Neanderthal brains measure up to ours—literally

If you look at a Neanderthal skull and a Homo sapiens skull, they’re visibly different: Neanderthal skulls are lower and longer, whereas ours tend to be rounder. However, those differences probably don’t say much about the brains within them, according to a recent study, which compared MRI scans of modern people’s brains with casts of the inside of Neanderthal skulls.

The results suggest that there’s more variation in brain size among modern people than between Neanderthals and Pleistocene Homo sapiens. And because brain size is actually a terrible way to predict cognitive capability, Neanderthals could have been a lot more like us than some previous studies have claimed, which definitely fits what the archaeological record tells us about how they lived. It would also mean that our species probably didn’t out-compete the Neanderthals by being smarter or more adaptable.

Neanderthal brains fit within the modern human range

Years after you die, the inner vault of your skull will hold the shape of your brain; if future archaeologists make a cast of that inner space, they’ll get a neat resin model of the outer contours of your brain, called an endocast. (Sediment that filled the skull of an Australopithecus africanus child who died 2.8 million years ago did this naturally, creating an endocast that’s half rocky brain-sculpture and half sparkling crystal.) For years, researchers have studied endocasts of Neanderthal skulls, trying to piece together how their brains were different or similar to ours. And that has been a matter of some debate.

A 2018 study compared endocasts from four Neanderthals and four early members of our species, measuring the volumes of 13 major brain regions. That study’s authors suggested that, despite having larger total cranial capacity (more room in their skulls), Neanderthals, on average, had smaller cerebellums than Homo sapiens. (A small structure at the back of the brain, the cerebellum plays a role in motor control, emotional regulation, and attention, among other things.) And while that’s technically true—based on, admittedly, a very small sample size—it wasn’t the whole story.

“The inferred differences were not put into the context of modern human populational variation in brain anatomy,” wrote Indiana University cognitive scientist P. Thomas Schoenemann and his colleagues. In that same paper, they decided to take a stab at doing so. Schoenemann and his colleagues performed the same size comparison using MRI scans of 400 modern people’s brains: 200 US residents of European descent and 200 ethnic Han Chinese people who had volunteered to be scanned as part of the Human Connectome project.

It turns out that, when it comes to brain size, the differences between our species and Neanderthals are on par with the differences within our species. For nine of the 13 regions measured, Schoenemann and his colleagues found bigger differences in volume between some modern people than the earlier study found between Neanderthals and Pleistocene Homo sapiens. “Our analysis shows that Neanderthal differences in brain and cognition would fit comfortably within the range of differences seen among modern humans,” wrote Schoenemann and his colleagues.

In other words, we’re a diverse species, and the size and shape of Neanderthal brains fit into the range of that diversity (which arguably lends some support to the paleoanthropologist who argues that maybe we shouldn’t think of Neanderthals and Denisovans as separate species at all). And all of those size differences are too small to have any effect on cognitive ability, so Neanderthals could easily be on par with our species there, too.

When does size matter?

Conventional wisdom gives the credit for humans’ evolutionary success to our intelligence and our “big brains,” but what does that even mean?

Decades’ worth of research have found that brain volume—whether we’re talking about the whole brain or the size of a particular region—has little to no connection to how well a person performs on cognitive tests compared to other people. Or as Schoenemann and his colleagues put it, “cognitive implications of neuroanatomical size differences are very weak in modern humans, when found at all.” In other words, when it comes to intelligence, brain size doesn’t matter.

(When we talk about “intelligence,” we’re describing something complex and, frankly, sort of nebulous; it’s impossible to really quantify, but that hasn’t stopped generations of scientists from trying. Researchers who study cognition break it down into specific areas: attention, inhibition, cognitive flexibility, speech production and speech comprehension, working memory, and episodic memory. Some of those abilities are associated with particular sections of the brain, but those relationships are often complicated.)

So, when looking at brain size and intelligence, the differences among human brains are relatively small compared to the differences between a human brain and any other great ape brain. For example, our closest relatives, the chimpanzees, have brains that average just 400 cubic centimeters; the average adult human brain takes up about 1,350 cubic centimeters. (And there’s a wide range, from about 1,100 to 1,500 cubic centimeters.)

So total brain volume is “empirically the best predictor of behavioral and cognitive abilities among primates,” but only if you’re comparing different primate species. Within species, the differences aren’t pronounced enough to matter.

If you’re comparing, say, crows to dolphins, you’ve got to factor in the size of the brain relative to the size of the whole animal, which scientists call the encephalization quotient; according to Schoenemann and his colleagues, that’s less relevant for primates, where it’s all about size.

With that in mind, a group of early hominins called Australopithecus afarensis, who lived about 3.2 million years ago, had about 500-cubic-centimeter brains. That’s a big enough difference that we can make some guesses that they were cognitively more like chimpanzees than like us. On the other hand, the average group of Neanderthals had a brain capacity that’s consistent with them scoring about the same on cognitive tests as their Homo sapiens neighbors.