by Henry Gee
In many situations, bacteria of different kinds work together in sheets or mats called “biofilms.”10 The first large organisms—reefs and mounds of mineralized bacterial biofilms called stromatolites—are built of colonies of different bacteria working together.11 Before the evolution of animals that could graze on them, stromatolites were common (they still live in isolated places where the water is too rich in salt or other minerals for other creatures to tolerate), and bacterial biofilms coated the ocean floor.12 Biofilms are still with us, thriving in, among other places, the lungs of people with cystic fibrosis, where they contribute to the deadly pathology of that disease.13
Beyond biofilms, though, there is much evidence that complex cells, such as those that make up our own bodies, were originally formed from associations of several different kinds of bacteria that became so commingled that they could no longer function independently. The mitochondria—small sausage-shaped bodies in all cells—are relatively closely related to a group of bacteria called proteobacteria.14 They even retain a vestige of their own DNA. The chloroplasts—the green bodies that give plant cells their green color (which also have their own DNA)—are distant relatives of the free-living, light-harvesting blue-green bacteria that contribute to stromatolites.15 The DNA complements of mitochondria and chloroplasts, though, are mere scraps compared with those of their free-living relatives, as most mitochondrial and chloroplast functions have devolved to the nucleus, in which almost all the DNA of cells is archived.16 Mitochondria and chloroplasts cannot function as free-living entities. By the same token, the nucleus—possibly the vestige of another kind of bacterium—depends on bodies such as mitochondria for its energy needs. This kind of union, known as “endosymbiosis,” is now known to have happened many times in evolution. There are some algae whose cells bear witness to not just one but two separate, independent symbiotic events,17 as if these cells were Russian dolls.
Complexity exists, and complex cells evolved from simpler ones. My thesis that evolution shows no definite trend in the direction of improvement would appear to have run into a sticky patch. Au contraire, say I.
I shall explain.
If evolution by natural selection can be said to have any “point” at all, it is that a creature should do all it can to improve the chances of its own offspring living long enough to reproduce. Why, then, would a simple cell, working perfectly well on its own, subsume its life and many of its functions in a larger collective, in whose stake it would have at best a slice of the action, rather than the whole cake?
The reason, I think, is all about energy, economics, and risk. Reproduction entirely on one’s own terms is an expensive and exhausting business, and the expenditure might not always pay off. Economies of scale apply as much to living organisms as to human industry. The net benefits of working together might outweigh those of continuing as an individual, and these benefits, such as gains in overall efficiency, might include surplus resources that allow greater specialization among the members of a collective, which in turn improves energetic efficiency still further.
These rewards might also include the ability, perhaps, to colonize new ecological niches that might be inaccessible to one’s competitors, and to do so speedily and efficiently; and, crucially, therefore, the capacity to perpetuate one’s genetic heritage far more effectively than one might manage if working alone.
To take just one example: the first plants to colonize the land more than 400 million years ago were small, encrusting things. But the competition for soil nutrients and light was so intense that plants soon formed associations with soil fungi called mycorrhizae to help them get the best out of the earth. The mycorrhizae, living around the roots of a plant, would extend that plant’s network into the soil, helping it extract water and vital nutrients. In return, the plants would feed the mycorrhizae the sugars created during photosynthesis. Plants with mycorrhizae would grow better than plants without, colonizing more and different habitats and increasing opportunities for their offspring to grow—and for their attendant mycorrhizae to prosper. In turn, the mycorrhizae would enable the plants to grow in soils in which they might otherwise wither. Today, land plants and mycorrhizae are totally dependent on one another.18 Meanwhile, the plants themselves soon evolved specialized cells that created hard tissues capable of supporting stalks and trunks that could grow upward quickly. Within a geological eyeblink, forests of tall trees sprang up, each tree trying to outdo the other for a share of the sunshine. And so below, with the mycorrhizae around the trees’ roots forming a wood-wide web of underground nutrient transport.19 Plants and mycorrhizae have achieved far more by working together than either could have managed alone.
The associations between bacteria to form biofilms and then cells; the association of cells to form organisms, in which the cells can then specialize; the further association of different organisms into systems of mutual benefit, as with plants and mycorrhizae, and even into entire interdependent ecosystems as Darwin described so eloquently with his picture of the tangled bank—on the surface, these can all be seen as step changes in complexity. Complexity just seems to ratchet up and up, so it’s no wonder that people tend to see evolution as a ladder that can be climbed, or as a chain in which there might be “missing links.”
Except, of course, that it’s not as simple as that.
As I alluded to above, organisms benefit from forming associations with other organisms, but that benefit comes at a cost. When organisms associate, it is because the benefits of living in a group outweigh those of living alone. The cost, though, is the sacrifice of immediate control of one’s fate. The only thing that matters in the calculus of evolution is that the benefits outweigh the costs, however marginally—which means that if the benefits are large, then the costs will be only fractionally less.
Consider, if you will, a mitochondrion in a cell. Eons ago it was a free-living bacterium with its own DNA and could reproduce entirely on its own terms. Now, though, almost all its DNA has migrated to the nucleus, which regulates almost every aspect of its life. The mitochondrion cannot function on its own, and has been reduced to an energy factory, producing power not just for itself but for the whole cell. To be sure, the mitochondrion benefits from the fact that much of the work of its own maintenance has been contracted out to other parts of the cell, but the cost for this convenience is its former autonomy—and with that, its complexity.
The increase in complexity of the whole, therefore, is paid for by the complexity of the individual parts, and, in terms of the numbers of individual organisms subsumed into the greater whole, the total amount of complexity might be said to have decreased. If the net benefits to all organisms of living in an association have increased to allow specialization of its members, it follows that complexity is traded for efficiency. After all, a simple cell in which the mitochondria, chloroplasts, and nucleus are still, more or less, separate organisms will be both more complex and less efficient than a single organism with a pooled resource of DNA and a division of labor among much simpler components.
My contention therefore is that the seeming rise in complexity hides a deeper truth—evolution is not just about gain, but about loss. Once one gets away from the idea that evolution does not, in its own nature, demand an increase in complexity, one can see that any apparent increase in overall complexity is driven by a loss of complexity among the individual components that make up the whole.
This makes sense once you think of natural selection not as a driver of improvement as a matter of destiny, but the sum of all those circumstances that keep a creature alive only according to its present needs. Natural selection will ensure that organisms will do just enough—and no more—to exploit an advantage, however minuscule, for their progeny. If this means that they will lose a great deal of complexity in return for the marginally improved likelihood of passing on their genes that symbiosis or association might offer, then they will make that trade.
The fact is that bodies are expensive to build and maintain, and
any creature that can get someone or something else to do the work instead will have the edge on a creature that insists on doing everything itself. There is a selective advantage, therefore, in being as simple as possible.
When people think of examples of the perfection of evolution (or, as it may be, the designs of the Creator), they tend to think of the evolution of beautiful structures such as the human eye or the tail of the peacock. Appearing very much further down the bill are parasites, creatures whose existence derives from the exploitation of other creatures, sometimes with grotesque, painful, and even lethal consequences.
Parasitism as a habit is hardly unusual. If you have ever dissected or gutted a wild animal—not a creature carefully bred for sport, science, or the table—you would no doubt have been amazed by the sheer load of parasites carried by an otherwise quite normal wild animal.20 I remember as a schoolboy slitting open a freshly caught dogfish, and finding that its insides mostly consisted of worms. The animal was so full of worms that the poor fish resembled nothing so much as a sports holdall filled with wet spaghetti.
As a consequence of living off the efforts of others, parasites often become much less complex in form than their free-living relatives. Examples abound: one of my favorites is a creature called Sacculina that parasitizes crabs.21 The mature adult is no more than a featureless blob, living on the crab and sending rootlets throughout the hapless host to extract its juices while it still lives. If you had to guess at the affinities of Sacculina, you’d probably say it was a fungus, but the truth is far more surprising. The true nature of Sacculina is betrayed only by its free-living larval stage, showing that it is, in fact, a kind of barnacle, but after the larva finds a crab to infect, it loses its shell and limbs, and indeed any obvious trace of its heritage, and becomes devoted to living off its host.
Sacculina might lose its shape, but it still consists of cells and tissues, and has its own complement of digestive enzymes and so on, all the better to consume its host. It has contracted out the services of locomotion, feeding, and much else to the crab, and for this gain it has traded its own limbs, mouthparts, sense organs, shell—just about every trace of its own crustacean heritage.
But parasitism can go a lot further than that.
Mycobacterium tuberculosis, the bacterium that causes tuberculosis, is a close relative of the leprosy bacillus, Mycobacterium leprae. But compared with the tubercle bacillus, the agent of leprosy has lost most of its genes. The tuberculosis bacterium has around 4,000 genes, compared with the 2,700 or so of the leprosy bacterium—of which at least 1,100 are known to be nonfunctional.22 With little capacity to provide very much for itself, the bacterium relies on its human host for the means to go on living. It is, in fact, so feeble that it can hardly manage to reproduce on its own. Given that drugs against bacteria work best when bacteria are reproducing, this explains why this bacterium, weak though it is, is very hard to kill. Far from being a matter of survival of the fittest, the evolution of leprosy shows that there are advantages in weakness. The race does not always go to the strong. As a parasite, the leprosy bacillus has gone much further than Sacculina, which still, at least, maintains its own metabolism. But by contracting many metabolic services out to its host, and shedding many of its genes, M. leprae has arguably become less complex than its relative M. tuberculosis—and has become a more perfect parasite.
But not as perfect as it might be.
If the leprosy bacillus is alive, if sickly, some even smaller parasites can be described as hardly living at all. These are the viruses.23 In general, viruses stand to bacteria as walnuts to watermelons. These creatures (I use the term loosely) are reduced to a few genes packaged into a protein coat. They have no digestive enzymes, no prospect of acquiring nutrients or digesting them, and no means of reproduction. They are in fact completely inert unless they can infect a cell (whether a bacterium or something more complex), whereupon they hijack the host’s own biochemical machinery to produce more viruses. Viruses, then, look like the perfect parasites. They have lost just about everything except the inviolable essence of their existence—their genetic material—and use the services of other creatures to duplicate that material and spread it around. Given that viruses can’t exist without more complex cells to parasitize, it is likely that they evolved from more complex organisms, refining and honing and streamlining themselves until they had lost all but the essentials. What might these organisms have been?
Most viruses have only a few genes—less than half a dozen—but there are some large and peculiar viruses, the so-called mimiviruses, which have more than 1,000 genes, making them as complex, genetically, as some bacteria.24 This suggests that at least some viruses are stripped-down bacteria. However, it could be that other viruses are rogue genetic elements that have broken away from more from complex creatures.
It’s hard to imagine parasites more reduced—more perfect—than viruses. But they exist. Amazingly, mimiviruses can be infected by tiny viruses, known as virophages,25 and there are other viruses, the so-called satellite viruses, that cannot infect a cell unless riding shotgun with a more capable, larger virus.
And yet there are parasites more perfect still. As if to demonstrate the point that complexity is made possible by the simplification of its components, the ultimate parasites are part of us.26
Many genes in our own genomes once came from viruses that have completely lost the ability to create their own protein coats, and can reproduce only by inserting themselves into our own genomes. These creatures—entities—are called LINEs (short for long interspersed elements) and have sacrificed almost every shred of their separate identities. They were once retroviruses, that is, viruses whose genomes are made of RNA that is “reverse transcribed” into the DNA of the host. They contain just two genes—one for an enzyme called reverse transcriptase that effects this process, and another called endonuclease that cuts the host DNA, enabling the parasitic DNA copy to slip in. Although they can, in theory, jump around the genome like this, almost all LINEs known have long since lost this ability: they can only reproduce when the genome of the host does so. In effect, they have become part of the genome of the host. About a fifth of the DNA in the human genome consists of old LINEs strung together end to end, slowly mutating into randomness, like so many train carriages rusting, forgotten, in long-abandoned sidings.
But not even LINEs get the prize for being the ultimate parasite. That award goes to the so-called short interspersed elements, or SINEs, which are very short sections of DNA that lie in wait for a LINE endonuclease to make a nick in the host DNA to allow LINE insertion—and slip in ahead of it. LINEs in the genome are accompanied by retinues of SINEs in the way dogs have fleas, and SINEs make up around 11 percent of the human genome.
If LINEs have almost no genes, SINEs have none at all. All they have is a stretch of DNA (a sign of the SINE) that catches the attention of the host’s enzymes, which transcribe it into RNA; this RNA is then reverse-transcribed by LINE reverse transcriptase back into DNA, which is then tucked neatly into place by the LINE’s endonuclease. In this way, a LINE, a parasite with only two genes of its own, is parasitized by a SINE, which has no genes at all, but just the ultimate in self-reflexive identity, a genetic notice that says no more than “Pick Me! Pick Me!”
SINEs, therefore, are the perfect parasites. They are also the ultimate demonstration of my point—that as parasites devolve more and more of their own functions to their hosts, they lose more and more complexity, until there is virtually nothing left.
Now, you might regard as special pleading the idea that the complexity of a system can increase only at a cost of the complexity of its individual parts. You might likewise think of the example of parasitism, advanced in the cause of my argument, in like fashion—despite its ubiquity. However, the fact remains that evolution abounds with loss, and the more we discover about the evolution of various creatures, the more we see that loss has played a critical part in shaping the forms of life we see around us.
&nb
sp; If symbiosis seems somewhat obscure, and the examples of parasitism I’ve chosen a little technical (you might never have seen a parasitic worm, or Sacculina; you’ve probably seen individual bacteria and viruses only in micrographs in books; and the existence of such arcana as SINEs and LINEs you must perforce take largely on trust), one can hardly argue with the concreteness of (say) birds. Birds are part of our daily lives. The smallest child knows what a bird is, and people who’ve heard of neither SINEs nor Sacculina can probably name many different bird species. Because of their ubiquity, beauty, and undeniable charm, the birds constitute perhaps the most intensively studied animal group. If the study of (say) parasitic crustaceans that eat the tongues of their fish hosts, only to replace them with their own bodies, is confined to a rather small group of specialists of epicurean taste, then the study of birds could hardly be more different, attracting flocks of professionals and veritable armies of knowledgeable amateurs.
One of the distinguishing features of modern birds is flight. Flight is an expensive pastime, such that the shapes of birds have been largely molded and subsumed to its cause—or so one might assume. The skeletons of birds are streamlined, with many bones fused together to form a rigid airframe. The bones are strong but hollow, making them very light. This hollowness extends to much of the rest of the insides of birds, too. The lungs of birds are connected to a system of air sacs that penetrates the entire body, even the insides of the hollow bones. As well as contributing to lightness, this air-sac system allows for a highly efficient system of gas exchange, as well as the cooling of internal organs heated by the fast metabolism that flight requires. Birds, like mammals, are warm-blooded, and run hot.
The outsides of birds are equally distinctive, being clothed in feathers. These remarkable appendages27 permit the bright and varied coloration of birds—vital to their often complex social lives—as well as creating a smooth, drag-free external surface, vital for rapid movement through the air. In addition, many feathers are ideally shaped as airfoils, whether individually or acting together. The presence of feathers seems to be, quintessentially, the feature of birds that marks them out from any other creature.
