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Author Topic: The Origin of Sex: Cosmic Solution to Ancient Mystery  (Read 25431 times)
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« on: June 21, 2008, 03:11:19 AM »

By Robert Roy Britt
Senior Science Writer

Comets and asteroids have been blamed for a lot of things before. Shaping Earth. Jumpstarting life. Wiping out dinosaurs. Even possibly altering human evolution.

But never sex.

Roughly 1 billion years after the first organisms romped in the hay, the origin of sex remains one of biology's greatest mysteries. Scientists can't say exactly why we do it, or what triggered those initial terrestrial flirtations. Before sex, life seemed to manage fine by employing asexual reproduction -- the cloning of offspring without the help of a partner.

Now a new study out of Caltech and NASA's Jet Propulsion Laboratory has used digital organisms to simulate life before sex and yielded a possible mechanism for instigating Earth's first courtship.

Intimacy never sounded so stressful.

Comet or asteroid impacts could have stressed asexual organisms enough to send them down the path of sexual reproduction after forcing a flurry of genetic mutations, the study shows. Heavy doses of radiation might also have done the trick.

While these potential catalysts for mutations are highly speculative, researchers Claus Wilke and Chris Adami announced Monday night that they have determined with certainty one possible way that organisms could have managed such a chaotic environment to their advantage in opening the original door to sexual liberation.

The key to this mutation management, Adami told SPACE.com, is the discovery that when things get rough, a population of organisms adapts to handling a few mutations, while also ensuring that many mutations will be self-destructive.

"Mutations can and do still occur," he said, "but they lead to dead organisms and therefore do not affect the future."

Before sex

Sex never should have happened, biologists often say.

Though the ultimate act of affection has been around longer than anyone can remember, it wasn't always so. On the early Earth, all organisms reproduced asexually.

Any gardener is familiar with how asexual production works. Underground runners can create multiple clones (not to mention destroy a good lawn). Potatoes give up an eye to create another potato. Bulbs divide. Cacti, exhibiting no creativity in this area but managing to foster progeny nonetheless, simply let pieces of themselves fall to the ground and hope for the best.

Some animals get in on the asexual act, too. Sponges and sea anemones produce little ones via buds. Flatworms, if cut in two, grow a new head on one of their severed ends and a new tail on the other.

These are handy and powerful ways to leave a legacy.

For one thing, there's no need for a partner -- no butting of horns, no beating of the chest, no late nights at the bar. Reproduction is virtually guaranteed. Also, when desirable traits evolve, they are not quickly diluted by evolution. Your offspring are just like you. Exact clones.

Sex, on the other hand, combines myriad mutations with each pairing of genes, and the process "can wash out the good and accumulate the bad," Adami says. Just ask any failed child of successful parents.

The age of sex

Despite all these advantages for asexual reproduction, somewhere along the evolutionary line sex became all the rage.

Thankfully so, for we humans owe our existence to that first melding of the genes. Asexual reproduction provides for a plodding style of evolution, relying solely on accidental mutations to effect change. It's an evolutionary slow train that might never have gotten around to delivering humans. It can also limit a population's ability to survive severe environmental change.

Sex, on the other hand, allows plants and animals to evolve quickly, because the gene pool mixes and the fitter survive.

Yet as any parent knows, sex is a rather inefficient way to make babies. Biologically speaking, the man spends nine months doing absolutely nothing productive while the woman does all the work (in some households, this problem is known to persist far longer).

So in an evolutionary sense, why would sex ever have become so popular? More to the point, why would any asexual organism have bothered to try out sex in the first place?

We're all mutants

Researchers have long known that mutations rewrite portions of an organism's genetic code. Some mutations can be good, in fact helping a species to thrive at the expense of others. But the effect can sometimes be deadly. Since sex involves two parents, there is twice the number of mutations to muck up the genetic scripts.

Wilke and Adami created two different simple, computerized life forms that "share many characteristics with bacteria," then placed them in a stressful 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 a natural throttle to the number of mutations a population of asexual bacteria can handle. The throttle can be thought of as a conservation law. The law dictates that a population capable of adapting to the harmful effects of a few mutations cannot possibly handle a bunch of mutations. Past a critical limit, the accumulated mutations make gibberish out of the genetic code and the organisms die.

Conversely, the new law also shows that a population which can handle many mutations would be highly vulnerable to the first few. "In fact there are such organisms [today]," Adami said. "Sex could, however, never evolve" in such a population. The offspring would be too vulnerable to the initial flurry of mutations that would be written into its code, combined from two organisms.

The birth of sex

Now imagine simple organisms long ago that just happened to share genetic information in a loose and uncoordinated fashion. Such sharing goes on today without leading to reproduction.

If such a population of organisms were suddenly faced with the stress of high mutation rates, it would over the course of many generations develop a capacity to handle a few mutations. But by the new law, numerous mutations would be intolerable.

The effect of all this, Adami says, is that bad mutations would be weeded out of the population.

When multiple mutations are intolerable, bad mutations cannot accumulate, because each successive bad mutation has an increasingly deadly effect on an already weakened organism. Useful mutations, however, do not harm a population in these conditions, Adami said.

Put another way: "When multiple mutations are intolerable, bad mutations cannot accumulate, while the good ones still can."

This could pave the way for the benefits of sex to be enjoyed.

A theoretical door would be open to sexual freedom, and if a pair of organisms mutated enough to go behind that door, then their newfound ability to share beneficial mutations, via sex, would give them a Darwinian advantage over their asexual cousins in the highly stressful environment.

"You can imagine a path that leads from the uncorrelated exchange of genetic material to the completely orchestrated recombination process," he says, referring to the birth of sex.

Any number of catastrophes might have fueled a changed environment and a rate of high mutations, Adami explains. 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.

But Adami stressed that these possibilities, while useful to consider, were not a part of the study and so remain highly speculative.

Not actually living organisms

Clifford W. Zeyl, who studies evolutionary genetics at Wake Forest University, called the work surprising and interesting, but added 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," Zeyl said.

Adami is confident that the computer experiment renders an accurate picture, and he suspects that if such a test could be carried out on real organisms (it can't, because it would take too long) similar results might be found.

"The digital organisms actually live in the memory of the computer, so all we do is set up the experiment and then observe," he said. He added that some biologists are skeptical of any research carried out using digital organisms, but says there is "no reason whatsoever" to think that the findings would not apply in real-life situations.

Posts: 1788


« Reply #1 on: November 23, 2021, 08:08:50 AM »

The Origins of Sex

    July 7, 2015 in Science 0

In the landmark 1986 book Origins of Sex, biologist Lynn Margulis and science writer Dorion Sagan trace the first appearance of sex back billions of years, to bacteria. Here, they describe the complex evolutionary history that their book will seek to untangle. The following is an excerpt from the introduction.

Everyone is interested in sex. But, from a scientific perspective, the word is all too often associated with reproduction, with sexual intercourse leading to childbirth. As we look over the evolutionary history of life, however, we see that sex is the formation of a genetically new individual. Sex is a genetic mixing process that has nothing necessarily to do with reproduction as we know it in mammals. Throughout evolutionary history a great many organisms offered ad exchanged genes sexually without that sex ever leading to the cell or organism copying known as reproduction. Although additional living beings are often reproduced by a contribution of genes from more than a single parent, sex in most organisms is still divorced from growth and reproduction, which are accomplished by nonsexual means.

Biologically, sex is part of the rich repertoire of life. Any specific instance of a sexual event is complex. Each event in a sexual process—for example, fertilization in a plant or animal—has its own specific history. Originally unrelated phenomena, such as genetic exchange (as in DNA recombination) and cell reproduction, often became entangled after having evolved from separate beginnings. The story of sex starts with an account of the earliest life on Earth. The private activities of early cells are involved even today in courtship among human beings. The intimate behavior of single cells has simply been elaborated to include animals and their behaviors and societies. Mammalian sex is a very late and special variation on a far more general theme.

The origin of sex is a problem that has long perplexed. It lends itself to innovative mythmaking (mythopoiesis); many cultures have imagined a primordial unisexual oneness that, under the influence of a celestial personality, was split into light and dark, heaven and earth, male and female, and so on. In the march of knowledge, however, mythical accounts of the origin of sex have been abandoned. We now realize, thanks to the insights of Darwinian evolution, that the sexual differences that loom so large in the daily lives of men and women did not arise at some specific time in the history of the human species. Evolution takes us far beyond the origin of apes and men, who at their first appearance were undoubtedly already fully sexual. Sex itself arose even earlier than the many species of sexual creatures with which we are familiar. It was present on the Earth when microbes, organisms that cannot be seen without a microscope, totally dominated the planetary surface. Sex was here for hundreds of millions of years before the first animals or plants appeared.

What keeps organisms that have sexual differences from devolving into the asexual state is, as we shall show, a completely different matter from how sex came about in the first place. Biologists, although they have tried, have not been able to prove that sexual organisms have an intrinsic advantage over asexual ones. Many have struggled with the question of how sexual organisms can afford to expend the biological “cost” of mating in every generation. Asexual organisms, since they can have more offspring per unit time, are, in Darwinian terms, more “fit.” this sort of analysis implies that sexuality should disappear. But in animals sexuality is tenaciously maintained. We show here that this problem of the maintenance of sex (that which keeps animals and plants from becoming asexual) must be clearly distinguished from the problem of the origins of sex (the ways in which sex first evolved). There has been some confusion between these two aspects of sexual theory. The mix-up between remaining sexual and becoming sexual is one which we will try to steer well clear of throughout this book.

The origin of sex was not a one-time event. Sex is not a singular but a multifaceted and widespread phenomenon; it has developed several times, at the very least. The two most consequential appearances of sex were in tiny microbes—a half to about five micrometers long. Sex first appeared in bacteria. Later, in larger, more complex microbes called “protists,” a new and different kind of sex evolved. Sex in bacteria is a biological mixing and matching on the molecular level: the splicing and mending of DNA molecules. Bacterial sexuality is very different from the meiotic sex of protists, fungi, plants, and animals, and it evolved far earlier. Meiosis, or cell division resulting in reduction in the number of chromosomes, and subsequent fertilization, or reunion of cells to reestablish the original chromosomal number, first occurred in protists. Protists, microbes generally from ten to a hundred micrometers long, are ancestral to fungi, animals, and plants. As protists evolved and gave rise to these other groups of organisms, sex was preserved. From a cellular vantage point, human sex is almost identical to that of some of the protistan microbes.

Excerpted from Origins of Sex: Three Billion Years of Genetic Recombination by Lynn Margulis and Dorion Sagan. Copyright 1986.

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