BBC science reporter Paul Rincon reports on a landmark scientific study that suggests many people alive today possess some Neanderthal ancestry.

While the findings have surprised some experts, the study confirms that living humans overwhelmingly trace their ancestry to a small population of Africans who later spread out across the world.

The result comes from analysis of the Neanderthal genome and shows that between 1% and 4% of the Eurasian human genome seems to come from Neanderthals.

The Out of Africa theory of modern human origins is that the ancestors of living humans originated in Africa some 200,000 years ago with a small group later populating the rest of the world.

The Neanderthal genetic contribution found in people from Europe, Asia and Oceania is small, but is higher than previous genetic analyses have suggested.

Professor Chris Stringer, research leader in human origins at London’s Natural History Museum, said what was really surprising was the implication that there has been some interbreeding between Neanderthals and modern humans in the past.

Other experts have been surprised by the amount of genetic contribution from Neanderthals being as high as 4%.

The landmark sequencing of the Neanderthal genome is the product of a four-year-long effort led from the Max Planck Institute in Leipzig, Germany, but involving many other universities around the world.

Writing in Science journal, the researchers describe how they compared this draft sequence with the genomes of modern people from around the globe.

The results show that the genomes of non-Africans are closer to the Neanderthal sequence than are those from Africa with the most likely explanation being that there was limited "gene flow" between Neanderthals and the ancestors of present-day Eurasians.