Evolution Diary

The origins of modern humans continues to be one of the most hotly debated topics among anthropologists, and there is little consensus about where and when the first members of our species, Homo sapiens, became fully modern. While fossil evidence tells a complex tale of mosaic change during the African Stone Age, almost nothing is known about changes in human ‘life history,’ or the timing of development, reproductive scheduling, and lifespan. Research during the past two decades has shown that early fossil humans (australopithecines and early Homo) possessed short growth periods, which were more similar to chimpanzees than to living humans. However, it is unclear when and in which group of fossil humans the modern condition of a relatively long childhood arose.

The team of scientists examined, using the unique tool that is the X-rays of the ESRF, the tooth growth and eruption in and 8 year old child fossil, one of the earliest representatives of Homo sapiens, found in Jebel Irhoud, Morocco and dated to approximately 160,000
years ago.

North African Fossil - The North African fossil mandible superimposed over horizontal developmental lines on an incisor tooth, with the internal daily lines shown in the black and white image on the bottom. With this non-destructive technique it is now possible to accurately and non-destructively reconstruct how fossil teeth grew and how old juvenile individuals were when they died. The circular image on the left shows the organization of the third generation synchrotron at the European Synchrotron Radiation Facility in Grenoble, France, where the first “virtual tooth histology” images were captured from fossil humans. Credit: Paul Tafforeau/ESRF

North African Juvenile Fossil
A) The North African juvenile fossil mandible showing the location of the incisor tooth enamel (white box) sampled with the Grenoble synchrotron. B) Close up of enamel fragment, with the area of interest (on right) shown in the white box. C) Synchrotron image showing growth lines (white arrows) with 10 daily lines between them (white brackets). The scale bar is 200 microns, or 0.2 mm. Credit: Paul Tafforeau/ESRF

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The scientists come from the Max Planck Institute for Evolutionary Anthropology (Germany), the Laboratory of Geobiology, Biochronology and Human Paleontology in Poitiers (France), the ESRF (France), the School of Dental Sciences in Newcastle (United Kingdom), the Research School of Earth Sciences in Canberra (Australia) and from the Department of Geology of the Mohammed V-Agdal University in Rabat (Marocco).

Tooth growth, and most importantly first molar tooth eruption age, represents one of the most powerful clues to reconstruct growth processes in fossil humans. By using incremental growth lines in teeth, similar to annual rings in trees, developmental rate and time may be accurately established millions of years after death.

The researchers used synchrotron light at the ESRF and combined a new application of a technique called phase imaging with developmental analysis. “It is the first non-destructive approach to characterize dental development with a high degree of precision, as synchrotron images reveal microscopic internal growth lines without damage to the sample”, explains Paul Tafforeau, one of the authors and developer of the virtual dental paleohistology using synchrotron light. Information on tooth growth in the Moroccan child was compared to other fossil and living human populations to determine if the modern condition of prolonged dental development was present.

These findings are not in contrast with our current wisdom that suggest that an extended period of development, and, by implication, childhood, may be linked to the origins of social, biological, and cultural changes needed to support dependent children with greater opportunities for early childhood learning.

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