Thirteen ancient Neanderthal skeletons have been unearthed in the limestone depths of El Sidròn, a dark, 49,000-year-old limestone cavern on Spain’s northwest coast. To Luis Rios and Antonio Rosas, a pair of Spanish paleontologists exploring the cave, one skeleton belonging to a unusually well-preserved seven-year-old stood out from the rest. The boy, nicknamed El Sidròn J1, represented a rare glimpse into the development of a Neanderthal child.

El Sidrón J1 was pretty similar to modern kids — except for the decidedly unmodern fact that his brain was still growing at the time of his death.

“We had a basic question to answer, and it was: How did Neanderthals grow and mature in comparison with modern humans?”

Rios, a member of the paleontology group at Museo Nacional de Ciencias Naturales in Madrid, shared the team’s findings with reporters this week ahead of the publication of a study in the journal Science.

A largely intact skeleton from a neanderthal "tween" revealed major differences in the way their brains grew compared to modern humans.

Rosas took a look at El Sidròn J1’s smile to determine its age, and the results showed that he died when was 7.7 years old. (Teeth grow in microscopic layers, and if you look closely enough, you can count them like rings on a tree.) Often the teeth aren’t well enough preserved to figure out how a Neanderthal was their death, so the paleontologists got very lucky here. And once they determined the age, they could track all sorts of developmental factors.

Compared to the average 7.7-year-old human, Rios says there was “there was no noticeable difference in the growth and maturation of this Neanderthal juvenile in comparison with modern human juveniles.” In that respect, it seems, we aren’t so different from our evolutionary cousins.

“But we did observe some differences,” Rios said. Unlike human kids, whose brains are usually fully grown by the age of 7.7, El Sidròn J1’s brain was not.

Brain growth here refers to the increase in physical volume: The human brain grows incredibly fast, reaching 55 percent of its adult size in three months and near 90 percent by the age of six. However, brain development — the pruning and rewiring of the existing neurons — continues until age 25.

“One of the major differences we found with this individual was that at the age of [seven], curiously his brain was still growing,” Rosas said. “So we conclude that the brain of this individual was still growing at that age that the modern humans would have stopped growing then.”

Brain development during preadolescence is especially important to understand, Rios said, because that’s when kids learn how to interact socially — and develop the cognitive skills needed to do so. Because El Sidròn J1 seems to have had a relatively extended period of brain growth, it could suggest that he and other children his age spent more time than human kids acquiring social cognitive skills.

They also looked closely at individual components of the body — the spine and the skull, in particular — and it seemed that parts of El Sidròn J1 developed at an odd pace. His spine was underdeveloped by human standards and comparable to a human of about five or six.

Homo neanderthalensis, our closest extinct human relative, was the inspiration for pop culture’s caveman: short, stocky, with a protruding brow ridge and primitive thinking skills. But despite their reputation for being dumb, Neanderthals had curiously large brains — even bigger than ours.

Evolutionary biologists have never been sure how or why that came to be. Perhaps Neanderthal kids had a huge brain expansion moment early on, or maybe their brains grew steadily through adulthood. It’s been tough to compare them with humans because of one frustrating technical hurdle: It’s really hard to determine precisely how old a Neanderthal was when it died. But El Sidròn J1, with his exceptionally well-kept bones, gave Rios and Rosas the rare chance to do so.

Rosas with the exceptionally well preserved skeleton of El Sidròn J1. Andrés Díaz-CSIC Communication

Rosas hesitates to speculate on whether this meant that Neanderthal children were as a whole more vulnerable than humans at the same age, noting that there are many more life history variables — like nutrition, geography, and social structure — to take into account. Perhaps Neanderthal mothers, he suggested, simply had more time to spend with individual kids because they spaced out different births, thus drawing out the learning period: At such an early stage of research, it’s hard to tell.

What the team can confidently say is that any differences in development between El Sidròn J1 and human kids his age are so minor that they don’t indicate a drastic, fundamentally different pace of growth. Rather, they write, they “reflect Neanderthal physiology and ontogenetic energy constraints,” raising more questions about the specific factors that limited the growth of a Neanderthal embryo, or baby, or child.

The El Sidròn cave where Rosas and team dug up the juvenile skeleton. Joan Costa-CSIC Communication

Environmental factors aside, the findings suggest that we’re a lot more like our stumpy ancestors than we might expect — which those used to feeling superior to our primitive relatives might find a bit unsettling. But whether we like it or not, the growth patterns of El Sidròn J1 and the average human kid were “quite similar,” Rosas said, so it “probably means that we inherit this growth pattern.”

“Perhaps when we found that there are less differences than we were expecting, that conclusion probably makes it look less sexy,” said Rosas.

“But it’s really important to be aware that we share with Neanderthals features that can be considered human, and that we what we have considered unique for modern humans — for Homo sapiens — perhaps is not so unique.”

Members of the team excavating El Sidròn. Paleoanthropology Group MNCN-CSIC

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