Scientists in Britain believe that the reason there are only two sexes is due to a bacterial infection our ancestors caught about two billion years ago.

There is no end to the diversity of life on Earth, so why most species have only two sexes has been puzzling scientists around the world for many years.

Mushrooms have as many as 36,000 sexes, and a strange growth called slime mould has about thirteen. But these are rare exceptions to the almost universal rule that life on earth is divided into two sexes.

An evolutionary mystery

This presents an evolutionary mystery; If we had a hundred sexes, and could mate with any one of them, our chance of finding a partner in our surrounding environment would be 99%.

Professor Laurence Hurst, of Bath University, England, explains the problem: "For example, imagine that you are in a disco and the lights have gone out. You're looking for somebody to go home with and the law is the first person you bump into is the person you choose.

If there are only two sexes, then on average, half the time the person you meet is not going to be a potential mate."

Bacteria in the genes

So the question is why we only have two sexes, if it appears to make the survival of species more rather than less difficult. Professor Hurst believes it's all down to how we inherit a particular set of genes, called mitochondrial genes.

Unlike the genes carried in the nucleus or centre of our cells, mitochondria are found in material outside the nucleus. And unlike genes in the nucleus, mitochondrial DNA can copy itself very rapidly. Professor Hurst, speaking at the Festival of Science in Sheffield, believes mitochondrial genes are the remains of a bacterial infection caught by our distant ancestors.

"It looks as though there used to be bacteria where mitochondria come from. So we think that we got them about two billion years ago from bacteria taken into ourselves. So they became part of us, and that ability to replicate at will is left over from their bacterial ancestry."

Fewer mutations

Because mitochondrial DNA can reproduce so quickly, any mutation in it could spread rapidly if 99% of a population could mate with any other members. If the mutation was damaging the result could be catastrophic.

Instead, almost every species on Earth only inherits mitochondria from its mothers. Exceptions like mushrooms have evolved to avoid exchanging mitochondrial DNA when they reproduce.

For the rest of us, finding a partner may be a little difficult, but in evolutionary terms, this is set against the benefits of fewer mutations.