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LONDON, (Reuters) A team of scientists who have been getting at the root of one of the major features that sets humans apart from great apes said on Wednesday it is a fairly recent evolutionary development.

Unlike apes, which mature in about 12 years, humans have a prolonged growth and don't reach adulthood until 18-20.

Palaeontologists had assumed the distinctive change in development had begun a million or more years ago with Homo erectus, which walked upright.

But a new study of teeth fossils reported in the science journal Nature suggests it was Neanderthals, who roamed the earth only about 120,000 years ago, who were the first to have a human-like long development.

"It seems our prolonged period of growth and development may be a recent evolutionary acquisition that arose in step with a larger, modern human-sized brain and not as we thought with Homo erectus," said Christopher Dean of University College in London.

TEETHING PROBLEMS

Like long lifespan and delayed reproduction, modern humans' prolonged development is a variable that sets us apart from apes. But studying these variables and figuring out when they developed is difficult, which is why Dean and his colleagues used teeth fossils to determine when they occurred.

"Teeth grow in an incremental manner, like trees or shells, and tooth tissues (enamel) preserve a record of their growth in the form of incremental markings," Dean explained.The scientists studied 13 fossil tooth fragments ranging in age from five million years to more recent samples.

They had expected Homo erectus, which had smaller teeth and jaws and was closer in brain and body size to humans, to have the first signs of prolonged growth.

But teeth markings show the shift did not happen until much later - with Neanderthals.The scientists also calculated how long it would have taken teeth belonging to Homo erectus to form and the likely age at which they would have first cut through the gums.

They estimated the first permanent teeth of Homo erectus came through at 4.5 years of age, while in apes they emerge at 3.5 years old but not in modern humans until about six.

The research also changes the estimated age of a Homo erectus skeleton called "Turkana boy" found in Kenya, who scientists had thought was about 12 when he died.

Based on the tooth fossil findings, Dean and his colleagues believe the boy, despite weighing 50 kg (110 lb) and measuring 1.60 metres (5 feet 3 inches), was probably only about eight.

"The brain in Homo erectus was still not modern human-like in its size and given the fact that there is a close link between the length of the growth period and the time it takes to grow and learn to use a large brain, these findings in retrospect fit closely with what had been predicted on the basis of absolute brain size alone," said Dean.

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