v1.2.2 / chapter 10 of 28 / 01 aug 08 / greg goebel / public domain
* Life began on Earth as single-celled organisms, and such life-forms dominated the planet for much of its history. However, even before the rise of multicellular organisms, there was considerable diversification among single-celled organisms, including the highly significant development of sexual reproduction.
* The first organisms to arise on the planet were the bacteria, the simplest known organisms capable of self-replication. Viruses are simpler, but they can't reproduce on their own -- they are strictly parasites that propagate by infecting cells of proper organisms, and that means there's really no way viruses as such could have come first.
A bacterium seems deceptively simple, no more than a bag of biomolecules and some simple organelles. They reproduce "asexually", with one bacterium dividing into two new bacteria, though it's possible for bacteria to "horizontally" transfer little loops of genes called "plasmids" between each other, a process known as "conjugation". It should be noted that this process has nothing directly to do with reproduction, so it's somewhat fuzzy to think of conjugation as a form of sex.
However, there is considerable diversity among bacteria. For example, some bacteria are "anaerobic", meaning they don't require oxygen to live, in fact find it toxic, unlike oxygen-using "aerobic" bacteria. It is clear that anaerobic bacteria came first, for the simple reason that the Earth's atmosphere didn't originally contain free oxygen. The anaerobic bacteria seem to have mostly obtained their energy from hot springs, volcanic vents, and the like, which no doubt somewhat limited their distribution over the Earth. (The Sun was actually substantially dimmer in those days and wasn't as good a source of energy as it is now, but on the other hand the Earth was much more geologically active then.)
The Earth's oxygen atmosphere didn't start to appear until about two billion years ago, and wouldn't resemble the modern atmosphere for another half-billion years. The original agents of this change were the "cyanobacteria", sometimes called "blue-green algae" -- though most taxonomists believe that's stretching the definition of "algae". In any case, they were the first organisms to perform "photosynthesis", or the conversion of sunlight into energy, with the accompanying conversion of carbon dioxide into biomass and oxygen. The oxygen atmosphere made aerobic bacteria possible.
Whatever the details, life, if an unspectacular form of it, became commonplace on the planet long ago. Fossil evidence for bacteria goes back over 3 billion years, particularly in the form of "stromatolites", which are basically just big layered mats of different kinds of bacteria. Stromatolites still persist in various odd parts of the Earth, for example some bays in western Australia, but they are rare, only surviving in locales where there isn't much else around to gobble up such a convenient food source. In the Precambrian they were not rare. They left behind highly distinctive layered fossil formations, some in the form of cones tens of meters high.
* The evolution of the bacteria was paralleled by the evolution of the "archaea", which were discovered in the 1970s by the groundbreaking American microbiologist Carl Woese (born 1928). They were originally thought to be simply strange sorts of bacteria and were called "archeabacteria". They have since become generally recognized through the identification of their unique molecular biology as being highly distinct from bacteria, with their own distinct tree of evolutionary development. In fact, the bacteria and archaea are regarded as two of three domains of life, and some even split them down into multiple separate families that are at an organizational level making them as distinct as plants and animals -- though that's not done here, once again if just for the sake of simplicity.
The archaea were long thought of as of marginal interest, freakish "extremophiles" associated with extreme environments -- extreme heat, extreme salinity, extreme acidity, and so on -- but further investigation shows they are more common than was once thought, just having been overlooked because biologists weren't on the lookout for them.
It's the third domain of life, the "eukaryotes", where the story starts to become of more general interest. The bacteria and archaea are both "prokaryotes" -- bags of biomolecules with fairly undeveloped internal structures. The eukaryotes, in contrast, are about ten times bigger in diameter than prokaryotes and have a central nucleus that contains their genetic material. They also have a much more sophisticated set of cellular "organelles". Some of these organelles are very interesting. The "mitochondrion" of most eukaryotic cells is used in energy processing, and has its own little distinct genetic code, separate from the main cellular code in the eukaryotic nucleus. Plants also add an organelle called a "plastid" or "chloroplast", which performs photosynthesis, and also has its own little genome. The genomes of these organelles are no longer sufficient to direct all the organelle's functions, with the genome of the organelles interacting with the genome of the host cell to get the job done.
The general belief among biologists is that the eukaryotic cell arose as a "mutualistic" or "symbiotic" relationship between various prokaryotes. The basic idea was originally proposed by a Russian biologist named Konstantin Merezhovsky (1855:1921) and fleshed out by an American biologist named Ivan Wallin (1883:1969) in the 1920s. It was generally regarded as a wild idea, and it certainly wasn't well supported by the evidence available. The modern understanding of DNA and molecular genetics opened the door to a more detailed examination of the matter, and in 1963 an American biologist named Lynn Margulis (born 1938) made a case for the idea that eventually proved persuasive.
In fact, although some have claimed the symbiotic origin of the eukaryotic cell was a wild improbability, the evidence suggests that it was almost inevitable. As Margulis pointed out such symbiotic associations clearly took place more than once, since plant cells have both mitochondria and chloroplasts -- and there may have been other events as well. Symbiosis often starts from parasitism, the parasite then finding it advantageous to help a host instead of harm it, and the mitochondrion and chloroplast appear to have bacterial ancestry. The evidence is strong that chloroplasts began life as cyanobacteria. There is actually a modern single-celled eukaryotic organism, Cyanophora paradoxa, that was once thought to contain chloroplasts, but the "chloroplasts" turned out to be cyanobacteria. The arrangement strongly suggests the "reinvention" of photosynthetic eukaryotes all over again, and certainly provides a persuasive clue into their origins.
While the mitochondria and the chloroplasts seem to have been derived from bacteria, biochemical clues hint that the eukaryotic host cell was derived from the archaea, suggesting that both families of the prokaryotes contributed to the creation of the eukaryotes. In any case, the eukaryotes would lead to multicellular organisms and the plants and animals we see around us.
The symbiotic origins of the eukaryotic cell -- not to mention horizontal gene transfer in prokaryotes, which can take place between very different species -- tends to complicate the branching tree of life, in that the tree is like a mangrove, based on a complex of twisted roots at the microorganism level. Some critics of modern evolutionary theory have claimed that this complex of roots completely undermines the notion of Darwinian common descent, but though it does add a fair amount of fine print, common descent remains a perfectly valid concept above the level of microorganisms. The fine print only amounts to an expansion of the Darwinian vision, not a refutation of it.
Margulis, along with her son (with astronomer popstar Carl Sagan) Dorion Sagan (born 1959), has pushed the idea that symbiosis is actually the primary source of variation for evolution, not mutation, but this idea has had few takers. Although symbiosis is clearly an important evolutionary process and Margulis deserves great credit for highlighting it, as Ernst Mayr pointed out there is little evidence in the species diversity of modern macro-organisms to suggest that this diversity arose directly from symbiotic processes.
* The rest of this survey concerns eukaryotes, but that shouldn't be interpreted as dismissing prokaryotes as unimportant in any sense. There is a vast range of diversity among the prokaryotes and we're still learning much about them. Prokaryotes have been around for a long time and calling them "primitive" or "simple" can be misleading. All organisms are highly elaborate, and in many cases the prokaryotes are capable of performing some very intriguing tricks. Much of the Earth's biomass is prokaryotic, and in fact there are ten times more prokaryotic cells in our bodies -- in the form of intestinal flora -- than eukaryotic cells. Even if the prokaryotic cells are about a thousand times less massive, that's still a revelation.
However, there's no way to investigate prokaryotes in great detail in a short survey. This tour of the "city of life" is not at all balanced and cannot pretend to be; there's no way a survey of the subject could be and still remain short. It does try to cover the entire "city", but it necessarily focuses in much more detail on what amounts to the popular tourist attractions.
* Eukaryotic cells came on the scene about 2 billion to 1.5 billion years ago. Single-celled eukaryotes form the third of the six kingdoms of life and are referred to as "protists" -- the other three eukaryotic kingdoms being the fungi, plants, and animals. The earliest of the modern protists date back to about 1.2 billion years ago, and are now seen as bewilderingly diverse.
The protists include dozens of distinct groupings, some of which are so different at the molecular biology level, as different as plants are from animals, that the term "protist" is nothing more than a convenient umbrella label. Readers wishing to learn about taxonomy in more detail should realize that modern taxonomists break the protists into as many as 20 separate kingdoms. They are treated as a single kingdom here, but yet again, only in the interests of simplification. The simplification is all the more gross because the fungi, plants, and animals are all descended from protists, meaning the protists can't be considered a clade.
Some protists that live in anaerobic environments, such as protists included in our intestinal fauna, don't have mitochondria, performing metabolism by methods unique to themselves. Some can form spores, a trick also practiced by some bacteria such as anthrax, becoming inactive but armored against environmental insults, allowing them to survive for long -- sometimes very long -- periods of time until conditions allow them to go active again.
A number of protist groupings are plantlike, such as the "diatoms" or "golden algae". They contain "plastids", cellular organelles that can perform photosynthesis. The plastids come in different colors -- brown for brown algae, red for red algae, and green for green algae, with the green algae seen as the likely roots of modern plants. There are funguslike protists as well, which like fungi digest food from their exterior environment and cannot photosynthesize.
The animal-like protists are known collectively as "protozoans", though once again this is just a convenient label, since the protozoans only share the fact that they ingest their food and don't photosynthesize. They include the "flagellates", "ciliates", and "amoeboids":
It should be noted anyone who claims that the fossil record is sparse hasn't considered the radiolaria and the forams: they form layers under the seabed, and some tropical beaches are made up of almost nothing but their shiny white remains. Their evolutionary history is laid out almost like the pages of a book, the only problem being that such organisms are hard to study in the lab and we remain somewhat ignorant of the lifestyles of living species. The radiolaria and the forams are one of the "markers" used to identify distinct geological layers, and indeed forams are used by oil companies to identify strata that could be associated with oil deposits.
It should also be noted that fossil evidence for protists only goes back about 800 million years, and that not all the modern groups, for example foraminifera, are necessarily that old. Just like more elaborate organisms, they have evolved in various directions, with new forms arising and old forms dying out. Popular science literature sometimes refers to modern organisms that can be presumed to be similar to ancient ones as "living fossils", but that's a bit of a loaded term. All organisms living today are descendants of ancient ancestors, and if some are very much different from their ancestors, that doesn't imply that others that haven't changed as much are simply carbon copies of their forebears. No examples of living species are parents of another species: they are all cousins, though some certainly may look far more like the grandparents than others.
* While the bulk of protists reproduce asexually by division, some do reproduce sexually, with two parents providing half their genome to create a new generation with a hybrid genome. It is generally believed that the protists "invented" sexual reproduction, and it's worthwhile to take something of a digression and inspect the subject of sex because of its importance in evolutionary science.
There has been a long debate among evolutionary biologists over the relative merits of asexual and sexual reproduction, but few dispute that by the evidence, the rise of sexual reproduction was a major event in the evolution of life. In fact, it is a serious oversimplification to say that mutational change drives Darwinian evolutionary processes, since the "recombination" of genomes through sex is a major factor in promoting genetic variability, in essence rearranging the "teams" of genes with every generation, with the effect of accelerating evolutionary change. If a particular mutation gives an advantage to one member of a species and another mutation gives an advantage to a second member of that species, then progeny of those two organisms can in the best case get a doubled advantage. In a sense, the evolution of sex was an example of the "evolution of evolution".
However, sexual reproduction is a bit puzzling. Evolution is not an end in itself; as noted in the discussion of punctuated equilibrium, a well-adapted organism may well continue to propagate for a long time with only relatively minor or obscure modifications that do not change its basic form and habits, until new selection pressures begin to affect its fat and happy existence. Evolution occurs in response to selection pressures -- it's much more driven than a driver. In addition, since sexual reproduction involves a fair amount of overhead, the selective advantage provided by sex had to be significant or it wouldn't have caught on. Asexual production permits rapid and efficient replication, and if the survival of species was driven strictly by the ability to reproduce, asexual reproduction would win hands-down. From the point of view of the "selfish gene", asexual reproduction ensures the maximum propagation of the "winning team" of genes.
It was William Hamilton who came up with the most popular, if not universally accepted, explanation of the evolutionary drive behind sexual reproduction, calling his theory the "parasitic Red Queen's race" -- after the animate chesspiece in Lewis Carroll's THROUGH THE LOOKING-GLASS who had to run as fast as she could just to stay in the same place.
In Hamilton's view, organisms featuring sexual reproduction tend to be relatively long-lived, with much longer generation times than the asexual pathogens that infect them. The fast-breeding pathogens are likely to mutate and stumble onto some means of breaking through the defenses of their hosts in time, in a sense "breaking the combination on the lock" of those defenses. Sexual reproduction ensured that the hosts could scramble the "combination" and stay just one step ahead of the pathogens; importantly, the genes in control of the mammalian immune system are highly polymorphic, ensuring that there is a wide range of "combinations". Even when a pathogen manages to crack the "combination", it will not be able to take down the entire population of a genetically-diverse species.
In other words, the fitness peak underneath large organisms is continually being pulled down by pathogens, the "winning team" will not go on winning indefinitely, and the large organisms have to keep up. From the point of view of the "selfish gene", asexual reproduction would be better. The problem is that pathogens will move in on that racket sooner rather than later, and so organisms that survive rearrange their teams of genes on a regular basis instead. The science-fiction notion of a "satan bug" that could take out every single human on the planet is, or at least we have good and hopeful reasons to believe it is, a fantasy. Even during the bubonic plague pandemics of the Middle Ages, the fatality rate in affected areas was "only" one in four.
A interesting example in this case is that of the modern cultivated banana. Wild bananas have fruit with large seeds, while cultivated bananas are mutants with vestigial seeds. Cultivated bananas can only be crossbred in the lab, and it requires considerable effort. Commercial production of bananas involves the replanting of cuttings -- or in effect the cultivated banana reproduces asexually, with humans as the agent of reproduction. There are a number of different varieties of cultivated bananas, such as the starchy plantain, but outside of the tropics the selection of bananas available to consumers is narrow. Up to the 1950s, the primary commercial banana was the "Gros Michel" AKA "Big Mike", which was all but wiped out worldwide by a fungal infection. The current commercial banana is the "Cavendish", and it is now being threatened by a mutant of the fungus that wiped out the Gros Michel. The asexual cultivated banana is at an inherent disadvantage in the Red Queen's race with its fungal parasite.
* Of course, asexual production does persist. Dandelions generally reproduce asexually -- the flower is vestigial -- and get away with it by having short, rapid life-cycles, breeding wildly before pathogens infect them and kill them off. Some organisms will have phased sexuality, reproducing asexually for a few generations and then, under some circumstances, reproducing sexually. Other organisms that are normally sexual reproducers may have a species among their ranks that reproduces asexually -- one interesting example being an asexual species of crayfish that has been adopted by commercial fish farmers -- though such species tend to be a minority since they are vulnerable to being culled out by pathogens. As would be expected under Darwinian evolution, asexual reproduction keeps getting "reinvented" again and again, but never proves successful enough to take back over. Asexual reproducers are doomed by selection pressures to forever remain a minority, at least among large organisms with long life-cycles.
Trying to figure out the variations on the theme of asexual versus sexual reproduction has been a challenge to biologists, and in fact sexual reproduction itself involves a number of intriguing puzzles. For example, how could "trial & error" evolutionary processes have created a male organism and a female organism simultaneously that worked together? That would seem impossible, and in fact it is: obviously, sexual reproduction emerged in phases. Bacteria, as mentioned, have a limited ability to transfer genetic information, and it seems plausible that a scheme along this line could have led ultimately to sexual reproduction.
At the outset such organisms would likely have been hermaphroditic -- that is, each both male and female. That's nothing unusual among relatively elaborate modern organisms, snails being a well-known example. That still leaves the question of how two distinct sexes emerged, but one could imagine hermaphrodites that cycled between genders, ultimately ending up with the gender fixed at birth.
That leads to another puzzle: why have two distinct sexes? Why aren't hermaphrodites the norm? It would seem that a species would reproduce more effectively if all its members were capable of bearing young. Hermaphrodites can also inseminate themselves, though "selfing" tends to lead to unhealthy in-breeding and hermaphroditic species tend to have mechanisms to prevent it when they can find partners. However, from an evolutionary point of view selfing is much healthier than dying out.
The question of males and females turns out to be particularly devious. Traditionally, the explanation was that it made sense to have males who generate their sperm rapidly and scatter their genomes readily, as well as females who actually have to deal with the overhead of bearing young. The first problem with this notion is that, while it is "cheaper" to generate one sperm than one egg, it usually takes far more than one sperm to inseminate one egg. In fact, since the female reproductive system can be hostile to sperm, it may take millions of sperm to inseminate one egg.
This hostility is puzzling, since it is in the evolutionary interests of species to encourage their own reproduction. One possible answer is that the female reproductive system is necessarily very "nurturing", which means that it's also a nurturing environment for pathogens. That results in an evolutionary race to raise the barrier of entry to pathogens without denying access to sperm, and the barrier gets raised to whatever level that the males can still overcome on a predictable basis. In any case, it isn't necessarily easier, at least from a broad point of view, to make sperm than it is to make eggs.
In addition, in many species the males do most of the work of child-rearing, one of the extreme cases being the seahorses and their elongated relatives, the pipefish, in which the female lays her eggs in a pouch provided by the male -- the female has a "penis" or ovipositor to do the job -- and the babies come to term in the male.
The classic argument for the existence of males and females is not entirely wrong. Imagine a horror-movie scenario in with a pathogen was introduced that decimated the human population, and suppose it was selective with regards to sex. If killed off most of the women, from the point of view of rebuilding the population, it might as well have killed off most of the men, too, since the surviving women would effectively breed no faster than they would have if as many of the men had been killed off. However, if it killed off most of the men, the population would be rebuilt within a generation. Could one healthy man keep ten fertile women pregnant? Sure. A hundred women? Possibly, though it might be somewhat exhausting. The surviving males would win the genetic jackpot.
However, the case would be exactly the opposite for seahorses. So then, why males and females? There may be actually be a simpler and more Darwinian explanation, and it relates to the fact that bisexual reproduction hasn't displaced hermaphrodism.
The traditional concept is that hermaphrodism occurs in immobile species -- plants or sessile animals -- or species that aren't highly mobile, such as snails. In such circumstances, coming into sexual contact with another member of the same species is relatively unlikely, and having two different sexes would halve the odds of doing so. Being hermaphroditic guarantees that any adult member of the same species is a candidate sex partner. Flowering plants are generally hermaphroditic, since they are sexually dependent on symbiotic pollinators like bees, hummingbirds, and the like. It might not be impossible for a pollinator to acquire different behaviors to handle two distinctly different plant genders, but it would certainly make things troublesome.
This scenario is true, but it's also not the complete truth, because there are plenty of hermaphroditic species that can get around perfectly well. Possibly the answer to the puzzle of bisexual reproduction versus hermaphrodism is that they are just two systems that happen to work. As noted, bisexual reproduction likely originated from hermaphrodites, and in fact there are existing species where the two "genders" are hermaphrodites and males. The hermaphrodites can reproduce among themselves or can reproduce with males.
The males, from this point of view, are "broken" hermaphrodites; further "breakage" could produce females. If the hermaphrodites were at a small selective disadvantage from having to maintain two reproductive systems, they would gradually be eliminated from the gene pool. It's much easier for bisexual reproduction to evolve from hermaphrodism through the loss of the reproductive system by halves, than for bisexual reproducers to evolve into hermaphrodites by developing a new reproductive system. Bisexual reproduction is likely to emerge from hermaphrodism on occasion just by the odds.
It is not so hard to figure why there are usually roughly equal numbers of males and females in a given bisexual species. It's a question of market saturation: if more human males were born than females, given that humans are basically monogamous, the excess males would simply "go to waste" and from an evolutionary point of view they would not be a good investment of effort. Humans are of course only one species, but Ronald Fisher performed a general analysis that showed that the optimum ratio of males and females for any bisexual species was, under stable circumstances, 50:50.
It might seem that harem animals, such as antelopes or seals, in which only a fairly small portion of the males ever get to reproduce, imply a violation of the 50:50 rule. In fact, mother seals bear as many males as females. Even though most of those males won't reproduce, the parents still have an incentive to commit resources to the game in the hope of winning. A harem system is simply a modification of the sex contest in which fewer prizes are handed out, but the prizes are bigger. It is interesting, however, that in harem-oriented species the males tend to be bigger as adults than the females, with the difference in size growing with the average size of the harem: bull elephant seals have unusually big harems and are monsters compared to the cows. The emphasis on size is not surprising, considering that under a harem scheme it is literally only the biggest bullies that get to sire offspring.
A fourth, somewhat absurd, question about sex is: why not have three or more sexes? The notion gets played up in sci-fi stories on occasion, usually as a gag -- on a planet with a dozen sexes, getting laid on a Saturday night would be hideously troublesome. In reality there are organisms that have been described as having more than two sexes, in fact sometimes hundreds of them. The details are too complicated to discuss here, and even consideration of this fine print still reveals that no organism has more than two genetic parents, no matter how confusing the sexual arrangements of the parents are in practice.
The reason is that sexual fusion of two germ cells into a composite zygote cell is complicated. This is why there's the female egg / male sperm asymmetry in sexual reproduction: it's simpler to combine the complicated egg with minimalist sperm and get a working hybrid than it would be to hybridize two germ cells as complicated as eggs, with all their cellular organelles. Trying to hybridize three germ cells of any configuration and get something that works would be nightmarishly complicated. An evolutionary path to such a complicated scheme would be much more elaborate than the path that led to two sexes, and it's hard to identify any advantage of three sexes over two that could provide selection pressures to drive evolution along such a path. Three or more sexes really are troublesome.
* Incidentally, the 50:50 gender rule has some fine print -- it actually means that the optimum level of relative investment in males versus females is 50:50. If males require as much investment as females, then the number of males matches the number of females; but in a species where males are twice as easy for a mother to produce as females, then the optimum ratio of males to females is predictably 2:1: "Two for the price of one!".
There are also some creatures that produce a few males and a large number of females -- but the brothers and sisters breed among themselves and such creatures can be seen as "virtual" asexual reproducers, clearly being descendants of sexual reproducers that "rediscovered" some of the benefits of asexual reproduction. In fact, in some species of mites, the offspring hatch inside the mother mite's body, with the male inseminating his sisters before they eat their way out of their unfortunate mother, the male then promptly dying. The female mites are "born pregnant", and on a casual inspection such a species might actually appear to be an asexual reproducer -- though in reality it has taken an astonishingly roundabout evolutionary route to get back to an ugly approximation of where its distant ancestors started from.
Another strange tale of sexual reproduction is that of the fish known as the blue-headed wrasse. Only the males are blue-headed, the females are dull-colored. They are a harem species, with one male controlling a school of females. However, as mentioned previously, some fish can change sex during their lives, and the blue-headed wrasse is one of them. If the male in control of the harem dies, the biggest female becomes a male and takes over. It's the best of both worlds: there are still males to spread around sperm, but if they can't run a harem, they get to reproduce as females.
Such tales hint at how complicated the matter of sex really is, and it must be emphasized that the discussion here is brief and glib -- not everyone agrees with the "parasitic Red Queen's race" idea, for example. There are special cases where it just doesn't seem to work, the most prominent case being the protists known as "bdelloid rotifers", all of which have been asexual reproducers for at least 85 million years, and which seem to be thriving. Sex turns out to be one of the most devious topics in evolutionary science, and it's likely to provide job security for biologists for a long time to come.
