Ancestors of modern-day humans are thought to have wiped out the ancient Neanderthals from Europe by passing on diseases and infections when they moved out of Africa and into the continent previously dominated by them.
The Neanderthals, who would only have developed resistance to the diseases of their European environment, are most likely to have been infected with a bacterium that causes stomach ulcers, the virus that causes genital herpes, tapeworms and tuberculosis.
The researchers said that some infectious diseases are likely to be many thousands of years older than previously believed.
The diseases and infections to which the hunter-gatherers were exposed would have made them less able to find enough food and remain healthy. The diseases would have spread through sexual contact between the two species.
"Humans migrating out of Africa would have been a significant reservoir of tropical diseases," said Charlotte Houldcroft from the University of Cambridge in Britain.
"For the Neanderthal population of Eurasia, adapted to that geographical infectious disease environment, exposure to new pathogens carried out of Africa may have been catastrophic," Houldcroft added.
The findings showed Helicobacter pylori -- a bacterium that causes stomach ulcers -- as highly likely to have been passed by humans to Neanderthals.
It is estimated to have first infected humans in Africa between 88,000 to 116,000 years ago, and in Europe 52,000 years ago.
Another likely candidate is herpes simplex 2 -- the virus that causes genital herpes. Evidence in the genome of this disease suggested that it was transmitted to humans in Africa 1.6 million years ago from another, currently unknown hominin species that in turn acquired it from chimpanzees.
The researchers have challenged the view that the spread of infectious diseases exploded with the evolution of agriculture about 8,000 years ago, which saw denser and more settled human populations coexisting with livestock.
Instead, genetic data showed that many infectious diseases have been "co-evolving with humans and our ancestors for tens of thousands to millions of years, and passed from them to the animals," the researchers noted in the paper published in the American Journal of Physical Anthropology.
"Hunter-gatherers lived in small foraging groups. Neanderthals lived in groups of between 15-30 members, for example. So disease would have broken out sporadically, but have been unable to spread very far. Once agriculture came along, these diseases had the perfect conditions to explode, but they were already around," Houldcroft maintained.
Recent theories for the cause of Neanderthal extinction range from climate change to an early human alliance with wolves resulting in domination of the food chain.
"It is probable that a combination of factors caused the demise of Neanderthals and the evidence is building that spread of disease was an important one," Houldcroft concluded.
