Neanderthals interbred with their close human relatives, homo sapiens, before becoming extinct 30,000 years ago.
That led to between 1% and 4% of the DNA of modern humans of non-African descent being made up of Neanderthal genes.
New research suggests the proportion could have ended up even higher.
A study has revealed how natural selection removed large numbers of disadvantageous Neanderthal gene variants from the human genome.
And that was only possible because there were many more of our ancestors than there were Neanderthals.
Had the Neanderthal genes mixed with a smaller population of humans, more of those genes might have survived.
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."
Neanderthals were shorter and stockier than humans, with angled cheekbones, prominent brow ridges and wide noses.
Although previously thought of as brutes, scientists have discovered that they used tools, buried their dead and controlled fire, among other intelligent behaviours.
Scientists remain divided about whether the Neanderthals were a separate human species or a sub-species that co-existed their more human counterparts.
Experts believe the more adaptable early modern humans out-competed the Neanderthals for resources.
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