Quirk of evolution limited Neanderthal genes in modern people
People living today would be a lot more Neanderthal had it not been for a quirk of evolutionary fate, research suggests.
The Neanderthals, who once colonised Europe and Asia, became extinct about 30,000 years ago - but not before interbreeding with their close human relatives, Homo sapiens.
As a result Neanderthal genes make up between 1% and 4% of the DNA of modern people of non-African descent.
The new study reveals how natural selection has purged the human genome of large numbers of weakly disadvantageous Neanderthal gene variants.
But this only happened because the ancestral Homo sapiens population was so much larger than that of the Neanderthals, scientists believe.
If the Neanderthal genes had mixed into a smaller population, more of them might have survived to the present day.
US researcher Dr Ivan Juric, from the University of California at Davis, said: "Selection is more efficient at removing deleterious variants in large populations.
"Weakly deleterious variants that could persist in Neanderthals could not persist in (early modern) humans. We think that this simple explanation can account for the pattern of Neanderthal ancestry that we see today along the genome of modern humans."
He added that if Neanderthals had been more numerous when they encountered our ancestors as they migrated out of Africa, we might have a different mix of Neanderthal and Homo sapiens genes.
More than 500,000 years ago, modern humans and Neanderthals had a common African ancestor. Neanderthals moved out first and entered Europe and Central Asia. Then 50,000 to 80,000 years ago, they were followed by their Homo sapiens cousins.
The two kinds of human - scientists are still divided about whether the Neanderthals were a separate human species or a sub-species - co-existed until the Neanderthals died out.
Experts believe the more adaptable early modern humans out-competed the Neanderthals for resources such as food and shelter.
The study, published in the journal Public Library of Science Genetics, also shows that East Asian people had higher initial levels of Neanderthal ancestry than Europeans.