New research suggests Neanderthals had sharper vision and bigger bodies than their early modern human cousins -- but that their brains had less space for the higher-level thinking needed to form large social groups.
The researchers say living in smaller social groups would mean fewer friends to come to the rescue in times of need, the BBC reports.
Because the Neanderthals had larger eyes than our species, more of their brain was devoted to seeing in the long, dark nights in Europe, at the expense of high-level processing.
So the human species, Homo Sapiens, was able to make warmer clothes and develop larger social networks, all of which helped survival during the ice age in Europe.
Neanderthals are a closely related species of human that lived in Europe from around 250,000 years ago. They coexisted and interacted briefly with our species until they went extinct about 28,000 years ago, in part due to an ice age.
Dark nights, murky days
The research team explored the idea that the ancestor of Neanderthals left Africa and had to adapt to the longer, darker nights and murkier days of Europe. The result was that Neanderthals evolved larger eyes and a much larger visual processing area at the back of their brains.
The humans that stayed in Africa, on the other hand, continued to enjoy bright and beautiful days and so had no need for such an adaption. Instead, these people, our ancestors, evolved their frontal lobes, associated with higher level thinking, before they spread across the globe.
Eiluned Pearce of Oxford University compared the skulls of 32 Homo sapiens and 13 Neanderthal skulls and found that Neanderthals had significantly larger eye sockets - on average 6mm longer from top to bottom.
This was enough for Neanderthals to use significantly more of their brain to process visual information.
"Since Neanderthals evolved at higher latitudes, more of the Neanderthal brain would have been dedicated to vision and body control, leaving less brain to deal with other functions like social networking," she says.
Oxford University's Prof Robin Dunbar, who supervised the study, said that the team wanted to avoid restoring the stereotypical image of Neanderthals.
"They were very, very smart, but not quite in the same league as Homo sapiens," he says.
"That difference might have been enough to tip the balance when things were beginning to get tough at the end of the last ice age," he said.