Comets and asteroids have been blamed for bringing life to Earth in the form of bacteria and wiping out the dinosaurs. Now scientists say they may also be responsible for sex.

The origin of sex remains one of biology?s greatest mysteries. Scientists can?t say exactly why we do it. Before sex, life seemed to manage fine with asexual reproduction. Researchers Claus Wilke and Chris Adami of Caltech and NASA?s Jet Propulsion Laboratory have used digital organisms to simulate life before sex and reveal a possible mechanism for the start of sex.

Comet or asteroid impacts could have stressed asexual organisms enough to send them down the path of sexual reproduction after forcing them to undergo many rapid genetic mutations, their study shows. Heavy doses of radiation would have caused these mutations. However, most mutations are self-destructive. ?Mutations can and do still occur,? according to Adami, ?but they lead to dead organisms and therefore do not affect the future.? However, some mutations can be good, helping a species to thrive at the expense of others. Sex was a mutation that worked.

Any number of catastrophes might have fueled a changed environment and a rate of high mutations. A cosmic impact could have altered Earth’s atmosphere for millions of years, exposing the planet to high doses of radiation. Increased volcanic activity is another possible source.

Biologists often say that sex never should have happened. On the early Earth, all organisms reproduced asexually. Gardeners are familiar with how asexual production works. Underground runners can create multiple clones. Potatoes give up an eye to create another potato, and bulbs divide. Cacti let pieces of themselves fall to the ground where they grow into new plants.

Some animals reproduce asexually as well. Sponges and sea anemones produce offspring from buds. Flatworms, if cut in two, grow a new head on one of their severed ends and a new tail on the other.

There are advantages to asexual reproduction. Energy isn?t wasted courting the opposite sex. Reproduction is virtually guaranteed. Also, when desirable traits evolve, they are not quickly diluted by evolution because your offspring are exact clones of you. Sex, on the other hand, combines many mutations with each pairing of genes, and the process ?can wash out the good and accumulate the bad,? Adami says.

Despite all these advantages for asexual reproduction, somewhere along the evolutionary line sex became common. We humans owe our existence to that mutation. Asexual reproduction provides for a slow pace of evolution, relying on accidental mutations to effect change. It might never have gotten around to the development of human beings.

Sex, on the other hand, allows plants and animals to evolve quickly, because the gene pool mixes and the fittest survive. Yet sex is a rather inefficient way to make babies. So in an evolutionary sense, why would sex ever have become so popular? Why would any asexual organism have bothered to try it out in the first place?

In order to study this, Wilke and Adami created two different simple, computerized life forms that ?share many characteristics with bacteria,? then placed them in a stressful computer environment where the rate of mutations was high. By studying digital creatures, they were able to zip through many generations in a short time.

The scientists found there is a natural limit to the number of mutations a population of asexual bacteria can handle. Past a critical limit, the accumulated mutations destroy the genetic code and the organisms die. However, useful mutations can accumulate under the right conditions. If a pair of organisms developed sexual reproduction, then their new ability to share beneficial mutations, via sex, would give them a Darwinian advantage over their asexual relatives in a highly stressful environment.

Clifford W. Zeyl, who studies evolutionary genetics at Wake Forest University, calls the work surprising and interesting, but adds a further caution. ?Since the idea came from a study of digital organisms and not from any historical evidence that such stresses actually acted on living organisms, or that they would have had the effect of selecting for sex, I think it?s highly speculative,? he says.

Adami is confident that the computer model works and says there is ?no reason whatsoever? to think that the findings would not apply in real-life situations.

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