Power, Sex, Suicide: Mitochondria and the Meaning of Life (28 page)

So we can now finally appreciate the full set of barriers to large size and complexity in bacteria. Bacteria replicate as quickly as they can, and are limited at least partly by the speed at which they can generate ATP. They generate ATP by pumping protons across their external membrane. They can’t grow larger because their energetic efficiency tails away as their size increases. This fact in itself makes the predatory eukaryotic lifestyle unlikely, because phagocytosis requires a combination of large size with abundant energy that is precluded by respiration across the outer membrane. Some bacteria developed complex internal membrane systems. However, the area of these is several orders of magnitude less than that of the mitochondrial membranes in a single eukaryotic cell, because without gene outposts bacteria can’t control the speed of respiration over a wider area. Given the strong selection pressures for fast reproduction and efficient energy generation, any of the possible transition states
en route
to establishing such genetic outposts would likely have been selected against whenever they arose. Only endosymbiosis was stable enough to provide the long-term conditions necessary for respiratory control on a wider scale.

Would things have happened differently somewhere else in an infinite universe? Anything is possible, but it seems to me unlikely. Natural selection is probabilistic: similar selection pressures are most likely to generate similar outcomes anywhere in the universe. This explains why natural selection so often converges on similar solutions, such as eyes and wings. Despite 4000 million years of evolution, we know of no single example of bacteria that succeeded in becoming eukaryotes by natural selection alone, or for that matter, of any mitochondria that lost all of their genes and still functioned as mitochondria. I doubt whether such events would happen any more often anywhere else either.

What of a eukaryotic-style chimera? We saw in
Part 1
that the eukaryotic cell evolved here on earth just once, by way of what seems to have been a deeply improbable chain of circumstances. Perhaps a similar concatenation would be
repeated elsewhere, but I see nothing in the laws of physics to suggest that the rise of complexity was inevitable. Physics is stymied by history. At best, the evolution of multicellular complexity seems to have been improbable; and without a kernel of complexity, intelligence is unthinkable. Yet once the loop that had kept bacteria simple was broken, the birth of that first large, complex cell, the first eukaryote, marked the beginning of a road that led, almost inexorably, to the spectacular feats of bioengineering that we see all around us today, including ourselves. This path was just as dependent on mitochondria as the origin of the eukaryotic cell itself, for the existence of mitochondria made the evolution of large size and greater complexity not just possible, but probable.

Size and the Ramp of Ascending Complexity

Does life inherently become more complex? There may be nothing in the genes to push life up a ramp of ascending complexity, but one force lies outside the genes. Size and complexity are usually linked, for larger size requires greater genetic and anatomical complexity. But there is an immediate advantage to being bigger: more mitochondria means more power and greater metabolic efficiency. It seems that two revolutions were powered by mitochondria—the accumulation of DNA and genes in eukaryotic cells, giving an impetus to complexity, and the evolution of warm-blooded animals, which inherited the earth.

The more the merrier—mitochondrial numbers dictate the evolution of size and complexity

Size is a dominating bias in biology. By and large, we are mostly interested in the largest life-forms—the plants, animals, and fungi that we can actually see. Our interest in bacteria or viruses tends to be anthropocentric, a morbid curiosity, probing into the horrors of the diseases that they cause, and the more gruesome the better. Necrotizing bacteria that chew up whole limbs in a matter of days can hardly but attract more attention than the myriad microscopic plankton that exert such a profound influence on our planet’s climate and atmosphere. Textbooks on microbiology tend to focus disproportionately on pathogens, despite the fact that only a tiny proportion of microbes actually cause disease. When we search for signs of life in space, we are really seeking extraterrestrial intelligence: we want proper aliens with twisting tentacles, not microscopic bacteria.

In the last few chapters, we have considered the origins of biological complexity: why it was that bacteria gave rise to our own remotest ancestors, the first eukaryotes—morphologically complex cells with nuclei and organelles such as mitochondria. I have argued that the fundamental mechanism of energy generation in cells made symbiosis necessary for the evolution of complexity: eukaryotic cells almost certainly could not have evolved by natural selection alone. Generating energy using mitochondria inside the cell made this leap possible. While symbiosis is commonplace in eukaryotic cells, however, endosymbiosis in bacteria (in which one bacterium lives inside another) is far less common. It seems that bacterial endosymbiosis gave rise to the complex eukaryotic cell on just one occasion, perhaps by way of the improbable train of events discussed in
Part 1
.

Yet once the first eukaryotes had evolved, we can legitimately talk about a ramp of ascending complexity: the progression from single cells to human beings certainly looks like a ramp, more than a little dizzying, even if we are deceived by appearances. Now a larger question looms: what drove the eukaryotes to acquire greater size and complexity? One answer that was popular in Darwin’s day, and which enabled many biologists to reconcile evolution and religion, is that life innately becomes more complex. According to this line of reasoning, evolution leads to greater complexity in the same way that an embryo develops into an adult—it follows instructions, ordained by God, in which each step approaches closer to Heaven. Many of our turns of phrase, such as ‘higher organisms’ and the ‘ascent of man’, hark back to this philosophy, and are in common currency today despite the admonitions of evolutionists
right back to Darwin himself. Such metaphors are powerful and poetic, but can be profoundly misleading. Another visually striking metaphor, that electrons orbit the nucleus of an atom in the same way that planets orbit the sun, long concealed the fantastic mysteries of quantum mechanics. The idea that evolution is akin to embryonic development conceals the fact that evolution has no foresight: it
cannot
operate as a program (whereas the development of an embryo is necessarily programmed by the genes). So complexity can’t have evolved with the distant goal of approaching closer to God, but only as an immediate payback for an immediate advantage.

If the evolution of complexity was not programmed, are we to believe that it occurred merely by chance, or was it an inevitable outcome of the workings of natural selection? The fact that bacteria never showed the least tendency to become more complex (morphologically) argues against the possibility that natural selection inevitably favours complexity. Numerous other examples show that natural selection is as likely to favour simplicity as complexity. On the other hand, we have seen that bacteria are stymied by their respiration problem, but eukaryotes are not. Did complexity perhaps evolve in eukaryotes just because it could? Ridding himself of higher religious connotations, Stephen Jay Gould once compared complexity with the random meanderings of a drunkard: if a wall blocks his passage on one side of the pavement, then the drunkard is more likely to end up in the gutter, simply because there is nowhere else for him to go. In the case of complexity, the metaphorical wall is the base of life: it is not possible to be any simpler than a bacterium (at least as an independent organism), so life’s random walk could only have been towards greater complexity. A related view is that life became more complex because evolutionary success was more likely to be found in the exploitation of new niches—an idea known as the ‘pioneering’ theory. Given that the simplest niches were already occupied by bacteria, the only direction in which life could evolve was towards greater complexity.

Both these arguments imply there was no intrinsic advantage to complexity—in other words, there was no trait inherent to the eukaryotes that encouraged the evolution of greater complexity—it was simply a response to the possibilities offered by the environment. I don’t doubt for a moment that both of these theories account for certain trends in evolution, but I do find it hard to swallow that the entire edifice of complex life on Earth was erected by what amounts to evolutionary drift. The trouble with drift is its lack of direction, and I can’t help but feel there is something inherently directed about eukaryotic evolution. The great chain of being may be an illusion, but it is a compelling one, one that held mankind in its sway for 2000 years (since the ancient Greeks). Just as we must account for the apparent evolution of ‘purpose’ in biology (the heart as a pump, etc), so too we must account for the apparent
trajectory towards greater complexity. Can a random walk, stopping off at vacant niches on the way, really produce something that even
looks
like a ramp of complexity? To twist Stephen Jay Gould’s analogy, how come so many meandering drunkards didn’t end up in the gutter, but actually succeeded in crossing the road?

One possible solution, inherent to eukaryotic cells but not to bacteria, is sex. That there is a link between sex and complexity has been argued persuasively by Mark Ridley in
Mendel’s Demon
. The trouble with asexual reproduction, says Ridley, is that it is not good at eliminating copying errors and harmful mutations in genes. The larger the genome, the greater the probability of a catastrophic error. The recombination of genes in sexual reproduction may lower this risk of error, and so raise the number of genes an organism can tolerate before undergoing a mutational meltdown (although this has never been proved). Clearly, however, the more genes an organism accumulates, the greater its possible complexity, so the invention of sex in eukaryotes might have opened the gates to complexity. While there is almost certainly some truth in this argument, there are also problems with the idea that sex stands at the gateway to complexity, as Ridley himself concedes. In particular, the number of genes in bacteria is well below the theoretical asexual limit, even if they relied on asexual reproduction alone, which they do not (lateral gene transfer in bacteria helps restore genetic integrity). Ridley acknowledges that the data are ambivalent, and the asexual limit to gene number may fall somewhere between fruit flies and human beings. If so the gates of complexity could hardly have been thrown open by the evolution of sex. Something else must have been the gate-keeper.

I do think there was an inherent tendency for eukaryotes to grow larger and more complex, but the reason relates to energy rather than sex. The efficiency of energy metabolism may have been the driving force behind the rampant ascent of eukaryotes to diversity and complexity. The same principles underpin energetic efficiency in all eukaryotic cells, giving an impetus to the evolution of larger size in both unicellular and multicellular organisms, whether plants, animals, or fungi. Rather than being a random walk through vacant niches, or a march driven by the imperative of sex, the trajectory of eukaryotic evolution is better explained as an inherent tendency to become larger, with an immediate payback for an immediate advantage—the economy of scale. As animals become larger, their metabolic rate falls, giving them a lower cost of living.

I am here conflating size with complexity. Even if it is true that greater size is favoured by a lower cost of living, is there really a connection between size and complexity? Complexity is not an easy term to define, and in attempting to do so we are inevitably biased towards ourselves: we tend to think of complex beings in terms of their intellect, behaviour, emotions, language, and so on,
rather than, for example, a complex life cycle, as in an insect with its drastic morphological transitions, from caterpillar to butterfly. In particular, I am not alone in my bias towards larger size: for most of us, I suspect, a tree appears more complex than a blade of grass, even though, in terms of photosynthetic machinery, grasses might be said to be more highly evolved. We insist that multicellular creatures are more complex than bacteria, even though the biochemistry of bacteria (as a group) is far more sophisticated than anything we eukaryotes can muster. We are even inclined to see patterns in the fossil record implying an evolutionary trend towards greater size (and presumably complexity), known as Cope’s Rule. While accepted with little question for a century, several systematic studies in the 1990s suggested that the trend is nought but an illusion: different species are equally likely to become smaller as they are larger. We are so mesmerized by our fellow large creatures that we easily overlook the smaller ones.