Dr. Rosas and his team studied ancient Neanderthal remains recovered from a cave system in Spain known as El Sidrón, where archaeologists have found the remains of more than a dozen individuals, including the child’s mother and younger brother.
The first thing the researchers needed to do with their child specimen was determine how old he was. So they peeked inside his mouth, which had a mix of 30 baby and adult teeth. By cutting into his teeth they were able to use a microscope to count bands in the enamel, which grow similarly to tree rings.
From their investigation they determined the child was just under eight years old when he died. They did not find any signs on his bones that would have clued them into the cause of his death.
By investigating the boy’s cranium, the researchers found that it was only 87.5 percent the size of a full grown Neanderthal’s cranium. That differs from anatomically modern human children, who at age seven have craniums that are about 95 percent the size of an adult’s.
Because cranium size is a good indicator of brain size, the findings suggest that Neanderthals’ large brains took longer to grow to adult size than our brains do.
Though the team did not have the child’s complete skull, they were able to compare the available fragments with a skull from a different Neanderthal and reconstruct the missing parts.
The team also found the Neanderthal child still had several unfused vertebrae. In modern human children, those vertebrae are fused around the ages of four to six.
Despite the differences in brain and spine development, the team found that in many ways the Neanderthal child was no different from a modern human child, especially in the elbows, wrists, hands and knees.
Both seem to have experienced similar growth patterns, like having arms and legs that grew slowly between infancy and puberty, according to Dr. Rosas.
As he went through the whole skeleton, comparing it to skeletons of Homo sapiens, these particular differences stood out, contributing to the mystery of what accounts for the early differences between the two species.
“We were surprised because we were expecting some differences,” said Dr. Rosas. “But it was, ‘Similar, similar, similar — oh, different — similar, similar — oh, different.’”
Luis Ríos, a paleoanthropologist also at The National Museum of Natural Sciences in Madrid and co-author on the paper, said at a news conference that the new finding about growth rates fits with the generally held idea that Neanderthal brains were bigger.
But to confirm that hypothesis, they will need to further investigate the craniums in the cave, looking for remains between childhood and adulthood, to complete their life cycle of Neanderthals.