The Panel of Hands created by Neanderthals in El Castillo Cave in Spain. REUTERS/Image courtesy of Pedro Saura/Handout

A new U.K. study argues that Neanderthals went extinct because their bulging eyes — much larger than modern-day humans' eyes — took up more brain power.

The study, published in the Proceedings of the Royal Society B journal, says Neanderthals brains were also larger and organized differently than ours.

"Neanderthals have significantly smaller adjusted endocranial capacities," the study says.

Previous research has focused on the size and shape of their brains, but didn't look at differences in how the brains were organized.

Neanderthal's larger brain is partly because of their larger bodies, but also due to their larger eyeballs, the study found. The Neanderthal's brain was also organized to process more visual information than modern humans.

Because they were more visually focused, Neanderthal's may have lacked cognitive ability to adapt to changes such as the Ice Age and likely led to their demise, the researchers said.