, that occur during the daily deposition of enamel by ameloblast cells (Fig. 1). The number of cross-striations per millimetre provides an estimate of the rate of enamel formation. Another form of periodic marking, known as the striae of Retzius, is used to examine crown formation. Counting the associated surface features (perikymata) provides estimates of the duration of crown formation.
Dean et al.3 applied this second kind of analysis to a wide range of fossil hominin species, including Australopithecus anamensis (which dates to 4.2 million to 3.9 million years ago), Neanderthals (300,000 to 28,000 years ago), and the ape Proconsul (18 million years ago). The study covers complete specimens — as in the teeth of the famous 'Turkana boy', an almost complete specimen attributed to H. ergaster10 — and isolated teeth and even fragments of a single tooth. The evidence indicates that, in terms of enamel-formation rates and crown-formation times, australopiths and early members of the genus Homo (H. habilis and H. erectus) resemble modern and fossil apes more closely than they do modern humans.
The results confirm the similarities of H. habilis and Australopithecus7,11. More intriguing, however, are the implications for interpreting the life history of H. erectus. Dean et al.3 propose that the developmental processes in H. erectus differ only slightly from those of earlier hominins (australopiths). This proposal is in agreement with the view that H. erectus/H. ergaster is only slightly different from australopiths in its relative brain size11,12 (there is suggested to be a relationship between an increase in brain size and extension of the postnatal period of development12,13). However, it has been argued that H. erectus/H. ergaster resembled modern humans in terms of its posture, body size and limb proportions11. So it seems that the human brain and body structures evolved in a mosaic pattern, with human-like rates of dental development and a marked increase in relative brain size apparently occurring later than other human-like body structures. 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.
What about hominin species intermediate in time between H. erectus and Neanderthals? On the basis of dental developmental patterns, it has been suggested that 0.8-million-year-old hominin remains from Spain (attributed to the species H. antecessor) had a developmental profile like that of modern humans14. But Dean et al.3 warn us that this does not necessarily imply human-like rates of growth. Because dental developmental patterns7,14 tell only part of the story, studying the rates of tooth development in Middle Pleistocene hominins should be revealing.
A cautionary note, however, is that when dental developmental data are not the only source of information on the growth process, different lines of evidence may lead to different conclusions. For example, combined estimates of the age at death of the Turkana boy — based on analysis of dental and skeletal development and on reconstruction of the stature of this individual — have suggested that his overall pattern of growth was within the normal range of variation seen in modern humans15.
Finally, the results of Dean et al.'s study3 have other implications. First, they show that the thick enamel of modern human teeth may not be homologous to the thick enamel of early hominins, being the result of a different developmental process. This finding underlines the importance of incorporating information on developmental mechanisms when interpreting the significance of certain traits in analysing evolutionary relationships16. Second, the new results support the idea that specific morphological characters (such as brain size, enamel thickness or bipedalism), as observed or inferred in fossil specimens, cannot be interpreted in isolation to indicate affinities with modern humanity. It is becoming increasingly clear that many features thought to be typical of modern humans may have evolved more than once. Dean et al.'s analysis raises new challenges in the search for fossil evidence of those characteristics that define both our genus11 and our species.
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Passing the knife edge in adolescence: Brain pruning and specification of individual lines of development
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