Humans are not having had any new types of virus embedded in their DNA than other mammals, scientists have found out. The reason for such strong protection from viruses is that human ancestors started using weapons to fight and hunt instead of biting their enemies and animals they hunted.

Certain viruses known as "retroviruses" are capable of incorporating themselves into an animal's genetic code and being passed down from one generation to the next. Surprisingly these "endogenous retroviruses" (ERVs) are not a serious threat to humans, but can result in diseases such as cancer in other animals.

In order to explain this fact, scientists compared the number of times ERVs remained in the genomes of animals and 39 other mammalian species, including chimpanzees, dolphins and great pandas.

They found that compared with other animals, far fewer retroviruses had been incorporated into the DNA of humans and great apes over the last 10 million years. Humans were unique in not having acquired any new ERVs in 30 millennia.

One reason could be a change in behavior as humans evolved, said the scientists. Fewer close-up and personal bloody fights using teeth and claws meant less exposure to the viruses, while we know that many viruses are transmitted through blood.