Power, Sex, Suicide: Mitochondria and the Meaning of Life (6 page)

Most biologists are more cautious, or less religious. Evolutionary biology holds more cautionary tales than just about any other science, and the erratic meanderings of life, throwing up weird and improbable successes, and demolishing whole phyla by turns, seems to owe more to the contingencies of history than to the laws of physics. In his famous book
Wonderful Life
, Stephen Jay
Gould wondered what might happen if the film of life were to be replayed over and over again from the beginning: would history repeat itself, leading inexorably each time to the evolutionary pinnacle of mankind, or would we be faced with a new, strange, and exotic world each time? In the latter case, of course, ‘we’ would not have evolved to see it. Gould has been criticized for not paying due respect to the power of convergent evolution, which is the tendency of organisms to develop similarities in physical appearance and performance, regardless of their ancestry, so that anything which flies will develop similar-looking wings; anything that sees will develop similar-looking eyes. This criticism was propounded most passionately and persuasively by Simon Conway Morris, in his book
Life’s Solution
. Conway Morris, ironically, was one of the heroes of Gould’s book,
Wonderful Life
, but he opposes that book’s sweeping conclusion. Play back the film of life, says Conway Morris, and life will flow down the same channels time and time again. It will do so because there are only so many possible engineering solutions to the same problems, and natural selection means that life will always tend to find the same solutions, whatever they may be. All of this boils down to a tension between contingency and convergence. To what extent is evolution ruled by the chance of contingency, versus the necessity of convergence? For Gould all is contingent; for Conway Morris, the question is, would an intelligent biped still have four fingers and a thumb?

Conway Morris’s point about convergent evolution is important in terms of the evolution of intelligence here or anywhere else in the universe. It would be disappointing to discover that no form of higher intelligence had ever managed to evolve elsewhere in the universe. Why? Because very different organisms should converge on intelligence as a good solution to a common problem. Intelligence is a valuable evolutionary commodity, opening new niches for those clever enough to occupy them. We should not think only of ourselves in this sense: some degree of intelligence, and in my view conscious self-awareness, is widespread among animals, from dolphins to bears to gorillas. Humanity evolved quickly to fill the ‘highest’ niche, and a number of contingent factors no doubt facilitated this rise; but who is to say that, given a vacated niche and a few tens of millions of years, the kind of foraging bears that break into cars and dustbins could not evolve to fill it? Or why not the majestic and intelligent giant squid? Perhaps it was little more than chance and contingency that led to the rise of
Homo sapiens
, rather than any of the other extinct lines of
Homo
, but the power of convergence always favours the niche. While we are the proud possessors of uniquely well-developed minds, there is nothing particularly improbable about the evolution of intelligence itself. Higher intelligence could evolve here again, and by the same token anywhere else in the universe. Life will keep converging on the best solutions.

The power of convergence is illustrated by the evolution of ‘good tricks’ like flight and sight. Life has converged on the same solutions repeatedly. While repeated evolution does not imply inevitability, it does change our perception of probability. Despite the obviously difficult engineering challenges involved, flight evolved independently no less than four times, in the insects, the pterosaurs (such as pterodactyls), the birds, and the bats. In each case, regardless of their different ancestries, flying creatures developed rather similar-looking wings, which act as aerofoils—and we too have paralleled this design feature in aeroplanes. Similarly, eyes have evolved independently as many as forty times, each time following a limited set of design specifications: the familiar ‘camera eye’ of mammals and (independently) the squid; and the compound eyes of insects and extinct groups such as trilobites. Again, we too have invented cameras that work along similar principles. Dolphins and bats developed sonar navigation systems independently, and we invented our own sonar system before we knew that dolphins and bats took soundings in this way. All these systems are exquisitely complex and beautifully adapted to needs, but the fact that each has evolved independently on several occasions implies that the odds against their evolution were not so very great.

If so, then convergence outweighs contingency, or necessity overcomes chance. As Richard Dawkins concluded, in
The Ancestors Tale
: ‘I am tempted by Conway Morris’s belief that we should stop thinking of convergent evolution as a colourful rarity to be remarked and marvelled at when we find it. Perhaps we should come to see it as the norm, exceptions to which are occasions for surprise.’ So if the film of life is played back over and over again, we may not be here to see it ourselves, but intelligent bipeds ought to be able to gaze up at flying creatures, and ponder the meaning of the heavens.

If the origin of life amidst the fire and brimstone of early Earth was not as improbable as we once thought (more on this in
Part 2
), and most of the major innovations of life on Earth all evolved repeatedly, then it is reasonable to believe that enlightened intelligent beings will evolve elsewhere in our universe. This sounds reasonable enough, but there is a nagging doubt. On Earth, all of this engineering flamboyance evolved in the last 600 million years, barely a sixth of the time in which life has existed. Before that, stretching back for perhaps more than 3000 million years, there was little to see but bacteria and a few primitive eukaryotic organisms like algae. Was there some other brake on evolution, some other contingency that needed to be overcome before life could really get going?

The most obvious brake, in a world dominated by simple single-celled organisms, is the evolution of large multicellular creatures, in which lots of cells collaborate together to form a single body. But if we apply the same yardstick of repeatability, then the odds against multicellularity do not seem particularly high. Multicellular organisms probably evolved independently quite a few
times. Animals and plants certainly evolved large size independently; so too (probably) did the fungi. Similarly, multicellular colonies may have evolved more than once among the algae—the red, brown, and green algae are ancient lineages, which diverged more than a billion years ago, at a time when single-celled forms were predominant. There is nothing about their organization or genetic ancestry to suggest that multicellularity arose only once among the algae. Indeed, many are so simple that they are better viewed as large colonies of similar cells, rather than true multicellular organisms.

At its most basic level a multicellular colony is simply a group of cells that divided but failed to separate properly. The difference between a colony and a true multicellular organism is the degree of specialization (differentiation) among genetically identical cells. In ourselves, for example, brain cells and kidney cells share the same genes but are specialized for different tasks, switching on and off whichever genes are necessary. At a simpler level, there are numerous examples of colonies, even bacterial colonies, in which some differentiation between cells is normal. Such a hazy boundary between a colony and a multicellular organism can confound our interpretation of bacterial colonies, which some specialists argue are better interpreted as multicellular organisms, even if most ordinary people would view them as little more than slime. But the important point is that the evolution of multicellular organisms does not appear to have presented a serious obstacle to the inventive flow of life. If life got stuck in a rut, it wasn’t because it was so hard to get cells to cooperate together.

In
Part 1
, I shall argue that there was one event in the history of life that was genuinely unlikely, which was responsible for the long delay before life took off in all its extravagance. If the film of life were played back over and over again, it seems to me likely that it would get stuck in the same rut virtually every time: we would be faced with a planet full of bacteria and little else. The event that made all the difference here was the evolution of the
eukaryotic cell
, the first complex cells that harbour a nucleus. An esoteric term like ‘eukaryotic cell’ might seem a quibbling exception, but the fact is that all true multicellular organisms on earth, including ourselves, are built only from eukaryotic cells: all plants, animals, fungi, and algae are eukaryotes. Most specialists agree the eukaryotic cell evolved only once. Certainly, all known eukaryotes are related—all of us share exactly the same genetic ancestry. If we apply the same rules of probability, then the origin of the eukaryotic cell looks far more improbable than the evolution of multicellular organisms, or flight, sight, and intelligence. It looks like genuine contingency, as unpredictable as an asteroid impact.

What has all this to do with mitochondria, you may be wondering? The answer stems from the surprising finding that all eukaryotes either have, or once had, mitochondria. Until quite recently, mitochondria had seemed
almost incidental to the evolution of eukaryotes, a nicety rather than a necessity. The really important development, after which eukaryotes are named, was the evolution of the nucleus. But now this is perceived differently. Recent research suggests that the acquisition of mitochondria was far more important than simply plugging an efficient power-supply into an already complicated cell, with a nucleus brimming with genes—it was the single event that made the evolution of complex eukaryotic cells possible at all. If the mitochondrial merger had not happened then we would not be here today, nor would any other form of intelligent or genuinely multicellular life. So the question of contingency boils down to a practical matter: how did mitochondria evolve?

The void between bacterial and eukaryotic cells is greater than any other in biology. Even if we begrudgingly accept bacterial colonies as true multicellular organisms, they never got beyond a very basic level of organization. This is hardly for lack of time or opportunity—bacteria dominated the world for two billion years, have colonized all thinkable environments and more than a few unthinkable ones, and in terms of biomass still outweigh all multicellular life put together. Yet for some reason, bacteria never evolved into the kind of multicellular organism that a man on the street might recognize. In contrast, the eukaryotic cell appeared much later (according to the mainstream view) and in the space of just a few hundred million years—a fraction of the time available to bacteria—gave rise to the great fountain of life we see all around us.

The Nobel laureate Christian de Duve has long been interested in the origin and history of life. He suggests in a wise final testament,
Life Evolving,
that the origin of the eukaryotes may have been a bottleneck rather than an improbable event—in other words, their evolution was an almost inevitable consequence of a relatively sudden change in the environmental conditions, such as a rise in the amount of oxygen in the atmosphere and oceans. Of all the populations of proto-eukaryotes living at the time, one form simply happened to be better adapted and expanded rapidly through the bottleneck to take advantage of the changing circumstances: it prospered, while less well-adapted competing forms died out, giving a misleading impression of chance. This possibility depends on the actual sequence of events and selection pressures involved, and can’t be ruled out until these are known with certainty. And of course, when we are talking of selection pressures exerted two billion years ago, it is unlikely that we can ever be certain; nonetheless, as I mentioned in the Introduction, it is possible to exclude some of the possibilities by considering modern molecular biology, and to narrow down a list of the most likely possibilities.

Despite my enormous respect for de Duve, I don’t find his bottleneck thesis very convincing. It is too monolithic, and the sheer variety of life weighs against it—there seems to be a place for almost everything. The whole world did not change at once, and many varied niche environments persisted. Perhaps most
importantly, environments lacking in oxygen (anoxic or hypoxic environments) persisted on a large scale, and do so to this day. To survive in such environments calls for a very different set of biochemical skills from those needed to survive in the new oxygenated surroundings. The fact that some eukaryotes already existed should not have precluded the evolution of a variety of different ‘eukaryotes’ in different environments, such as the stagnant sludge at the bottom of the oceans. Yet this is not what happened. The astonishing fact is that all the single-celled eukaryotes that live there are related to the oxygen-breathing organisms living in fresh air. I find it highly improbable that the first eukaryotes were so competitive that they annihilated all competition from every environment, even those most unsuited to their own character. Certainly the eukaryotes are not so competitive that they annihilated the competition of bacteria: they took their place alongside them, and opened up new niches for themselves. Nor can I think of any parallels elsewhere in life, on any scale. The fact that the eukaryotes became the masters of oxygen respiration did not lead to its disappearance among the bacteria. And more generally, many types of bacteria persisted for billions of years despite unceasing and unforgiving competition for the same resources.

Let’s consider a single example, the methanogens. These bacteria (more technically, Archaea) scratch a living by generating methane gas from hydrogen and carbon dioxide. We’ll consider this briefly as the methanogens are important to our story later on. The problem for methanogens is that, though carbon dioxide is plentiful, hydrogen is not: it quickly reacts with oxygen to form water, and so is not found in oxygenated environments for any long period. The methanogens can therefore only survive in environments where they have access to hydrogen gas—usually environments totally lacking in oxygen, or with constant volcanic activity, replenishing the source of hydrogen faster than it is used up. But the methanogens are not the only type of bacteria that use hydrogen—and they are not particularly efficient at extracting hydrogen from the environment. Another type of bacteria, so-called
sulphate-reducing bacteria
, makes a living by converting (or reducing) sulphate into hydrogen sulphide—the gas that stinks of rotten eggs (in fact rotten eggs reek of hydrogen sulphide). To do so, they too can use hydrogen gas, and they usually out-compete the methanogens for this scarce resource. Even so, the methanogens have survived for three billion years in niche environments, where the sulphate-reducing bacteria are penalized in some other respect—usually for the lack of sulphate. For example, because freshwater lakes are impoverished in sulphate, the sulphate-reducing bacteria can’t establish themselves; and in the sludge at the bottom of such lakes, or in stagnant marshes, the methanogens live on. The methane gas they emit is known as swamp gas, and at times it sets alight with a mysterious blue flame that plays over the marshes, a phenomenon known as
‘will-o-the-wisp’, which explains many ‘sightings’ of ghosts and UFOs. But the productions of the methanogens are far from insubstantial. Anyone who advocates switching from exploiting oil reserves to natural gas can thank the methanogens—they are responsible for essentially our entire supply. Methanogens are also found in the guts of cattle and even people, as the hindgut is exceedingly low in oxygen. The methanogens thrive in vegetarians because grass, and vegetation in general, is low in sulphur compounds. Meat is much richer in sulphur; so sulphate-reducing bacteria usually displace methanogens in carnivores. Change your diet, and you will notice the difference in polite company.