“What we see in this Neanderthal is the general pattern of growth is very similar to that of modern humans,” said Luis Ríos, a paleoanthropologist at the National Museum of Natural History of Spain, during a news conference on Wednesday.
Katerina Harvati, an expert in Neanderthal evolution at the University of Tubingen in Germany who was not involved with the study, said that “one must be careful in extrapolating findings from one individual. Nevertheless, this specimen is very complete, making it a particularly good case study.”
Between 2000 and 2013, excavators removed more than 2,500 fossilized bones from the Spanish cave system known as El Sidron. Researchers have so far identified 13 individuals. This includes several adults, as well as the young boy and a child of about 2 or 3 years old. The bones were jumbled and must be pieced together, jigsawlike. Some of the bones have cut marks made after death, suggesting postmortem cannibalism.
Yet these Neanderthals were no knuckle-dragging carnivores. There's evidence from earlier studies that they ate nuts and fungus as part of a mostly vegetarian diet. They medicated themselves with plants that acted like natural painkillers. The sophistication of the stone tools found in El Sidron indicate these archaic humans lived during the Middle Paleolithic period, about 49,000 years ago.
About 36 percent of the juvenile Neanderthal's skeleton was still intact. El Sidron J1, which is what the researchers called him, had a complete lower jaw, 30 teeth, bits of skull, backbone, ribs, arms and a knee. He would have been about 3-foot-8 and 60 pounds. Though tests of his ancient DNA were inconclusive, the teeth size and bone shape indicated the Neanderthal was male.
Ríos and his colleagues used J1's teeth to determine the boy's age. This was “established mostly through the dental histology,” said study author Antonio Rosas, a paleoanthropologist at Spain's National Museum of Natural History. Which is to say the researchers used microscopes to track growth lines in the tooth enamel. They checked the dental age against the skeleton's maturation, particularly in the best-preserved areas: his elbow, wrist, hand and knee. The bones agreed with the dental estimate. J1 was 7.7 years old, give or take a month, when he died.
That's young, even for a Neanderthal. By one recent estimate, one in four Neanderthals lived past the age of 40, a life span on par with early humans.
J1, when compared with the development of modern kids, fell mostly within the expected human range. There were two major exceptions. Based on his cranial bones, the volume of J1's brain was about 88 percent that of the average adult Neanderthal's. He would have been underdeveloped if he were one of us. A 7-year-old modern human's brain is 95 percent of an adult's. Also, the bones in J1's spine had not yet fully fused, something that happens to human children around age 5 or 6.
The authors argue that these changes did not signal a fundamental difference in the pace of Neanderthal development. The body shape and large Neanderthal skull might have taken more energy to finish growing. But overall, they concluded, J1 reveals that Neanderthals grew up like modern human children. That is, slowly.
Humans are late bloomers compared with other primates. Chimpanzees grow much faster after birth; studies suggest they do not have an adolescent growth spurt. Our bodies pump the brakes after infancy, only to speed up again after a long pause. Biologists hypothesize this pause allows young brains to soak in socially complex environments over a period of years, rather than maxing out in size too early.
The authors acknowledged that their conclusions were limited by the fact J1 was their only specimen in this research and said they hope to add data from other juveniles. Still, Ríos said, using the best fossil evidence — in this case, one skeleton — “is the only way to move forward” in the study of archaic humans.
Harvati said this evidence will also have to be reconciled with other research, which hints at faster growth rates in Neanderthal children, at least based on their teeth. "It maybe that the differences are more subtle than initially thought," she said, "with Neanderthals falling within the range of modern human variation, albeit at the faster end of the spectrum."
Rosas said that the finding “has to be put in context of our evolution.” Our delayed adolescence was thought to be uniquely human, he said. After all, it gave us “this big brain.” But, if both species bloomed late, this trait likely appeared somewhere in our common evolutionary history, he said, on the shared path to Neanderthals and Homo sapiens.
This post has been updated.