Neanderthals never invented written language, developed agriculture or progressed past the Stone Age. At the same time, they had brains just as big in volume as modern humans’. The question of why we Homo sapiens are significantly more intelligent than the similarly big-brained Neanderthals—and why we survived and proliferated while they went extinct—has puzzled scientists for some time.

Now, a new study by Oxford researchers provides evidence for a novel explanation. As they detail in a paper published today in the Proceedings of the Royal Society B, a greater percentage of the Neanderthal brain seems to have been devoted to vision and control of their larger bodies, leaving less mental real estate for higher thinking and social interactions.

The research team, led by Eiluned Pearce, came to the finding by comparing the skulls of 13 Neanderthals who lived 27,000 to 75,000 years ago to 32 human skulls from the same era. In contrast to previous studies, which merely measured the interior of Neanderthal skulls to arrive at a brain volume, the researchers attempted to come to a “corrected” volume, which would account for the fact that the Neanderthals’ brains were in control of rather differently-proportioned bodies than ours ancestors’ brains were.

One of the easiest differences to quantify, they found, was the size of the visual cortex—the part of the brain responsible for interpreting visual information. In primates, the volume of this area is roughly proportional to the size of the animal’s eyes, so by measuring the Neanderthals’ eye sockets, they could get a decent approximation of their the visual cortex as well. The Neanderthals, it turns out, had much larger eyes than ancient humans. The researchers speculate that this could be because they evolved exclusively in Europe, which is of higher latitude (and thus has poorer light conditions) than Africa, where H. sapiens evolved.

Along with eyes, Neanderthals had significantly larger bodies than humans, with wider shoulders, thicker bones and a more robust build overall. To account for this difference, the researchers drew upon previous research into the estimated body masses of the skeletons found with these skulls and of other Neanderthals. In primates, the amount of brain capacity devoted to body control is also proportionate to body size, so the scientists were able to calculate roughly how much of the Neanderthals’ brains were assigned to this task.

After correcting for these differences, the research team found that the amount of brain volume left over for other tasks—in other words, the mental capacity not devoted to seeing the world or moving the body—was significantly smaller for Neanderthals than for ancient H. sapiens. Although the average raw brain volumes of the two groups studied were practically identical (1473.84 cubic centimeters for humans versus 1473.46 for Neanderthals), the average “corrected” Neanderthal brain volume was just 1133.98 cubic centimeters, compared to 1332.41 for the humans.

This divergence in mental capacity for higher cognition and social networking, the researcher argue, could have led to the wildly different fates of H. sapiens and Neanderthals. “Having less brain available to manage the social world has profound implications for the Neanderthals’ ability to maintain extended trading networks,” Robin Dunbar, one of the co-authors, said in a press statement. “ are likely also to have resulted in less well developed material culture—which, between them, may have left them more exposed than modern humans when facing the ecological challenges of the Ice Ages.”

Previous studies have also suggested that the internal organization of Neanderthal brains differed significantly from ours. For example, a 2010 project used computerized 3D modeling and Neanderthal skulls of varying ages to find that their brains developed at different rates over the course of an individual’s adolescence as compared to human brains despite comparable brain volumes.

The overall explanation for why Neanderthals went extinct while we survived, of course, is more complicated. Emerging evidence points to the idea that Neaderthals were smarter than previously thought, though perhaps not smart enough to outmaneuver humans for resources. But not all of them had to—in another major 2010 discovery,a team of researchers compared human and Neanderthal genomes and found evidence that our ancestors in Eurasia may have interbred with Neanderthals, preserving a few of their genes amidst our present-day DNA.

Apart from the offspring of a small number of rare interbreeding events, though, the Neanderthals did die out. Their brains might have been just as big as ours, but ours might have been better at a few key tasks–those involved in building social bonds in particular—allowing us to survive the most recent glacial period while the Neanderthals expired.