November 16, 2010

Neanderthals Had Shorter Childhood Than Modern Humans

A new analysis of teeth from 11 Neanderthal and early human fossils shows that modern humans are slower than our ancestors to reach full maturity.

The findings also suggest that modern kids' lengthy childhoods are a relatively recent phenomenon unique to our own species, and may even have given early humans an evolutionary advantage over Neanderthals.

The study is the latest to demonstrate the small, but vital, differences in early development between modern humans and Neanderthals, who became extinct about 28,000 years ago.

Scientists examine the growth of teeth because it serves as a proxy for overall development.

"Teeth are remarkable time recorders, capturing each day of growth much like rings in trees reveal yearly progress," said Tanya Smith, assistant professor of human evolutionary biology at Harvard and the study's lead author.

"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 early humans, other primates have shorter gestation, faster childhood maturation, younger age at first reproduction and shorter lifespans.  However, scientists are unclear about precisely when, during the 7 million years since our evolutionary split from non-human primates, this shift occurred.  Scientists have also debated whether various development patterns differed between Neanderthals and homo sapiens, from which modern humans evolved.

Previous studies have pointed to differences in maturation and lifespan between early humans and apes, whose females have shorter pregnancies and whose offspring mature faster and reproduce at younger ages than humans. Chimpanzees, for instance, bear their first babies at age 13 on average, compared with age 19 in humans.

Smith and her colleagues found that young Neanderthal tooth growth was significantly faster than in humans, 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.

Humans' extended maturation may also have facilitated additional learning and complex cognition, possibly giving early Homo sapiens an advantage over their Neanderthal cousins, the researchers said.

The study involved some of the most famous Neanderthal children ever discovered, including the first hominid fossil, discovered in Belgium in the winter of 1829-30. This individual was previously believed to have been four to five years old at the time of death.  However, using powerful synchrotron X-rays developed at the European Synchrotron Radiation Facility in Grenoble, France, along with the biological rhythms inside teeth, the scientists discovered the child was only three years old.

While counting lines in teeth isn't a new method, Smith says, doing it "virtually" using synchrotron micro-computed tomography is.

"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.

The study adds to the growing body of evidence that subtle developmental differences exist between humans and our Neanderthal cousins. 

The recent sequencing of the Neanderthal genome, in particular, has provided exciting genetic clues pointing to differences in cranial and skeletal development between Neanderthals and modern humans. 

A separate study released last week revealed that the brains of Neanderthals and humans were similar at birth, but developed differently during the first year of life.

The current study, 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.


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