It could easily be double the value estimated by the researchers, whose starting assumptions were deliberately cautious. It turns out that this rate of mutation is extremely high, especially for a species, such as humans, with a rather low rate of reproduction. If each deleterious mutation acted independently, the human race would have become extinct. It is likely, though, that mutations tend to be weeded out preferentially when several occur together. This possibility, says James F. Crow of the University of Wisconsin, Madison, offers an explanation for the existence of sexual reproduction, which shuffles mutations with each generation.
Sex loads the mutational dice in favour of some offspring rather than others.
The unlucky ones will contain several mutations at once and die before they can reproduce, ridding the general population of these particular mutations. Either way, the effect will be to enhance random sampling effects at the expense of natural selection. The effect of this may have been that, over the millennia, a large number of slightly deleterious mutations have become fixed in the human population.