Scientists have reconstructed the oldest known human genome after finding the bones of a 45,000-year-old man in Siberia.

The findings allowed scientists to further confirm that early humans and Neanderthals mixed and had children, said the researchers of the study published in Nature.

"What's exciting about this paper is that it's looking at a very ancient modern human who would have lived around the same time as Neanderthals," said Sarah Tishkoff, a geneticist at the University of Pennsylvania, who was not on the research team.

The bone and genetic sequence showed the man had both human and Neanderthal genes.

The one mystery that remains is what happened after the two interbred. Humans survived and Neanderthals died out but scientists still don't understand why.

The study also alludes to humans leaving Africa 60,000 years ago, much later than previously thought.

"We have caught evolution red handed!" said Svante Pääbo, a director at the Max Planck Institute for Evolutionary Anthropology and lead on the study.