London: The key immunity genes that help us fight viruses today may have been passed down by our extinct relatives, including Neanderthals, scientists have claimed.
Cross-breeding between ancestors of modern humans and their extinct close relatives passed down specific genes which can still be found in our DNA, the researchers believe.
They think that improvements in the immune system of Homo sapiens` may also have been inherited from the Denisovans – a now-vanished human sub-species from eastern Asia, the Daily Mail reported.
Last year, scientists discovered that ancient intimate relations meant as much as four percent of the DNA of some people living today was Neanderthal. A similar genetic study showed that up to six per cent of the modern human genome, or genetic code, was Denisovan in origin.
Lead researcher Dr Peter Parham, of Stanford University, California, said cross-breeding contributed to our gene pool.
He said: "The cross-breeding wasn`t just a random event that happened, it gave something useful to the gene pool of the modern human."
Neanderthals, who lived in western Asia and Europe, co-existed with early modern humans for several thousand years before dying out around 30,000 years ago.
It is believed that Homo sapiens, Neanderthals and Denisovans split into different populations from a common African ancestor around 400,000 years ago.
For the research, published today in the journal Science, the researchers focused on immune system elements called HLA genes, which are critical to the body`s ability to identify and destroy harmful foreign invaders.
They are among the most variable and adaptable of human genes and being flexible helps them stay ahead in the arms race with rapidly evolving viruses.
By comparing the HLA genes of modern and extinct humans, the scientists were able to show that certain HLA carried by people today were inherited from Neanderthals and Denisovans.
The ancient genes were analysed by looking at DNA extracted from fossil bones.
Within one class of HLA gene, the experts estimated that Europeans owe half their variants to interbreeding with Neanderthals and Denisovans, Asians 80 percent, and people from Papua New Guinea up to 95 per cent.
The same pattern of inheritance was not seen in other regions of the human genome.
"The HLA system is unique in its diversity and the strength of natural selection acting on it," said co-author Dr Laurent Abi-Rached.
"But it`s possible that other gene systems, particularly the ones under similar pressure for variation, could show a similar pattern," Dr Abi-Rached added.