Cosmic Apprentice: Dispatches from the Edges of Science (8 page)

Panspermia has the power to reinvigorate our notion of dust from a figure of neglect and unimportance to one of essential substance. Working with Sir Fred Hoyle (who coined the term
big bang
), the astronomer Chandra Wickramasinghe found decades ago that the infrared signatures of dust seen in all directions by peering astronomers matches the wavelengths of dried bacteria, suggesting that the cosmos might be full of bacterial dust. More recently, with his colleagues at the Cardiff Centre for Astrobiology, Wickramasinghe calculated that radioactive material such as aluminium-26, injected into our solar system during its formation from shock waves emanating from a nearby supernova explosion, heated the frozen centers of comets to produce subsurface oceans. Provided a liquid medium, bacterial dust may thus have come to life inside these hurling comets. Rocky on the outside, watery on the inside, they invert the composition of playground bullies’ rock-filled snowballs. These astral bodies may be legion among the hundreds of billions of comets flying about just in our own solar system. Photos such as those of comet Tempel 1 taken by the Deep Impact probe in 2005 show evidence of ice core melting, which might happen as comets approach stars or burn through atmospheres. So maybe bacterial dust can stow away, sometimes coming to life inside spherical lakes, where they live in the dark, inside whizzing comets, for perhaps millions of years. And if one of the comets hits—voilà! They have light and the opportunity to evolve photosynthesis. Within a starry firmament filled with particles, a cosmos of violence and transformation, everything, even germs careening through space, may someday get the opportunity to shine.

CHAPTER 5

A QUICK HISTORY OF SEX

IN HIS
Letters to His Son on the Art of Becoming a Man of the World and a Gentleman,
Lord Chesterfield, the eighteenth-century British statesman and man of letters, offered the following concise account of sex: “The expense is damnable, the pleasure momentary and the position ludicrous.”

Despite the droll nature of his quip, Chesterfield’s observation highlights some deep truths about our status as living, breeding beings on this planet. The damnable expense—which in Chesterfield’s case doubtless refers to the money and time spent in wooing, dating, and engaging in matrimony—theoretically applies to all sexually reproductive organisms.

Considering that some organisms can simply clone themselves—a well-fed amoeba grows and splits to produce two new amoebas—what is the point of making reproduction dependent on an intricate set of shenanigans with a member of the opposite sex but the same species?

When evolution can take one organism and create two, why make matters more difficult for itself by developing a method of reproduction that requires two organisms to make one? This added expense, in Chesterfield’s terms, has led to a bevy of evolutionary theories on why sex exists.

So, too, the pleasure Chesterfield dismisses as momentary, and the position he describes as ludicrous, may have their origins in evolutionary theory.

WHEN WE ASK THE QUESTION “why sex?” the answer comes thundering back: “to reproduce.” But that just begs another question: “Why reproduce?”

Sex and reproduction are not necessarily connected, even though they are strongly linked in us as well as in most plants and animals. In biological terms, sexual reproduction can be defined as the formation of new individuals from the genes of at least two different sources (i.e., parents). Simple reproduction, by contrast, is an increase in the number of individuals—they don’t necessarily have to have any new genes.

Bacteria have been exchanging genes, without strictly needing to do so to reproduce, long before the evolution of plants, animals, fungi, or even amoebas. In some cases, one “parent” in an act of bacterial sexuality is not even alive; it’s simply a raw gene—a DNA molecule in solution. This phenomenon was first demonstrated by the British medical officer Frederick Griffin in 1928 and called the “transforming principle.” Griffin found that even dead bacteria of one strain could pass on their genetic material to live bacteria of another strain, thus “transforming” their offspring into the strain of the dead bacteria. It was later discovered that the “transformation” was actually caused by the living bacteria absorbing the DNA of the dead bacteria and using it to replicate.

We now know that viral DNA, genetic elements called plasmids, and whole bacteria with an entire set of genes may also serve as “parents” in bacterial sex. But no additional individual need be produced: the result of bacterial sex is not duplication but the same bacterium with a new set of genes. Then, when it actually does reproduce, it has the abilities or traits conferred by the new genes.

If we had sex like this, it would be like going swimming with brown eyes, picking up a gene for green eyes in the pool, and then passing that trait on to your children. Although it evolved on Earth billions of years ago, this kind of nonreproductive sex has lately been tapped en masse by the biotech industry. But the original biotechnology innovation was pioneered by bacteria.

There is tantalizing evidence to suggest that this primitive form of sex evolved in the billions of years before the Earth was enveloped in a protective ozone layer. This thin layer of O3 is thought to have appeared only one to two billion years ago as a by-product of the metabolic process of green photosynthesizing cyanobacteria. The ozone layer around the Earth today shields life from the vast majority of damaging ultraviolet radiation.

However, bombard modern bacteria with levels of ultraviolet radiation similar to those before the ozone layer formed, and they disperse bits of naked and protein-coated DNA and RNA into the surrounding medium. This would have resulted in a veritable orgy of bacterial recombination in broad daylight.

One intriguing prospect is that strands of DNA damaged by ultraviolet radiation may have originally been fixed when primitive organisms shed loose genetic material and that this repair process was appropriated for recombination. But the sex that humans engage in to reproduce is decidedly different from that practiced by bacteria. In fact, the biggest distinction between life-forms on Earth is not between plants and animals but between prokaryotes (bacteria) and eukaryotes (all other organisms).

Bacteria have no nuclei and no true chromosomes, whereas eukaryotes do. Eukaryotes range from single-celled amoebas and
Paramecia
to trees, fish, and humans. Whereas bacteria can receive anything from a single gene floating loose in the surrounding medium to all of another individual bacterium’s genes, the reproductive sex of cells with nuclei involves reception of half the genes from each of two parents. In truth, the search for the origins of our kind of sex must be sought in a study of these primitive eukaryotes.

CYCLES OF HUNGRY DOUBLING and genetic exchange—the origin of reproductive sex—are thought to have arisen at least three times in the history of life: in the swimming single cells called choanomastigotes thought to be ancestral to animals; in the swimming water mold cells called chytrids ancestral to fungi; and in the swimming populations of green algae ancestral to plants.

Once sexual reproduction arose, it went through many fascinating permutations and variations. Originally in ancestral microbes, gametes, or sex cells, were of the same size. The divergence into a relatively larger, more stationary gamete (the egg) and smaller, more numerous, faster-swimming ones (the sperm) set the stage for the separation into the bodies that housed them, that is, male and female animal bodies.

The original requirement for eggs to be fertilized in an aquatic habitat gave way to the rise of the amniote egg, which provided a protective environment in which the young could develop without being directly in the water. Whereas fish spawn their sperm and eggs directly in the water, selection naturally rewarded males who could deliver their sperm to eggs before they left the female body.

Penises evolved as an efficient sperm delivery technique. Geoff Parker, an evolutionary biologist at the University of Liverpool, put the idea of sperm competition, a form of natural selection at the level of the gamete, forward in 1970. Other things being equal, the competition among sperm of two or more males for the fertilization of an ovum rewards males with more sperm, larger penises, or sperm that displaces previously deposited sperm or has qualities such as extreme stickiness which forms a barrier, or plug, preventing subsequent suitors from delivering their genetic goods.

Banana slugs and Dungeness crabs, as well as many species of insects and rodents, are known to have sperm plugs. The damselfly uses his complex penis to scrub females free of rival sperm, that his own may enter her reproductive tract.

Nor are humans impervious to this form of rivalry at the level of sex organs and secretions: Experiments show that men who suspect their partners of cheating produce far more sperm per ejaculation than males persuaded of their partner’s fidelity. Thus it is that jealousy, physiologically at least, is an aphrodisiac.

SCIENTISTS DISTINGUISH between the
origin
and the
maintenance
of sexuality, which is to say, why it exists despite, as Chesterfield put it, the “damnable expense.” Ideas to explain the origins of sex—such as bacteria taking fragments of each other’s genes on an ultraviolet-saturated primitive Earth—are not that easy to test experimentally. Scientists have thus tended to focus more on
why
sex exists.

Scientists believe that, although sexual reproduction is more complicated than simple cell division or cloning, it confers genetic benefits on the many species of plants and animals that engage in it. By requiring the fusion of genes in each new individual, sex provides an opportunity to pool advantageous traits and mutations. Additionally, when deleterious mutations come together in unfit individuals, those individuals die, thus ridding the population of negative traits.

There is some observational and experimental evidence to support these theses. In 1973 Leigh van Valen, an evolutionary biologist at the University of Chicago, put forward the “Red Queen” hypothesis, the forerunner of one of the most widely accepted theories of why sex exists. Van Valen’s theory discusses how evolutionary change begets further change by altering the environment at large. The hypothesis is named after the queen in Lewis Carroll’s
Through the Looking-Glass,
who says, “It takes all the running you can do, to keep in the same place.”

Another evolutionary biologist, Graham Bell of McGill University in Montreal, explored the same principle. By shuffling their genes with each offspring, sexually reproducing organisms are better at eluding parasites. Evidence for this theory comes from experiments with minnows—small freshwater fish—in Mexico. Researchers separated asexually cloning and sexually reproducing minnows into different ponds. They then counted black spots caused by a parasite and found that the asexual minnows were far more prone to disease than their genetically more varied, sexed relatives. By shuffling their genes each and every time they reproduce, sexual reproducers appear to be better able to elude the evolving bevy of potential inner assailants.

Hybrid vigor—the noted superior hardiness of organisms with genetically distinct parents—is also associated with beauty, at least in humans. The chances of facial and body symmetry that we find beautiful increase when separate genetic stocks are brought together. This phenomenon can also be seen from the inverse; inbred fruit flies have less symmetry. Cheetahs, having little genetic diversity among remaining populations, tend to have asymmetrical facial bones.

Clear, disease-free skin is a key trait in human estimations of beauty. Thus, when lovers are attracted to each other, they may be unconsciously estimating each other’s freedom from, or ability to ward off, potentially damaging parasites, which have a harder job attacking the moving target of obligatory genetic recombinants.

Yet, while much ink has been spilled over what “maintains” sex (why it continues to exist in sexually reproducing organisms), it’s not as though we humans, and other plants, fungi, and animals, can just opt out of sexual reproduction. In multicellular beings, sexual reproduction is the only way a new being can form.

Sexual fusion of eggs and sperm is required to start the embryo, and, except for certain exceptions such as the green algae and the artificially selected triploid banana, all plants produce embryos, as do all animals. On paper, it is easy enough to say we would prefer to eliminate the “damnable expense” and just be cloned. But, as technology stands, even the wealthiest misandrist (or hater of men) cannot access the technology necessary to bypass males if she wishes to reproduce.

A similar situation appears to be the case in the animals officially classified as “asexual.” Careful study reveals that these organisms—such as the all-female populations of whiptail lizards that inhabit the southwestern United States—still undergo chromosome sex processes at the cellular level. In the case of the female whiptails, they also mount each other and produce more eggs when another female has mounted them. Thus asexual animals appear to be evolutionary rarities that have devolved from a sexually reproducing state, rather than truly “asexual” beings.

To understand the depths of sexuality, we don’t just need to study animals and microbes, for the roots of sexual reproduction lie in events that are many hundreds of millions of years old. The Red Queen, and other theories explaining why sex is genetically worth the trouble, assumes that most species of plants and animals could simply lose the trait of sexual reproduction if it were adaptive. But this may simply not be the case.

We may wish we could fly because it would be more “adaptive,” but the physiological option is simply not open to us. Sexual reproduction may be so old that most species are unable to lose it. And those that do may become vulnerable to rapidly spreading parasites, and therefore removed from the gene pool. Simon Robson, a biologist at James Cook University in Queensland, says of zoologists who theorize on the maintenance of sex without regard for its nonanimal history: “They’re working from a data set two billion years out of date.”

In fact, in the 1940s the Harvard University biologist Lemuel Roscoe Cleveland solved the mystery of the origin of fertilization and how cells with one set of chromosomes (haploid cells) came to merge with each other and develop two sets of chromosomes (becoming diploid). Cleveland noticed that in some cases, hungry amoebalike cells would incompletely devour each other, merging cell membranes and cytoplasm and pooling their chromosomes in a single nucleus. He reasoned that the first fertilization was the result not of a sexual but a starvational urge to merge.

This theory is borne out by evidence from other quarters. Some cells today reproduce asexually . . . at least until they are faced with starvation, such as from lack of nitrogen in their environment. In this case, they fuse together to make a diploid. Primordial sexual fertilization, which Cleveland witnessed among microbes called hypermastigotes, may have had a most unromantic genesis. Cleveland also carefully chronicled and photographed the necessary separation from the state of doubleness, which he called “relief from diploidy.” We undergo such relief when we produce haploid sperm cells or ova from diploid body cells; aside from these sex cells, the rest of the cells in our body remain in a diploid state, with forty-six chromosomes, twenty-three each from mum and dad.

THE KINKIEST HUMANS are utterly unimaginative compared with the sexual variety on display in the animal world. African bedbug males, for example, routinely pierce females through any part of their carapace in order to impregnate them.

Natural selection is amoral. Which dysfunctional human family is comparable to the behavior of
Adactylidium,
the nasal mites whose mothers give rise to 99 percent females? Inside the mite’s womb, one unborn male inseminates all his unborn sisters, then dies. The incestuously impregnated females grow, eating their mother’s body from the inside out to provide themselves with nourishment. They are born pregnant, beginning the cycle again.