Scientists Compared Neanderthal Brains with Modern Humans and Found Something Surprising

Brain size can matter across the broad sweep of primate evolution but among humans isn't the best way to estimate intelligence.

When the first Neanderthal specimen was discovered in 1856 in Germany, scientists had never seen a human skull like it.

It is long where ours is round, low where ours is high, heavy-browed where ours is smooth. In the early 20th century, scientists stared at those differences and couldn’t help but deliver a harsh verdict. The brain inside, many assumed, must have worked differently. Perhaps worse. Perhaps badly enough to explain why Neanderthals vanished while Homo sapiens endured and thoroughly conquered this world.

Although more modern research shows Neanderthals had complex societies very similar to ours, including the use of symbolic art and sophisticated tools, the viewpoint of a cognitively inferior Neanderthal is still prevalent. A new study says this story rests on a shaky comparison.

Previous studies on Neanderthal brains were not measured against the full range of brains we see in living people today. When they are, the supposed gulf between them and us begins to look less like a chasm and more like ordinary human variation.

The study argues that estimated differences between Neanderthal and early modern human brains “fit comfortably within the range of differences seen among modern humans,” as lead author Thomas Schoenemann and colleagues put it.

“Putting estimated Neanderthal differences into the context of modern human variation does not support the view that they were cognitively challenged,” Schoenemann, a cognitive scientist and anthropologist at Indiana University Bloomington, said in the university press release.

The Skull Does Not Necessarily Reflect Intelligence

The old logic was tempting because it seemed obvious. Neanderthals had longer, lower skulls. Modern humans had rounder, more globular ones. The space inside the skull — known as the endocranium — preserves a rough impression of the brain’s outer shape. Scientists can make endocasts from fossils and use them to estimate the size of different brain regions.

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In 2018, one influential study used such reconstructions to compare four Neanderthals with four early members of our own species. It concluded that Neanderthals had smaller cerebellums, despite often having larger overall cranial capacities. The cerebellum, tucked at the back of the brain, helps coordinate movement but also contributes to attention, memory, language, and emotional regulation.

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It follows that Neanderthals went extinct and were outcompeted by Homo sapiens because they were less mentally flexible, less social, less verbal, and less able to plan.

But Schoenemann and his colleagues noticed a missing step. The 2018 study compared Neanderthals with early modern humans, but it never asked how large those differences were compared with differences among living humans.

This is an important distinction because human brains vary a lot. They vary in size, shape, and the volumes of individual regions. Yet scientists do not treat that variation as evidence that one living population has meaningfully different cognitive abilities from another.

So, the team repeated the earlier method using MRI scans from 400 living people. The data included 200 Americans of European descent and 200 ethnic Han Chinese participants from the Human Connectome Project. They measured 13 brain regions, then compared the differences between these two modern groups with the previously reported differences between Neanderthals and Pleistocene Homo sapiens.

In nine of the 13 regions, the differences between the two modern human groups were larger than the estimated differences between Neanderthals and early modern humans.

“The differences between modern human and Neanderthal brains do not meaningfully exceed those among different modern human populations,” Schoenemann said.

“Nobody argues that American people and Chinese people are cognitively different in any meaningful sense, evolutionary or otherwise, so why should we assume anything about similar-sized differences between Neanderthals and early modern humans?”

Small Differences, Smaller Implications

Brain size can matter across the broad sweep of primate evolution. The average chimpanzee brain is roughly 400 cubic centimeters. Meanwhile, the average adult human brain is about 1,350 cubic centimeters, with living humans varying widely around that mark.

But within our species, brain volume is a poor guide to mental ability. The relationship between the size of a region and the skill associated with it is usually weak, messy, and indirect. A bigger brain region does not work like a bigger engine.

Even so, Schoenemann’s team tried to quantify what the Neanderthal differences would mean if one accepted the existing links between brain anatomy and cognition. Their estimate was tiny. The largest predicted cognitive difference between Neanderthals and early modern humans was only 0.14 standard deviations.

In plain terms, if you lined up 100 Neanderthals and 100 early modern humans, the modern human group might have roughly five more people above the Neanderthal average. At the high end of ability, the difference would amount to about one person.

“It seems likely that any average cognitive differences that existed would have been very subtle, if detectable at all,” Schoenemann said.

That does not prove Neanderthals and Homo sapiens thought identically. No fossil can do that. The study also does not claim that anatomy is irrelevant. Small differences in behavior can, in principle, have evolutionary consequences.

But the authors argue that the brain differences alone are too small, and too poorly tied to cognition, to carry the heavy explanatory burden long placed on them.

The Neanderthals Had Already Answered

The thing is, we don’t have just fossils to form a picture of what Neanderthals were like.

They made tools that required planning and practice. They hunted large animals in groups. They used pigments in art and crafted jewelry. They buried their dead with rituals. They lived for hundreds of thousands of years across harsh, shifting landscapes in Europe and western Asia.

None of this fits the old caricature of a dim, doomed cousin stumbling before smarty pants Homo sapiens.

This caricature is seductive because it offers a simple explanation for a difficult problem. Neanderthals disappeared around 40,000 years ago, not long after modern humans expanded into parts of Eurasia. If two human groups met and only one remained, it was easy — perhaps too easy — to say that one outsmarted the other.

But extinction rarely has a single cause. Climate change, new diseases perhaps brought by fresh off the boat Homo sapiens, or plain bad luck, may all have played a hand, besides competition over resources with us. Most people alive today outside Africa carry a small amount of Neanderthal DNA, a genetic reminder that the story was not clean replacement but entanglement.

A Cautionary Tale in Brain Science

This is not just a story about Neanderthals. It is also a story about us, and about the hazards of leaning too much on anatomy to make hierarchical judgements.

Science has a long and ugly history of using skulls and brains to rank human groups. Measurements that looked objective often carried prejudice in disguise. Read more about scientific racism and it should make you mad.

Neanderthals had brains shaped differently from ours. They also had lives shaped differently from ours. The mistake was assuming that the first difference explained the second — and then using both to explain their disappearance.

The new paper does not solve the mystery of the Neanderthal extinction, but it narrows it. It tells scientists where not to look for easy answers. The cause of their extinction probably lies less in the roundness of our skulls than in the slow arithmetic of population size, migration, interbreeding, immune system, and survival.

The new findings were published in the Proceedings of the National Academy of Sciences.