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Death, for bacteria, is not inevitable. Protect a bacterium from predators, and provide it with adequate food and space to grow, and it would continue living--and reproducing asexually--forever. But a paramecium (a slightly more advanced single-cell organism), under the same ideal conditions, would stop dividing after about 200 generations--and die. Death, for paramecia and their offspring, is inevitable. Unless they have sex. If at any point during that 200 or so generations, two of the progeny of our paramecium have sex, their clock will be reset to zero. They and their progeny are granted another 200 generations. Those who fail to have sex eventually die. Immortality for bacteria is automatic; for all other living beings--including humans--immortality depends on having sex. But why is this so? Why must death be inevitable? And what is the connection between death and sexual reproduction? In Sex and the Origins of Death, William R. Clark looks at life and death at the level of the cell, as he addresses such profound questions as why we age, why death exists, and why death and sex go hand in hand. Clark reveals that there are in fact two kinds of cell death--accidental death, caused by extreme cold or heat, starvation, or physical destruction, and programmed cell death, initiated by codes embedded in our DNA. (Bacteria have no such codes.) We learn that every cell in our body has a self-destruct program embedded into it and that cell suicide is in fact a fairly commonplace event. We also discover that virtually every aspect of a cell's life is regulated by its DNA, including its own death, that the span of life is genetically determined (identical twins on average die 36 months apart, randomly selected siblings 106 months apart), that human tissue in culture will divide some 50 times and then die (an important exception being tumor cells, which divide indefinitely). But why do our cells have such programs? Why must we die? To shed light on this question, Clark reaches far back in evolutionary history, to the moment when inevitable death (death from aging) first appeared. For cells during the first billion years, death, when it occurred, was accidental; there was nothing programmed into them that said they must die. But fierce competition gradually led to multicellular animals--size being an advantage against predators--and with this change came cell specialization and, most important, germ cells in which reproductive DNA was segregated. When sexual reproduction evolved, it became the dominant form of reproduction on the planet, in part because mixing DNA from two individuals corrects errors that have crept into the code. But this improved DNA made DNA in the other (somatic) cells not only superfluous, but dangerous, because somatic DNA might harbor mutations. Nature's solution to this danger, Clark concludes, was programmed death--the somatic cells must die. Unfortunately, we are the somatic cells. Death is necessary to exploit to the fullest the advantages of sexual reproduction. In Sex and the Origins of Death, William Clark ranges far and wide over fascinating terrain. Whether describing a 62-year-old man having a major heart attack (and how his myocardial cells rupture and die), or discussing curious life-forms that defy any definition of life (including bacterial spores, which can regenerate after decades of inactivity, and viruses, which are nothing more than DNA or RNA wrapped in protein), this brilliant, profound volume illuminates the miraculous workings of life at its most elemental level and finds in these tiny spaces the answers to some of our largest questions.
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Ewelina Sienkiewicz
reviewed Sex and the origins of death on April 21, 2014 Sex and the origins of death
Why we die and how to beat it.
From the outset, what UCLA's Wm. Clark reports is staggering: Death is "not an obligatory attribute of life," he writes, and did not appear with the advent of it. Cellular aging resulting in death may not have occurred for more than a billion years after life's first entry on earth. Programmed cell death (PCD) which we suffer (displayed through wrinkles and forgetfulness) seems to have arisen about the time cells were experimenting with sex.
Sex is an energy costly activity, engaged in because it rolls the genetic dice, inviting variations with each new offspring. An advantage because with environmental change what was well suited in the old world is often not suited for the new. Gene variations result, and through natural selection, a few offspring amongst the dying progenitors may survive to save the species. For example, bacteria reproduce though cloning themselves, and can do so at a rate of 16 million per hour from one parent (take your antibiotics). But when the environment becomes harsh, bacterial parents spontaneously engage in sex, swapping genes with others as a gamble on survival.
In a description of catastrophic cell death, Clark displays a talent to meet or exceed even Sagan's best - clear, rich, compelling. Here heart attack, and the wonder of cell machinery resist the inevitable as systems and their backups struggle to counter power failures and starvation in a chain reaction of fading miracles. Like a community, some components are wholly unaware of disaster while others sacrifice themselves, transferring energy to last lines of defense - pumps stationed in cell walls countering a siege of water pressing in about to wash them away.
Such stunning, intentioned actions of this tiny, helpless, complex organism, the cell (of which we possess about 100 trillion - as many cells as there are stars in the nearest 400 spiral galaxies including the Milky Way!) is starkly contrasted against our cell's decision to commit suicide. This happens when life is late, or as early as the womb when ancient relics of evolution are flushed out of us - like reminders of an ocean origin when interdigital webbing of our onetime fins are removed through PCD, leaving what's left between our fingers. Once the nucleus decides to pull the trigger, one last set of instructions emerge as its DNA begins disassembling. All the while a stack of unread commands are being executed by unwary elements of the cell. The cell detaches from its neighbors, undulates, breaking into globules while still ignorant workers in these blobs work away, floating into a void, devoured by immune systems. Awful…
But there are rays of hope for immortality. "Growth factors" are given to cells like lymphocytes to put a safety on their trigger. And there are executioners in this tragedy, T-Cells. Having spotted an invader they do not murder the foreigner, they command the interloper to kill itself, orders dutifully followed. T-Cells know the security code.
Clarks notes an important difference between us and other "primitive" life forms. For example, paramecium dodge death by letting their macro-nuclei run the show while a micro-version lays dormant. After enough cell splitting, it has sex with another paramecium. Its macro-nuclei suffers PCD and the micro takes over as a newly minted micro-nucleus goes to sleep. Once eukaryotic cells (what we're made of) became multicellular, reproductive DNA would be not only kept in separate nuclei (as the paramecium) but in separate cells - our germ cells (sperm, egg). The rest of us, our bodies, are their guardians, not only redundant and irrelevant but we turn dangerous with too many divisions. When our germ cells meet others, clocks are reset just as they are for paramecium. Sex can save our germ cells but it cannot save us.
These growth factors, security codes, telemeres or some other mechanism may finally be commandeered to salvage us from oblivion. For now, as Clark writes, we must die and there are many mechanisms built into us to make sure we do. Death does not just happen, it is worked toward, with safeguards to assure cells don't backslide into immortality - as cancer cells do, a recipe for disaster. The winner is our species because germ cells are immortal through sex as we contribute molecular chains of ourselves to the future and whoever is made of us. Clark reveals this and so much more. A pure joy to read.
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