Image: CHRISTOPHER DEAN/Pennsylvania State University
Christopher Dean of University College, London and colleagues examined 13 fossil tooth fragments from specimens dating between four million and 120, 000 years ago. "Dental development is a good measure of overall growth and development," co-author Alan Walker of Pennsylvania State University explains. "Teeth grow in an incremental manner like trees or shells, preserving a record of their growth with daily marks along the prisms that make up the enamel." By examining patterns (see image) within the enamel of both the fossil and modern teeth, the scientists calculated and compared rates of growth.
The first dental evidence for a modern human-like growth period, the scientists discovered, was in a Neanderthal fossil from around 120,000 years ago. What's more, the researchers found that Homo erectus, despite its complement of many modern human characteristics such as body proportions, weight and small teeth and jaws, did not exhibit a slow growth period. They suggest this finding may be linked to the fact that, compared to modern humans, Homo erectus still had a small brain, which would not require as much time to grow and learn. "It seems our prolonged period of growth and development may be a more recent evolutionary acquisition that arose in step with our comparatively recent development of a larger, modern, human-sized brain," Walker says.
In an accompanying commentary in the same issue, Jocopo Moggi-Cecchi of the University of Florence writes, "it is becoming increasingly clear that many features thought to be typical of modern humans may have evolved more than once." The present study he adds, "raises new challenges in the search for fossil evidence of those characteristics that define both our genus and our species."