Neanderthals went extinct because they had larger eyes than modern humans that monopolized brain resources to see in the long nights in Europe, scientists say.

Modern humans, on the other hand, spent more time in Africa where longer, brighter days required no such adaption and allowed our frontal lobes, associated with higher-level thinking, to evolve further before we moved out of Africa and spread across the globe, they said.

Eiluned Pearce of Britain's Oxford University, comparing Neanderthal skulls to Homo sapiens found Neanderthals had significantly larger eye sockets.

Although the difference was slight -- about a quarter of an inch -- it was sufficient for Neanderthals to use significantly more of their brains to process visual information, she said.

"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 told BBC News.

Researcher Chris Stringer, an expert in human origins at the Natural History Museum in London, agreed.

"We infer that Neanderthals had a smaller cognitive part of the brain and this would have limited them, including their ability to form larger groups," Stringer said. "If you live in a larger group, you need a larger brain in order to process all those extra relationships."

Thus the Neanderthals' more visually-focused brain structure might have limited their ability to innovate and to adapt to the ice age believed to have contributed to their demise, the researchers said.

"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," Stringer said.