"Is Earth Exceptional?
By Mario Livio and Jack Szostak
Basic, 336 pages, $32
Life as No One Knows It
By Sara Imari Walker
Riverhead, 272 pages, $29
Becoming Earth
By Ferris Jabr
Random House, 304 pages, $30
In 1973, Julia Child made "primordial soup" for a display at the Smithsonian National Air and Space Museum. The chef said she was demonstrating "what some scientists think could have been nature's recipe for life on this Earth, and perhaps life in the rest of the universe."
Child was re-creating an experiment first performed by Stanley Miller and Harold Urey in 1952. The two chemists had made a mixture of gases thought to have been present in Earth's early atmosphere; they suspended it above water (representing a primeval ocean) and applied electrical sparks to simulate lightning. The resulting soup was found to hold traces of amino acids, the basic constituents of proteins.
As Mario Livio and Jack Szostak point out in "Is Earth Exceptional? The Quest for Cosmic Life," the experiment also created thousands of other chemicals, which in a real-world situation would have hindered rather than helped any further step toward life. Mr. Livio, an astrophysicist, and Mr. Szostak, a professor of chemistry at the University of Chicago, prefer a theory proposed in 2009 that suggests life first developed not in the ocean but on land.
Among the chemical compounds abundant in Earth's atmosphere during the planet's first 100 million years of existence would have been cyanide, raining down onto iron-rich rocks. With the help of ultraviolet light from the sun, sulfur from volcanic eruptions, and cycles of wet and dry weather, a series of reactions could have taken place. Laboratory studies have shown that these reactions can lead to the formation of nucleosides, key components of DNA. According to Messrs. Livio and Szostak, "cutting-edge research has replaced the old-fashioned concept of a 'prebiotic soup' with a series of steps in which intermediates are stabilized while in solution, and purified by crystallization." The process is called cyanosulfidic photoredox chemistry, and the first half of "Is Earth Exeptional?" explains it in considerable detail.
A different hypothesis, widely publicized since the 1990s, sees deep-sea volcanic fissures as the place where life began. Messrs. Livio and Szostak call the idea a "classic example" of a "deep-seated misconception . . . diverting attention and resources from more realistic scenarios." They admit, however, that the newer theory they espouse also faces "crucial unsolved problems."
The second half of "Is Earth Exceptional?" looks at the prospect of finding evidence of extraterrestrial life. Attempts to detect radio signals from alien civilizations have so far drawn a blank. The National Aeronautics and Space Administration remains hopeful that signs of microbial life might be found within our solar system, perhaps fossilized on Mars or swimming in the oceans of the moons of Jupiter and Saturn. Messrs. Livio and Szostak suspect that any life existing anywhere in our solar system except on Earth likely descended from microbes thrown from our young planet by meteor impacts.
The authors look instead toward planets known to be orbiting other stars. Thousands have been discovered, but their distance means that any life to be found there can only be inferred indirectly. The glint of starlight grazing an atmosphere, for instance, can be studied as a way of deducing what gases may be present. "There is little doubt that the first signs of simple life to be detected on exoplanets are likely to be in the form of biosignature gases," the authors say. Which gases to look for, however, is still a matter of debate. "Is Earth Exceptional?" will be challenging for the lay reader, but it gives a good impression of how much has been achieved, and how much further there is to go.
In "Life as No One Knows It," Sara Imari Walker, an astrobiologist and theoretical physicist at Arizona State University, tackles the most basic question of her discipline: What counts as "life"? We might consider growth or reproduction as characteristic of living things, but crystals grow and fire makes more fire. Ms. Walker cites a common definition -- "life is a self-sustaining chemical system capable of Darwinian evolution" -- and finds fault there too. Parasites aren't self-sustaining, they need a host. And are you personally capable of Darwinian evolution?
"I'll know it when I see it" is what Ms. Walker often hears from colleagues when the issue comes up, but she wants a more objective criterion and thinks she may have found it. What she and her co-workers have devised is a new approach called assembly theory. By way of analogy, imagine a jet aircraft and a pile of aircraft parts. Both are complex things made of essentially the same components, but a pile of junk can be rearranged in lots of ways and still be junk. There are far fewer ways of putting parts together to get a machine that flies, and each part must itself be properly assembled.
Ms. Walker believes that living things exhibit a similar kind of complexity, resulting from natural selection operating over countless generations. This leads to a new definition: "Life is the only thing in the universe that can make objects that are composed of many unique, recursively constructed parts."
Her enthusiasm for the idea sometimes borders on grandiosity. Ms. Walker describes assembly theory as "new physics," but it is hard to see the justification. The claim is that chemical complexity can be quantified by a number called the assembly index, or "the smallest number of physically possible steps necessary to produce an object." She maintains that the only things with an index of 15 or more are living ones. This seems to presuppose prior knowledge of everything that can possibly happen in chemistry. Can we be sure that a nonliving process of such complexity will never happen anywhere in the universe? Experts are divided on whether assembly theory really can solve the mystery of life, but "Life as No One Knows It" is a provocative and intriguing introduction to the ideas behind it.
James Lovelock is best known as the founder of the Gaia theory, which considers Earth and its organisms to be a single, self-regulating "superorganism." Lovelock was also a pioneer in the field of astrobiology. In "Becoming Earth," Ferris Jabr recalls meeting Lovelock soon after the scientist turned 100 (he died three years later, in 2022). Frail yet still "cheerful, articulate, and quick-witted," he told Mr. Jabr that "anything alive is capable of changing the planet." He encouraged NASA to look for those changes on Mars in the 1960s, and the modern search for exoplanetary biosignatures follows the same idea. The approach also underpins "Becoming Earth," which vividly describes how much life has shaped, and has been shaped by, our planet.
Mr. Jabr is an accomplished science journalist. He writes eloquently -- sometimes beautifully -- about his encounters around the world with biologists, environmentalists, farmers and others involved in some way with the life of the planet. The book is neatly arranged into sections on rock, water and air, with each section further divided to show how successive levels of life, from microbes to humans, interact with those environments.
The author begins deep underground, in a disused gold mine where scientists are discovering new organisms thriving in darkness. "These intraterrestrial microbes," he tells us, "are ancient and slow, reproducing infrequently and possibly living for millions of years." They could account for more than 90% of Earth's microbes, and may offer insights into how life first began. Mr. Jabr does not discuss the cyanosulfidic theory preferred by Messrs. Livio and Szostak, but he does mention deep-sea volcanic fissures as the place where a crucial stage in evolution -- the advent of photosynthesis -- may have occurred. "In lieu of sunlight," he writes, the first photosynthesizers "would have relied on the dim glow of magma and superheated water, breathing hydrogen sulfide and excreting sulfur." Their descendants harnessed sunlight to develop "a radical new version of photosynthesis," taking advantage of "highly abundant resources, spinning sunlight, water, and carbon dioxide into sugar and releasing oxygen as a byproduct" -- toxic to most early microbes. Many scientists believe this caused a mass extinction, after which some oxygen-loving bacteria paved the way for animals and eventually ourselves.
Mr. Jabr visits a Siberian reserve nicknamed Pleistocene Park, where scientists are trying to work out if it was hunting or climate change that wiped out the mammoth. He looks down on the Amazon rain forest from a steel mast as high as the Eiffel Tower -- and has to hurry down 1,500 steps before a thunderstorm arrives. He snorkels in a kelp forest, and in California he visits firefighters ("some of whom prefer to be called firelighters"), relearning indigenous wisdom about living with wildfires. At home in Portland, Ore., Mr. Jabr tends to his garden. "Our living planet typically requires centuries to create a single inch of fertile topsoil." Defining soil, he says, is as hard as defining life, because soil itself is "the most complex bio-material that we know of." Hard to imagine how life on any other planet could be as wonderful as what lies beneath our feet.
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Mr. Crumey is the author, most recently, of the novel "Beethoven's Assassins."" [1]
1. REVIEW --- Books: Is There No Place Like Home? Crumey, Andrew. Wall Street Journal, Eastern edition; New York, N.Y.. 19 Oct 2024: C.7.
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