Neanderthals had sharper eyes and bigger bodies than their early modern human cousins - but these traits only contributed to their downfall, new research has suggested.
Their brains had too much to do managing their vision and the physical demands of their bodies to spare resources for social networking.
The more socially adept modern humans were better able to cope with environmental change and compete for shelter and food. Eventually, the Neanderthals followed in the footsteps of the dinosaurs and became extinct, leaving our Homo sapiens ancestors to inherit the Earth.
Neanderthals had already long occupied Eurasia when early modern humans began migrating out of Africa about 70,000 years ago. For a time, the two models of human lived side-by-side, and many experts believe they even interbred. But by around 30,000 years ago the Neanderthals had vanished.
Although early modern humans and Neanderthals were similar in many ways they also had distinct differences. Neanderthals were physically bigger and stronger, their foreheads and chins sloped at more of an angle, and their noses were more pronounced. They also had larger eyes, which may have evolved to cope with lower light levels in northern latitudes.
Like modern humans, they used tools and weapons, and they may also have worn ornaments and painted cave walls. However, they were culturally much less sophisticated.
The new research, published in the journal Proceedings of the Royal Society B, shows that Neanderthals' brains were similar in size to those of modern humans, but structured differently.
Analysis of fossil data suggests that larger areas of the Neanderthal brain were devoted to vision and movement, leaving less room for the higher level thinking needed to form large social groups.
Lead researcher Dr Eiluned Pearce, from Oxford University's Institute of Cognitive and Evolutionary Anthropology, said: "'Since Neanderthals evolved at higher latitudes and also have bigger bodies than modern humans, 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."
"Smaller social groups might have made Neanderthals less able to cope with the difficulties of their harsh Eurasian environments because they would have had fewer friends to help them out in times of need. Overall, differences in brain organisation and social cognition may go a long way towards explaining why Neanderthals went extinct whereas modern humans survived."