Researchers probing prehistoric dental records have found that humans taking their sweet time to grow up is a relatively recent phenomenon.
The studies of growth patterns preserved in the enamel of fossil teeth indicate that, contrary to a long-held view, an early ancestor who stood up 1.5 million years ago, Homo erectus, matured more like an ape than a modern man or woman. The report is published in the British journal Nature.
While great apes reach adulthood in 11 or 12 years, humans need at least 18 to 20 to do so. It had been assumed that Homo erectus, characterized by a series of modern-day features, followed the people pattern of growth and development. The intriguing new results showed otherwise, suggesting the prolonged period of childhood -- which many consider a key event in human evolution, allowing extra time for learning -- may go hand in hand with the recent development of the large human brain, investigators told United Press International.
The scientists analyzed daily growth patterns preserved in the enamel of fossil teeth recovered from mostly young individuals representing a wide array of early human ancestors, whose remains had been unearthed in south and east Africa over the past 50 years.
"Of the 13 fossil tooth fragments we studied -- both those attributed to the earliest australopith hominins that lived roughly between 4 million 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," said team member Alan Walker, distinguished professor of anthropology and biology at Pennsylvania State University in University Park, Penn. "We found 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 evolution of an extended childhood had implications for human society and culture," said anthropologist Jacopo Moggi-Cecchi of the University of Florence in Italy, who wrote an accompanying News and Views article. "New analyses of dental development in fossil hominins suggest that our lengthy growth processes arose quite late in evolution."
Hominins include modern humans as well as extinct species that are more closely related to humans than to chimpanzees. What sets humans apart from other primates are such unique characteristics as brain size, movement style, long lifespan, late reproduction and delayed childhood, Moggi-Cecchi said.
"These life-history features do not leave direct traces in the fossil record, so it is incredibly difficult to figure out when and how they evolved," she said, noting clues present in teeth have given paleoanthropologists something to chew on.
"In primates, there is a correlation between some life-history variables and features of dental development," she explained. "Moreover, the eruption of certain teeth acts as a marker of growth; the appearance of the third permanent molar -- the wisdom tooth -- marks the end of the juvenile period."
Delving into dental development over the eons, the team extracted some insights into the roots of prolonged childhood.
"One of the things that sets modern humans apart from the living great apes is our long period of growth and development. Humans take 18 or 20 years to growth up while chimpanzees and gorillas take 11 or 12 years. A key question is when this prolonged human growth period arose during our long evolutionary history," lead study author Christopher Dean of University College in London told UPI.
Dental development provides a good measure of overall growth and development and is well preserved in the fossil record, the scientists said.
"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," said Walker, a pioneer in the field of fossil dentition and member of a 1984 expedition to Kenya that yielded the remains of a juvenile Homo erectus, a key find.
With the aid of a scanning electron microscope, the investigators made thin sections of modern and fossil teeth. They counted the daily incremental markings within the enamel of humans, apes and fossil hominin species, calculating and comparing their rates of enamel formation.
"Modern humans have a slower trajectory of enamel growth that keeps pace with our prolonged or extended growth period," Dean said. "The enamel growth trajectory in apes and fossil hominins is faster and reflects the fact that they form their teeth in a shorter period of time."
"This is a remarkable work, well designed and conducted," Moggi-Cecchi told UPI.
The results surprised the researchers.
"The expectation had been that Homo erectus -- the first fossil hominin to show a suite of modern human-like characters, such as modern human-like body proportions and body weights and much smaller teeth and jaws -- would show evidence of a modern human-like growth period," Dean said.
"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 said.
The studies also shed light on the age of the Homo erectus youth found on the western shore of Lake Turkana in Kenya.
"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," Dean said. "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."
