Life-history traits such as age at first reproduction and prolonged childhood are hard to establish from the fossil record alone. Yet they are important to know to understand the sequence of human evolution.
Studying the rate of tooth development and enamel growth can provide some clues. Traits such as brain size, age at first reproduction, and lifespan are closely allied with aspects of dental development.
"Tooth growth is finely tuned to the pace of how whole animals grow," said Schwartz.
Slower tooth formation is associated with the extended growth period of modern humans. The researchers studied teeth or tooth fragments from a range of hominins, beginning with four species of australopiths, including Australopithecus anamensis, which date from 4.2 million to 3.9 million years ago.
Within the genus Homo, they studied early members Homo habilis, a species dating from 2.3 million to 1.8 million years ago, and Homo erectus and Homo ergaster, species that date from 1.9 to 0.8 million years ago.
They also looked at teeth from a Neandertal who lived about 120,000 years ago, and the hominoid ape Proconsul, which lived about 18 million years ago.
Narrower Time Frames
The scientists found that enamel formation and crown development rates of the australopiths and early species of the Homo genus were more like the patterns of modern and fossil apes than those of modern humans.
Exactly when the slow rate of enamel growth typical of modern humans first appeared still has not been determined, but the findings point the way to future studies that should help answer that question, said Schwartz.
"We can't say unequivocally that Neandertals display modern human growth patterns; in fact there is evidence that they grew somewhat distinctly," he said. "We need to study other Neandertal specimens and more recent Homo species, especially Homo antecessor."
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