Power, Sex, Suicide: Mitochondria and the Meaning of Life by Nick Lane Read Free Book Online Page B

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 seemedalmost 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?

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The Deepest Evolutionary Chasm
     
    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 mostimportantly, 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