Europe's neanderthals were outnumbered by humans, forcing them into extinction.
There were 10 humans for every Neanderthal.
Faced with competition, the Neanderthals retreated into less attractive regions of the continent.
A final death blow may have been delivered by a harsh cold snap.
A swell of modern humans outnumbered Neanderthals in Europe by nearly 10 to one, forcing their extinction 40,000 years ago, suggests a study of French archaeology sites.
Scientists have long debated what caused the Neanderthals to die off rather suddenly, making way for the thriving population of more advanced Homo sapiens who likely moved in from Africa.
The latest theory, published in the journal Science, is based on a statistical analysis of artifacts and evidence from the Perigord region of southern France, where lies the largest concentration of Neanderthal and early modern human sites in Europe.
Researchers at the University of Cambridge found more sites where modern humans settled, larger settlement areas, greater densities of tools and bigger amounts of animal and food remains, suggesting Neanderthals were crowded out.
Homo sapiens also likely had more elaborate social networks and possibly sharper brains, as evidenced by the stone tools, jewellery and artwork they left behind which was much more advanced than Neanderthal creations.
Their arrival in such large numbers likely forced Neanderthals from their habitual settlements and into places where food and shelter were harder to find, says lead author Professor Paul Mellars of Cambridge University.
"It was clearly this range of new technological and behavioral innovations which allowed the modern human populations to invade and survive in much larger population numbers than those of the preceding Neanderthals across the whole of the European continent," he says.
"Faced with this kind of competition, the Neanderthals seem to have retreated initially into more marginal and less attractive regions of the continent."
The last traces of Neanderthals, who had survived on the continent for some 300,000 years, have been uncovered in caves in modern-day Spain and Gibraltar.
Mellars suggests a final death blow may have been delivered by a harsh cold snap, a theory that has been debated in the scientific community for many years.
The Neanderthals' extinction may have been "accelerated further by sudden climatic deterioration across the continent around 40,000 years ago," he says.