An analysis of a Neanderthal child's skeleton suggests that its brain was still developing at a time when the brains of modern human children are fully formed.
This is further evidence that this now extinct human was not more brutish and primitive than our species.
The research has been published in the journal Science.
Until now it had been thought that we were the only species whose brains developed relatively slowly. Unlike other apes and more primitive humans, Homo sapiens has an extended period of childhood lasting several years.
But a team led by Prof Antonio Rosas of the Museum of Natural Sciences in Madrid found that if anything, Neanderthal brains may have developed more slowly than ours.
"It was a surprise," he told BBC News. "When we started the study we were expecting something similar to the previous studies."
Prof Rosas and his team believe they are right and the previous studies are wrong because for the first time they were able to study a relatively complete skeleton of a child at a crucial stage in their development.
It was of a boy, who was nearly seven-and-a-half years old when he died. His bones were found in the 49,000-year-old site of El Sidrón, in Spain.
The boy's remains are exceptionally well preserved and include a mix of baby and adult teeth, which enabled the team to accurately determine his age.
This brain is estimated to have been 87.5% of the size of an average adult Neanderthal brain upon death. A modern human child at the same general age would have, on average, a brain that was 95% the size of an adult's.
The researchers also found that some of the small bones forming the boy's backbone were not fused. In modern humans, these bones tend to fuse by the time children reach the age of six.
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