Scientists have long known that some illnesses are hereditary, but a group of researchers has come up with a more general explanation for humankind's susceptibility to disease. They blame the small ancestral gene pool that spawned the species millions of years ago.

A study by researchers from the universities of Bath, Edinburgh and Sussex claims that there was not a big enough population of early humans to develop a wide and varied gene pool that would fight mutations.

They believe that if our ancestors had benefited from the vast choice of mates available to other species such as mice and rats, there would have been fewer of the mutations in our DNA that can trigger illness. More mates would have meant that many of the mutations would have been eradicated through natural selection.

The study, published in PLOS Biology, found that key regions of our DNA controlling when genes are switched on and off have been altered by around 140,000 naturally-occurring mutations over the past six million years. This has left modern humans with "sloppy" gene control mechanisms which can make us susceptible to illnesses.

The researchers suggest most of the "mildly harmful" mutations occurred when there was only a small population of about 10,000 early Hominids - the two legged primates that evolved into both humans and chimpanzees.

Had there been more early Hominids, most of these mutations would have been overridden by natural selection from a larger pool of available DNA.

"We are used to viewing us as the pinnacle of evolution, but seeing that rodents control their genes much more precisely is somewhat sobering," said Martin Lercher from the University of Bath.

"As a species, we have become used to ever increasing health care and nutrition.But if what we found is still ongoing, then these improvements might at some point be offset by the deterioration of our gene control regions."