This image of fossil enamel clearly shows the kind of daily growth the researchers used in their study.
Photo credit: Christopher Dean
The long period of development leading up to a modern human's adulthood arose relatively late in our evolutionary history, according to an analysis of growth patterns in fossil teeth in the 6 December issue of the journal Nature, written by Christopher Dean of University College, London, and colleagues including Alan Walker, distinguished professor of anthropology and biology at Penn State.
"One of the things that sets modern humans apart from the living great apes is our long period of growth and development," Dean explains. "While humans take a good 18 to 20 years to grow up, other primate species like chimpanzees and gorillas take just 11 or 12 years." A supporting article, written by Jacopo Moggi-Cecchi of the University of Florence in Italy, is included in the "News and Views" section of the journal. The research was designed to determine when the prolonged growth period we have today arose during our long evolutionary history.
"Dental development is a good measure of overall growth and development," says Walker, who pioneered the study of living primates as a basis for the analysis of fossils and was one of the first to use scanning electron microscope studies of fossil teeth. "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 making thin sections of modern and fossil teeth, the researchers were able to count the daily incremental markings within the enamel of humans, apes, and fossil "hominin" species in the human lineage in order to calculate and compare their rates of enamel formation.
"Of the 13 fossil tooth fragments we studied--both those attributed to the earliest australopith hominins that lived roughly between 4 and 1 million years ago, and those of the earliest members of our own Homo genus that lived about 1.5 million years ago--none showed the slower pattern of modern human enamel growth," says Walker, who in 1984 was a key member of the team that discovered a juvenile skeleton in Kenya from one of the earliest species in the Homo genus, Homo erectus. "We found that the first dental evidence for a modern human-like growth period appears much more recently, in a Neanderthal fossil that lived about 120,000 years ago."
The results are surprising because researchers had expected that Homo erectus--the first fossil human ancestor to show a suite of modern human-like characteristics including body proportions, body weight, and small teeth and jaws--would show evidence of a modern human-like growth period. However, because the brain in Homo erectus was still not as large as a modern human's and because a long growth period is linked with the time needed to grow and learn to use a large brain, the researchers say these findings are compatible with predictions that could be made on the basis of brain size alone.
As part of their research, the scientists used the incremental growth markings to calculate the formation times of individual teeth as a clue in the solution to another mystery about the age at death of the Homo erectus fossil found in Kenya.
"Using these tooth-formation times, we can speculate about the age at which key teeth emerged into the mouth in Homo erectus," Dean explains. "It seems likely that the first permanent molar tooth, which erupts at around 6 years in modern humans and about 3.5 years in apes, erupted between 4 and 4.5 years in Homo erectus. Previously, most people accepted this boy was close to 11 or 12 years of age, but now it seems more likely he was closer to 8 years of age, which is a surprise because he was already 5 feet 3 inches tall."
"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.
This research was supported by the Royal Society and the Leverhulme Trust.
A high-resolution image of fossil enamel, which clearly shows the kind of daily growth the researchers used in their study, is available from a link on the web at:
Allan Walker, 814-865-3122, firstname.lastname@example.org
Christopher Dean tel +44 20 7209 6158, email@example.com
Jacopo Moggi-Cecchi tel +39 055 239 8065, firstname.lastname@example.org
Barbara K. Kennedy (PIO), tel +814-863-4682, email@example.com
A PDF for the research paper and the associated "News and Views" article are available on the Nature journals press site at <http://press.nature.com> .