Life's Edge by Carl ZimmerNotable passages from
Life's Edge by Carl Zimmer

Penguin Audio, 2021

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"What we call 'life' is a certain quality, the sum of certain reactions of systems of matter, as the smile is the quality or reaction of the lips," [Nobel Prize winning Hungarian physiologist Albert] Szent-Györgyi once said [in a 1948 lecture at MIT].
   When he stopped to reflect more deeply on what he and his fellow biochemists were learning about life, Szent-Györgyi found it hard to offer a meaningful definition. If the definition of life involved something that sustained itself through chemical reactions, then a candle flame might be alive. What about a star, or a civilization?
   All living things, Szent-Györgyi explained to his audience at MIT, shared some hallmarks. But thinking too categorically about those hallmarks was a one-way ticket to absurdity. "One rabbit could never reproduce itself," Szent-Györgyi observed. "And if life is characterized by self-reproduction, one rabbit could not be called alive at all."
   We can find different features of life at different scales, Szent-Györgyi said, but only depending on the features of life we cherish most. "The noun 'life' has no sense," Szent-Györgyi declared, "there being no such thing."

"Life is a self-sustained chemical system capable of undergoing Darwinian evolution." [Scientists at a 1992 NASA meeting organized to come up with ideas for how to study the possibility of life on other worlds.]

Barge and a number of other scientists have grown especially curious about an Arizona-sized moon of Saturn called Enceladus. In 2005 the Cassini probe flew past the south pole of the moon and spotted a vast plume of vapor rising from giant fractures in the ice.
   That surprise led the engineers at JPL to steer a new course for Cassini . The probe made its way back to Enceladus for a closer flyby, then back again, returning twenty-three times in total. On each visit it gulped clouds of vapor and analyzed their contents. The plume, the scientists discovered, contains a mix of water, carbon dioxide, carbon monoxide, salt, benzene, and an assortment of other organic compounds.
   That deep-space mist offered a vision of what lay below the ice. Scientists eventually concluded that its frozen shell extends down about fifteen miles, serving as a roof for a salty ocean twenty miles thick. Even though Enceladus is only 314 miles across, its ocean is much deeper than ours. The deepest point in Earth's oceans, a place called Challenger Deep, is less than seven miles.
   Enceladus is 148,000 miles away from Saturn, but it takes the moon only thirty-three hours to complete an orbit around the planet. The gravitational force exerted by Saturn regularly stretches the core of Enceladus, a waterlogged ball of sand and gravel. The cycle of flexing creates enough friction to heat the core's water to a boil. It rises up to the ocean, reacting along the way with minerals and becoming a chemical-rich soup. The chill of space keeps the skin of Enceladus's ocean frozen. But Saturn's tides have broken the surface with crevices, out of which blast plumes of vapor from the warm ocean below.
   Liquid water, heat, organic compounds—Enceladus has a lot of the ingredients that seem to be essential for life.

Some critics found NASA's definition not just impractical but misleading. It narrowed the possibilities that life might take.
   Consider its requirement that life be capable of Darwinian evolution, for example. That's a very particular kind of change over time. It occurs when genes get precisely—but not perfectly—copied from generation to generation. Individuals with certain combinations of genes fare better at reproducing than others, and natural selection spreads those fitter versions. Over time, natural selection turns many mutations to produce new adaptations.
   But can we be so sure that evolution isn't unfolding somewhere else in some other manner? Is there no chance, for example, that a different kind of biology might allow for the inheritance of acquired traits—often called Lamarckian evolution? What if heredity could flow not just between the generations but between individuals in the same generation?
   This kind of dissatisfaction led to an explosion of hundreds of new definitions.

   Life is an expected, collectively self-organized property of catalytic polymers.
   Life is a metabolic network within a boundary.
   Life is a new quality brought upon an organic chemical system by a dialectic change resulting from an increase in the quantity of complexity of the system. This new quality is characterized by the ability of temporal self-maintenance and self-preservation.
   Life is the process of existence of open non-equilibrium complete systems that are composed of carbon-based polymers and are able to self-reproduce and evolve on the basis of template synthesis of their polymer components.
   Life is a far from equilibrium self-maintaining chemical system capable of processing, transforming and accumulating information acquired from the environment.
   The existence of the dynamically ordered region of water realizing a boson condensation of evanescent photons inside and outside the cell can be regarded as the definition of life.
   Life is a monophyletic clade that originated with a last universal common ancestor, and includes all its descendants.
And a frank one:
   Life is what the scientific establishment (probably after some healthy disagreement) will accept as life.

One philosopher has taken a far more radical stand. Carol Cleland argues that there's no point in searching for a definition of life or even just a convenient stand-in for one. It's actually bad for science, she maintains, because it keeps us from reaching a deeper understanding about what it means to be alive.…
   Life is different. It is not the sort of thing that can be defined simply by linking together concepts. As a result, it's futile to search for a laundry list of features that will turn out to be the real definition of life. "We don't want to know what the word life means to us," Cleland said. "We want to know what life is." And if we want to satisfy our desire, Cleland argues, we need to give up our search for a definition.…
   When it comes to life, Cleland argues, we are still alchemists. We use our intuitions to decide which things are alive or not and make arbitrary lists of the features they share. We paper over our ignorance with definitions that never manage to capture the thing we're trying to understand. The best thing that scientists can do right now, Cleland argues, is work toward a theory that explains life.

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