The last of the Neanderthals may have died out tens of thousands of years ago, but large stretches of their genetic code live on in people today.

Though many of us can claim only a handful of Neanderthal genes, when added together, the human population carries more than a fifth of the archaic human's DNA, researchers found.

The finding means that scientists can study about 20% of the Neanderthal genome without having to prise the genetic material from fragile and ancient fossils.

The Neanderthal traces in our genetic makeup are the lasting legacy of sexual encounters between our direct ancestors and the Neanderthals they met when they walked out of Africa and into Eurasia about 65,000 years ago.

The populations of both groups were likely so small that interbreeding was a rare event, but the benefits of some Neanderthal genes were so great that they spread through the population and linger on in modern non-Africans today.

Benjamin Vernot and Joshua Akey at the University of Washington in Seattle sequenced the genomes of more than 600 people from Europe and eastern Asia. They then used a computer analysis to find gene variants that bore all the hallmarks of having come from Neanderthals.

To see whether the technique worked, they checked the genes against the official Neanderthal genome, which was sequenced from fossil remnants in 2010 by researchers in Germany.

The researchers found that while most non-Africans carried 1 to 3% Neanderthal DNA, the total amount in modern humans reached about 20%. "Although Neanderthals are extinct, there's still a lot of genetic information about them floating around, in our own genomes. It's not necessarily useful in that it will cure cancer, but it helps us to learn about our history," Vernot told the Guardian. Details of the study are reported in Science.

The researchers now believe that even deeper mining of modern genomes could help to find genetic traces of other archaic humans.

In a separate study published in Nature, David Reich at Harvard University looked for Neanderthal genes in the DNA of more than 1,000 living people. He found that the Neanderthals left a mark in distinct regions of the modern human genome, but in others left no trace at all.

Many of the Neanderthal genes that live on in people today are involved in making keratin, a protein used in skin, hair and nails. Reich speculates that modern humans may have picked up Neanderthal genes that were better suited to the cold environment, perhaps because they produced more or thicker hair, or tougher skin.

More striking was that humans are missing Neanderthal DNA from many other regions of their genomes, which suggests that evolution steadily stripped the archaic DNA out until it vanished all together.

Reich found that hardly any Neanderthal genes were expressed in modern men's testes, and that the X chromosome was almost completely devoid of Neanderthal DNA. That would happen if males with Neanderthal and modern human parents were infertile, because the males would never get to pass on their single Neanderthal X chromosome.

"When Neanderthals and modern humans interbred they were actually at the edge of biological compatibility. They did interbreed, and Neanderthals left an important biological trace in modern humans, but nevertheless, the population had to sort out some problems afterwards, because certain Neanderthal variants led to reduced male fertility," said Reich. The finding suggests that most Neanderthal DNA found in humans today was passed down from females.

"Anything related to maleness in the Neanderthal has been purged from our genomes," said Chris Stringer, head of human origins at the Natural History Museum in London. "Neanderthal DNA has come down to us today, but that transmission was mainly through the female line, because the males would have been significantly less fertile, and possibly even sterile."