PRESS ASSOCIATION -- Neanderthal nookie thousands of years ago boosted the health of the human race, scientists believe.

Sex between ancestors of modern humans and their extinct close relatives led to improvements in the Homo sapiens' immune system that defend us against disease today, research suggests.

A similar benefit may have been inherited from the Denisovans, another now-vanished human sub-species from eastern Asia.

Scientists discovered last year that ancient intimate relations meant as much as 4% of the DNA of some people living today was Neanderthal. A similar genetic study showed that up to 6% of the modern human genome, or genetic code, was Denisovan in origin.

The latest research suggests that couplings between different human sub-species has had a positive effect on fitness.

"The cross-breeding wasn't just a random event that happened, it gave something useful to the gene pool of the modern human," said lead researcher Professor Peter Parham, from Stanford University in California, US.

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.

Homo sapiens, Neanderthals and Denisovans split into different populations from a common African ancestor around 400,000 years ago.

The new research, published on Thursday in the journal Science, focused on immune system elements called HLA genes which are critical to the body's ability to identify and destroy harmful foreign invaders.

HLA genes are among the most variable and adaptable genes in the human genome. 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.