The above is a human tooth. If you look closely you can see faint grooves running horizontally across it's surface. These grooves are called perikymata and represent growth. More specifically they represent growth cycles of about 6-14 days. Below is a high magnification detail from a thin section.
High magnification detail from the same thin section showing the regular growth structures found in enamel (polarised transmitted light microscopy). The approximately weekly growth layers, known as brown striae of Retzius, can be seen running from bottom left to top right. The enamel prisms run across from left to right, along which cross-striations can be seen (arrows). The cross-striations represent circadian (i.e. daily) growth markers and can be used to determine the precise timing of crown formation, as well as the timing of any disruption to this formation.
In theory one could count the number of perikymata on a tooth multiply it by 6-12 days and come up with an estimate of how long it took the tooth to form - getting at the same time an estimate of the length of childhood. Such studies have a long history in anthropology and recently this idea was applied to Neanderthal teeth. At issue is how long Neanderthals took to reach maturity - if they took less time than anatomically modern humans then that would obviously have bearing on the whole "Out of Africa/Multiregional Continuity" debate.
Recently a team of researchers performed a study on the perikymata of Neanderthal teeth:
For the study, the researchers used precise dental impressions Guatelli-Steinberg and Larsen made of 55 teeth believed to come from 30 Neanderthal individuals. These were compared to 65 teeth from 17 Inuit, 134 teeth from 114 southern Africans and 115 teeth from as many Newcastle residents. In all cases, the researchers tallied the number of perikymata on the enamel surface of the teeth.
Guatelli-Steinberg said that the results showed that the enamel formation times for the Neanderthals fell easily within the range of time shown by teeth from the three modern populations – a conclusion that did not support a shorter childhood for the Neanderthals.
Enticing though it may be, these new findings haven't convinced the researchers that a Neanderthal childhood was equal to a modern human's.
“The missing key bit of data to show that would be evidence for when the first molar tooth erupted in the Neanderthals, and we simple have no evidence of when that occurred,” she said.
The length of time is important, the researchers say, because unlike all other primates, humans have an extended period of childhood growth, during which brain matures both in size and through experiences. Some earlier hominids matured far more quickly than modern humans.
“The question is when exactly did that pattern of development evolve in the growth of humans,” she said.
