Power, Sex, Suicide: Mitochondria and the Meaning of Life (5 page)

Even the visionary Margulis was not correct about everything, luckily for the rest of us. Aligning herself with the earlier advocates of symbiosis, Margulis had argued that it would one day prove possible to grow mitochondria in culture—it was only a matter of finding the right growth factors. Today, we know that this is not possible. The reason was also made clear by the detailed sequence of the mitochondrial genome: the mitochondrial genes only encode a handful of proteins (13 to be exact), along with all the genetic machinery needed to make them. The great majority of mitochondrial proteins (some 800) are encoded by the genes in the nucleus, of which there are 30 000 to 40 000 in total. The apparent independence of mitochondria is therefore truly apparent, and not genuine. Their reliance on two genomes, the mitochondrial and the nuclear, is evident even at the level of a few proteins that are composed of multiple subunits, some of which are encoded by the mitochondrial genes, and others by the nuclear genes. Because they rely on both genomes, mitochondria can only be cultured within their host cells, and are correctly designated ‘organelles’, rather than symbionts. Nonetheless, the word ‘organelle’ gives no hint of their extraordinary past, and affords no insight into their profound influence on evolution.

There is another sense in which many biologists today still disagree with Lynn Margulis, and that relates to the evolutionary power of symbiosis in general. For Margulis, the eukaryotic cell is the product of multiple symbiotic mergers, in which the component cells have been subsumed into the greater whole to varying degrees. Her theory has been dubbed the ‘serial endosymbiosis theory’, meaning that eukaryotic cells were formed by a succession of such mergers between cells, giving rise to a community of cells living within one another. Besides chloroplasts and mitochondria, Margulis cites the cell skeleton with its organizing centre, the centriole, as the contribution of another type of bacteria, the
Spirochaetes
. In fact, according to Margulis the whole organic world is an elaboration of collaborative bacteria—the microcosm. The idea goes back to Darwin himself, who wrote in a celebrated passage: ‘Each living being is a microcosm—a little universe formed of self-propagating organisms inconceivably minute and numerous as the stars in the heavens.’

The idea of a microcosm is beautiful and inspiring, but raises a number of difficulties. Cooperation is not an alternative to competition. A collaboration
between different bacteria to form new cells and organisms merely raises the bar for competition, which is now between the more complex organisms rather than their collaborative subunits—many of which, including the mitochondria, turn out to have retained plenty of selfish interests of their own. But the biggest difficulty with an all-embracing view of symbiosis is the mitochondria themselves, which wag a cautionary finger at the power of microscopic collaboration. It seems that all eukaryotic cells either have, or once had (and then lost), mitochondria. In other words, possession of mitochondria is a
sine qua non
of the eukaryotic condition.

Why on earth should this be? If collaboration between bacteria were so commonplace, we might expect to find all sorts of distinct ‘eukaryotic’ cells, each composed of a different set of collaborative microorganisms. Of course, we do—there is a great range of eukaryotic collaboration, especially in the more obscure microscopic communities living in inaccessible places, such as the mud of the sea floor. But the astonishing finding is that all these far-flung eukaryotes share the same ancestry—and they
all
either have or once had mitochondria. This is not true of any other collaboration between microorganisms in eukaryotes. In other words, the collaborations that attained fulfilment in eukaryotic organisms are contingent on the existence of mitochondria. If the original merger had not taken place, then neither would any of the others. We can say this with near certainty, because the bacteria have been collaborating and competing among themselves for nearly four billion years, and yet only came up with the eukaryotic cell once. The acquisition of mitochondria was the pivotal moment in the history of life.

We are discovering new habitats and relationships all the time. They are a fabulously rich testing ground of ideas. To give just a single example, one of the more surprising discoveries at the turn of the millennium was the abundance of tiny, so-called
pico-eukaryotes
, which live among the micro-plankton in extreme environments, such as the bottom of the Antarctic oceans, and in acidic, iron-rich rivers, like the Rio Tinto in southern Spain (known by the ancient Phoenicians as the ‘river of fire’ because of its deep red colour). In general, such environments were considered to be the domain of hardy, ‘extremophile’ bacteria, and the last place one might expect to find fragile eukaryotes. The pico-eukaryotes are about the same size as bacteria and favour similar environments, and so generated a lot of interest as possible intermediates between bacteria and eukaryotes. Yet despite their small size and unusual predilection for extreme conditions, all turned out to fit into known groups of eukaryotes: genetic analysis showed they don’t challenge the existing classification system at all. Astonishingly, this new bubbling fountain of variations on a eukaryotic theme adds up to no more than
subgroups
to existing groups, all of which we have known about for many years.

In these unsuspected environments, the very places we would expect to find a tapestry of unique collaborations, we do not. Instead, we find more of the same. Take the smallest known eukaryotic cell, for example,
Ostreococcus tauri
. It is less than a thousandth of a millimetre (1 micron) in diameter, rather smaller than most bacteria, yet it is a perfectly formed eukaryote. It has a nucleus with 14 linear chromosomes, one chloroplast—and, most remarkably of all, several tiny mitochondria. It is not alone. The unexpected fountain of eukaryotic variation in extreme conditions has thrown up perhaps 20 or 30 new subgroups of eukaryotes. It seems that all of them have, or once had, mitochondria, despite their small size, unusual lifestyles, and hostile surroundings.

What does all this mean? It means that mitochondria are not just another collaborative player: they hold the key to the evolution of complexity. This book is about what the mitochondria did for us. I ignore many of the technical aspects that are discussed in textbooks—incidental details like porphyrin synthesis and even the Krebs cycle, which could in principle take place anywhere else in the cell, and merely found a convenient location in the mitochondria. Instead, we’ll see why mitochondria made such a difference to life, and to our own lives. We’ll see why mitochondria are the clandestine rulers of our world, masters of power, sex, and suicide.

The Origin of the Eukaryotic Cell

All true multicellular life on earth is made up of eukaryotic cells—cells with a nucleus. The evolution of these complex cells is shrouded in mystery, and may have been one of the most unlikely events in the entire history of life. The critical moment was not the formation of a nucleus, but rather the union of two cells, in which one cell physically engulfed another, giving rise to a chimeric cell containing mitochondria. Yet one cell engulfing another is commonplace; what was so special about the eukaryotic merger that it happened only once?

The first eukaryote—one cell engulfed another to form an extraordinary chimera two billion years ago

Are we alone in the universe? Ever since Copernicus showed that the earth and planets orbit the sun, science has marched us away from a deeply held anthropocentric view of the universe to a humbling and insignificant outpost. From a statistical point of view, the existence of life elsewhere in the universe seems to be overwhelmingly probable, but on the same basis it must be so distant as to be meaningless to us. The chances of meeting it would be infinitesimal.

In recent decades, the tide has begun to turn. The shift coincides with the mounting scientific respectability afforded to studies on the origin of life. Once a taboo subject, dismissed as ungodly and unscientific speculation, the origin of life is now seen as a solvable scientific conundrum, and is being inched in upon from both the past and the future. Starting at the beginning of time and moving forwards, cosmologists and geologists are trying to infer the likely conditions on the early earth that might have given rise to life, from the vaporizing impacts of asteroids and the hell-fire forces of vulcanism, to the chemistry of inorganic molecules and the self-organizing properties of matter. Starting in the present and moving backwards in time, molecular biologists are comparing the detailed genetic sequences of microbes in an attempt to construct a universal tree of life, right down to its roots. Despite continuing controversies about exactly how and when life began on earth, it no longer seems as improbable as we once imagined, and probably happened much faster than we thought. The estimates of ‘molecular clocks’ push back the origin of life to a time uncomfortably close to the period of heavy bombardment that cratered the moon and earth 4000 million years ago. If it really did happen so quickly in our boiling and battered cauldron, why not everywhere else?

This picture of life evolving amidst the fire and brimstone of primordial earth gains credence from the remarkable capacity of bacteria to thrive, or at least survive, in excessively hostile conditions today. The discovery, in the late 1970s, of vibrant bacterial colonies in the high pressures and searing temperatures of sulphurous hydrothermal vents at the bottom of the oceans (known as ‘black smokers’) came as a shock. The complacent belief that all life on earth ultimately depended on the energy of the sun, channelled through the photosynthesis of organic compounds by bacteria, algae, and plants, was overturned at a stroke. Since then, a series of shocking discoveries has revolutionized our perception of life’s orbit. Self-sufficient (autotrophic) bacteria live in countless numbers in the ‘deep-hot biosphere’, buried up to several miles deep in the
rocks of the earth’s crust. There they scrape a living from the minerals themselves, growing so slowly that a single generation may take a million years to reproduce—but they are undoubtedly alive (rather than dead or latent). Their total biomass is calculated to be similar to the total bacterial biomass of the entire sunlit surface world. Other bacteria survive radiation at the genetically crippling doses found in outer space, and thrive in nuclear power stations or sterilized tins of meat. Still others flourish in the dry valleys of Antarctica, or freeze for millions of years in the Siberian permafrost, or tolerate acid baths and alkaline lakes strong enough to dissolve rubber boots. It is hard to imagine that such tough bacteria would fail to survive on Mars if seeded there, or could not hitch a lift on comets blasted across deep space. And if they could survive there, why should they not evolve there? When handled with the adept publicity of NASA, ever eager to scrutinize Mars and the deepest reaches of space for signs of life, the remarkable feats of bacteria have fostered the rise and rise of the nascent science of astrobiology.

The success of life in hostile conditions has tempted some astrobiologists to view living organisms as an emergent property of the universal laws of physics. These laws seem to favour the evolution of life in the universe that we see around us: had the constants of nature been ever-so-slightly different, the stars could not have formed, or would have burnt out long ago, or never generated the nurturing warmth of the sun’s rays. Perhaps we live in a multiverse, in which each universe is subject to different constants and we, inevitably, live in what Astronomer Royal Martin Rees calls a
biophilic
universe, one of a small set in which the fundamental constants favour life. Or perhaps, by an unknown quirk of particle physics, or a breathtaking freak of chance, or by the hand of a benevolent Creator, who put in place the biophilic laws, we are lucky enough to live in a true universe that does favour life. Either way, our universe apparently kindles life. Some thinkers go even further, and see the eventual evolution of humanity, and in particular of human consciousness, as an inevitable outcome of the universal laws, which is to say the precise weightings of the fundamental constants of physics. This amounts to a modern version of the clockwork universe of Leibniz and Newton, parodied by Voltaire as ‘All is for the best in the best of all possible worlds.’ Some physicists and cosmologists with a leaning towards biology find a spiritual grandeur in this view of the universe as the midwife of intelligence. Such insights into the innermost workings of nature are celebrated as a ‘window’ into the mind of God.