By Steve Bradt and Montserrat Capellas
A sophisticated new examination of teeth from 11 Neanderthal and early human fossils shows that modern humans are slower than our ancestors to reach full maturity. The finding suggests that our characteristically slow development and long childhood are recent and unique to our own species, and may have given early humans an evolutionary advantage over Neanderthals.
Scientists Paul Tafforeau (ESRF), Tanya Smith (Harvard University & MPI-EVA), and Jean-Jacques Hublin (MPI-EVA) position the upper jaw (maxilla) of the Le Moustier 1 juvenile Neanderthal before passing powerful X-rays through the fossil to see inside its’ teeth. Modern human developmental models suggested that this individual would have been 15-16 years of age, while the current study demonstrates that this 11.6-12.1 year old individual matured more rapidly than modern humans. Fossil courtesy of the Museum fur Vor- und Frühgeschichte (Berlin). Photo credit Chantal Argoud (ESRF).
The research, led by scientists at Harvard University, the Max Planck Institute for Evolutionary Biology (MPI-EVA), and the European Synchrotron Radiation Facility (ESRF), is detailed in the Proceedings of the National Academy of Sciences.
“Teeth are remarkable time recorders, capturing each day of growth much like rings in trees reveal yearly progress,” said Tanya M. Smith, assistant professor of human evolutionary biology at Harvard. “Even more impressive is the fact that our first molars contain a tiny ‘birth certificate,’ and finding this birth line allows scientists to calculate exactly how old a juvenile was when it died.”
Compared to even early humans, other primates have shorter gestation, faster childhood maturation, younger age at first reproduction, and a shorter overall lifespan. It’s been unclear exactly when, in the 6 to 7 million years since our evolutionary split from non-human primates, the life course shifted, the scientists said.
One poorly understood change is our unique life history, or the way in which we time growth, development, and reproductive efforts. Compared to humans, non-human primate life history is marked by a shorter gestation period, faster post-natal maturation rates, younger age at first reproduction, shorter post-reproductive period, and a shorter overall lifespan. For example, chimpanzees reach reproductive maturity several years before humans, bearing their first offspring by age 13, in contrast to the human average of 19.
Smith and her colleagues found that young Neanderthals’ teeth growth — a proxy for overall development — was significantly faster than in our own species, including some of the earliest groups of modern humans to leave Africa some 90,000 to 100,000 years ago. This indicates that the elongation of childhood has been a relatively recent development.
Such studies add to the growing body of evidence that subtle developmental differences exist between us and our Neanderthal cousins. The recent sequencing of the Neanderthal genome has provided tantalizing genetic clues pointing to differences in cranial and skeletal development between Neanderthals and modern humans.
The current study involves some of the most famous Neanderthal children ever discovered, including the first hominin fossil, discovered in Belgium in the winter of 1829-30. This individual was previously thought, based on comparisons with modern humans, to have been four to five years old at the time of death. Now, powerful synchrotron X-rays and biological rhythms inside teeth have revealed the child was only three years old.
While counting lines in teeth isn’t a new method, Smith said, doing it “virtually” using synchrotron micro-computed tomography is.
This forensic approach to the past is possible with a super-microscope: extremely powerful X-ray beams produced at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, which is one of the largest synchrotron in the world.
“At the ESRF,” Paul Tafforeau said, “we are able to look inside invaluable fossils without damaging them by using the special properties of high energy synchrotron X-rays. We can investigate fossils at different scales and in three-dimensions, ranging from studies of overall 3D shape down to microscopic daily growth lines. This is currently the only place where these studies of fossil humans are possible.”
Scientists and curators have been quietly visiting the European synchrotron, often with some of the rarest hominin fossils in the world, for imaging with this state-of-the-art technique.
“These new methods present a unique opportunity to assess the origins of a fundamentally human condition: the costly yet advantageous shift from a primitive ‘live fast and die young’ strategy to the ‘live slow and grow old’ strategy that has helped to make humans one of the most successful organisms on the planet,” Smith said. Humans’ extended maturation may have facilitated additional learning and complex cognition, possibly giving early Homo sapiens an advantage over their Neanderthal cousins.
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