After analyzing the DNA of a 45,000-year-old human, scientists have found new clues about the first time that ancient humans interbred with Neanderthals, an extinct species of human who evolved outside of Africa.

According to the genome sequence from a thigh bone of a skeleton found in Western Siberia, the first time that the two mixed was between 50,000 and 60,000 years ago.

A new study published in Nature journal also supports the theory that modern humans originated from Africa around 60,000 years ago, before spreading across the world.

Prof. Svante Paabo and his colleagues at the Max Planck Institute in Leipzig, Germany used pioneering methods to extract DNA from ancient human remains and read its genetic code.

"The amazing thing is that we have a good genome of a 45,000 year old person who was close to the ancestor of all present-day humans outside Africa," Paabo told BBC News.

After analyzing the leg bone of a man who lived in Siberia 45,000 years ago, Prof. Paabo found that the man's DNA contained extracts from Neanderthals.

This pivotal discovery provides telling clues about how and when modern humans began expanding into Europe and Asia.

"Our analysis shows that modern humans had already interbred with Neanderthals then and we can determine when that first happened much more precisely than we could before," said the professor.

According to his study, the first interbreeding between modern humans and Neanderthals occurred between 50,000 and 60,000 years ago.

Back in 2010, Prof. Paabo and his team published a study that revealed that all non-African humans today have Neanderthal DNA.

According to Prof. Chris Stringer of the Natural History Museum in London, this interbreeding might present evidence about when human ancestors living outside of Africa began to settle in other continents. It also shows that our species evolved over 150,000 years ago, rather than 100,000 years ago as it was previously believed.

"There is a dispute as to when that 'Out of Africa' event happened and this fossil helps to look at that. It is close to the time I think that modern humans exited from Africa and gave rise to the populations in the rest of the world. I think that exit happened 60,000 years ago," he said.