Neanderthals could have gone extinct due to a slight drop in their fertility rates, a new study finds.

The last of the Neanderthals, the closest extinct relatives of modern humans, disappeared from Europe about 40,000 years ago.

Previous research estimated that at its peak, the entire Neanderthal population in both Europe and Asia was quite small, totaling 70,000 at most.

Scientists have long debated whether the dispersal of modern humans across the globe helped kill off Neanderthals, either directly through conflict or indirectly through the spread of disease.

"The disappearance of the Neanderthal population is an exciting subject — imagine a human group that has lived for thousands of years and is very well-adapted to its environment, and then disappears," study senior author Silvana Condemi, a paleoanthropologist at Aix-Marseille University in Marseille, France, told Live Science.

Today, thanks to the results of genetic analysis, we know that the encounters between Neanderthals and sapiens were not always so cruel, and that interbreeding took place — even today's humans have genes of Neanderthal origin."

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