Neanderthals' bigger eyes and bodies meant they had less brain space to dedicate to social networking, which may explain why they died out and Homo sapiens conquered the planet, a study said yesterday.
An enigmatic branch of the human family tree, Neanderthals lived in Europe, Central Asia and Middle East for up to 300,000 years, but vanished from the fossil record about 30,000 years ago.
Why they disappeared is one of the hottest topics in anthropology. Theories range from climate change to aggression from their H. sapiens cousins.
Now experts from Oxford University and London's Natural History Museum suggest the answer lies in available brainpower.
Neanderthals were stockier than anatomically modern humans who shared the planet with them at the time of their demise, but their brains were the same size, the team writes in the journal Proceedings of the Royal Society B, which deals with biological research.
As a result, Neanderthals "would have required proportionately more neural matter" to maintain and control their larger bodies, they say.
Comparing the skulls of 32 humans and 13 Neanderthals, the researchers also established the hominids had bigger eye sockets, indicating bigger eyes and visual cortices - those areas of the brain that regulate vision.
"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," said Oxford anthropologist and lead author Eiluned Pearce.
Among living primates and humans, the size of an individual's social network was constrained by the size of specific brain areas, she said. The larger these areas are, the more connections an individual can maintain.
The archaeological record seems to support the theory that Neanderthals were cognitively limited to smaller groups - they transported raw materials over shorter distances and rare finds of symbolic artefacts suggest a limited ability to trade.
The ability to organise a collective response would have been a key to survival in harsh times, like during the Ice Age, Pearce said. "If Neanderthals knew fewer people in fewer neighbouring groups, this would have meant fewer sources of help in the event of, for example, local resource failure," she said.
"Smaller groups are also more liable to demographic fluctuations, meaning a greater chance of a particular group dying out. Smaller groups are less able to maintain cultural knowledge, so innovations may be more likely to be lost."