"The researchers believe that most of the damaging mutations occurred when there was only a small population of early hominids - the two-legged primates who later evolved into humans and chimpanzees. At the time, there may have been as few as 10,000 hominids to breed with one another. Chris Tyler-Smith, Wellcome Trust Sanger Institute In comparison, rats and mice had plenty of mates to choose from and harmful DNA mutations were rapidly eliminated from the gene pool, according to the study authors from the Universities of Bath, Edinburgh and Sussex. Dr Martin Lercher, a co-researcher, said: "We are used to viewing us as the pinnacle of evolution, but seeing that rodents control their genes much more precisely is somewhat sobering."