It takes two to tango ... but why?

The genetic implications of reproduction date back hundreds of millions of years, and even baffled Charles Darwin.

It may well have escaped your notice, but we are now entering the last few days of the Year of the Reef, the Year of the Frog and the United Nations International Year of the Potato. Precisely how the last 360-odd days have benefited the causes of reefs, frogs and potatoes isn't entirely clear; doubtless the organisers will point to the "increased awareness" generated by their efforts. In which case, prepare to have your awareness increased about all manner of science over the coming year, with the impending dawn of the International Year of Astronomy, Darwin Year and the Year of Science 2009.

Scientists are supposed to welcome such jamborees, on the grounds they encourage the public to view science as exciting and cutting-edge. Yet all too often they come across as patronising exercises in intellectual self-satisfaction. Take the theme of the Year of Science 2009: "How we know what we know." As a sales pitch to the public, this is right up there with "How scientists have pretty much sorted out everything". Does anyone - apart from professors of epistemology - get a thrill out of being told how the known came to be known?

Over the coming year, the organisers of the Year of Science 2009 are planning lectures and exhibitions on 12 monthly themes, starting in January with the "process and nature of science", before moving on to evolution (coinciding with the bicentennial of the birth of Charles Darwin on Feb 12), and finishing with "science and health". All very worthy - not to say life-threateningly dull. How much more enticing it would be if the organisers ditched their theme in favour of "What we don't know". As for what they might cover each month, they should get themselves a copy of 13 Things That Don't Make Sense (Doubleday).

This baker's dozen of anomalies have been put together by former physicist and science writer Dr Michael Brooks, who describes them as "the most intriguing scientific mysteries of our time". Some will be familiar to regular readers of Frontiers, such as the quest to identify the so-called Dark Energy and Dark Matter which fill the cosmos. But Dr Brooks includes many more anomalies that are rarely discussed outside the scientific community.

One which you can bet won't figure prominently during the upcoming "Darwin Year" celebrations centres on that most basic of evolutionary processes, reproduction. Why have so many lifeforms evolved the need to include a second organism to reproduce, rather than just doing it all themselves? Consider: sexual reproduction leads to only half of an organism's genes being passed on - far less effective than asexual reproduction, where all the genes get transmitted. More puzzling still, sexually reproducing organisms should have been overwhelmed in the Darwinian battle for survival, as only half the population is producing offspring, compared to every one of their asexual counterparts.

For years the standard explanation has been that sexual reproduction injects much-needed variety into the gene pool, allowing organisms to cope with environmental change and other challenges to their survival. It was an argument that gained strength from studies suggesting that asexually-reproducing organisms quickly become extinct. It's now known, however, that some types of organisms have survived perfectly well using asexual reproduction for millions of years.

Other explanations for sex have been put forward, but none has succeeded in explaining why a reproductive strategy with such a high evolutionary cost - the halving of both genetic inheritance and reproductive population - has become so ubiquitous. The answer may come from computer simulations, which allow researchers to create their own Darwinian worlds and study the success of different reproduction strategies. But for the time being, the prevalence of sexual reproduction remains - in the words of Darwin himself - "hidden in darkness". Dr Brooks himself suspects it may even be the longest-standing scientific mystery of all, having defeated the best minds in biology for more than a century. He quotes one of the leading experts on evolution, the late Prof John Maynard Smith of Sussex University, UK, who described the existence of sexual reproduction as an "evolutionary scandal".

Dr Brooks describes one intriguing suggestion for explaining the existence of sex, which has its roots in a key turning-point in the early history of the Earth. Around 2.7 billion years ago, oxygen levels in the atmosphere began to soar - along with levels of toxic "free radicals", created from the gas by the action of sunlight. The result was catastrophic for many organisms, but as ever, some evolved ways of coping with the problem. Some bacteria even turned the oxygen-rich environment to their advantage, using it as an energy source.

Then a truly life-changing event took place: after around 300 million years, the organisms which could cope with the oxygen-rich atmosphere got together with the energy-generating bacteria. The result can be found in our bodies to this day: every one of our cells contains mitochondria - the distant ancestors of those primordial bacteria. While we still benefit from their energy-generating abilities, there is a deadly downside: the production of free radicals, causing potentially lethal damage to our genes. And this, says Dr Brooks, might explain the origin of sexual reproduction - which gives us access to new genes that may be more robust to the ravages of free radicals.

The possibility of an intimate link between sex and death dating back more than two billion years is an intriguing idea. It might even be right. At the very least, it deals with the kind of mystery that should be at the heart of the upcoming science "years". For as the late, great science writer Isaac Asimov once said, the most exciting exclamation in science is not "Eureka!" but "That's funny? ".

Robert Matthews is Visiting Reader in Science at Aston University, Birmingham, England.