British scientists have reopened the debate over why some organisms, including humans, reproduce sexually.

Evolutionary biologists have long puzzled over why sex evolved at all. Asexual organisms simply split themselves to produce offspring and can reproduce twice as fast as sexual organisms. So why go to the trouble of having sex?

Until now, the main theory has been that sexual reproduction, which requires combining both parents' DNA, helps eliminate harmful genetic mutations from the genome.

But researchers said on Thursday they had disproved this theory by estimating the mutation rate in related species, including humans, chimps and monkeys.

Good hand

Sex requires finding a mate and combining two genomes. An asexual organism, however, simply divides itself into two, something that is quicker and less costly to the organism.

One explanation for why sexual reproduction evolved is that it helps eliminate potentially lethal genetic defects.

When genomes of the male and female are shuffled together, some offspring will be dealt a good genetic hand and survive. However, those that inherit harmful mutations do not survive to pass their DNA on to offspring.

If this hypothesis is correct, the higher the mutation rate, the more advantageous sexual reproduction should be.

Mutation rate

Dr Peter Keightley, of the University of Edinburgh, and Dr Adam Eyre-Walker, of the University of Sussex, Brighton, tested this idea.

They calculated just how high the mutation rate would have to be for a range of organisms to realise the benefits of sexual reproduction.

They found that the organisms seem to have a lower than necessary mutation rate, suggesting that the harmful mutations theory does not explain the evolution of sex.

The research is reported in the journal Science.