The Stardust Revolution

by Jacob Berkowitz

  Benner is looking both backward and forward at life by trying to make it and by guiding the search for it beyond Earth. Researchers at FfAME are using molecular paleontology to work backward down the tree of terrestrial life in an attempt to identify the molecular missing links to the origins of life on our planet. They're doing modern-day Miller-Urey-type experiments to explore how the rain of cosmic molecules on the primordial Earth may have given rise to the simplest first life. They're trying to construct life in the lab from scratch via synthetic biology—they're not trying to re-create how life actually emerged on Earth but rather are trying to see if they can create life de novo. It's this “build it and you will understand it” approach that many biochemists believe is the key to unraveling life's chemical nature.

  The final slice of FfAME's research pie is devoted to considering how the question of what life is applies to the search for other life in the cosmos. It's this piece of his research that landed Benner the position of cochair of a US National Research Council committee called the Committee on the Limits of Organic Life in Planetary Systems, a subcommittee of the Committee on the Origins and Evolution of Life. The subcommittee's task, as requested by NASA, was to provide guidance for its scientists and engineers in the design of a new generation of life-detection experiments. In 2007, the committee, a veritable who's who of distinguished US astrobiologists, astronomers, microbiologists, chemists, and planetary scientists, released its report, The Limits of Organic Life in Planetary Systems.

  Benner, who cowrote the report's final draft, knew then that it wouldn't include one key point: a definition of life. “This is no accident,” he wrote, reflecting on the experience. “Early in the committee's deliberations a conscious decision was made not to include a definition of ‘life’ in the book. Perhaps this reflected cowardice. It may, however, be better viewed as an expedient based on wise experience. Nearly every member of the panel had spent hours in other committee meetings discussing that definition, with little productive outcome.” Fortunately, Benner has a good sense of humor, which undoubtedly helps him survive the slippery slopes of defining life. The committee report is dedicated to “non-human life forms, wherever they are.”

  As we sit poolside under the hot Houston afternoon Sun, Benner walks me through the history of why, in the Stardust Revolution, the definition of life has become such a nebulous topic. It wasn't always so. The problem is that the definition of life has evolved. “Life” used to be so simple. Indeed, from the eighteenth century to the first half of the twentieth century, biologists didn't worry about defining life. Life was like a beating heart: you just had to look and listen, and you could tell something was alive. Ironically, all this began to change with what was at first seen as the answer to a definition of life, the discovery of DNA's structure. On February 28, 1953, Cambridge University molecular biologists Francis Crick and James Watson left the university's Cavendish labs and headed for the Eagle pub to celebrate. There, Crick is reported to have raised his glass and announced to his pub mates, “We have discovered the secret of life!” They toasted their victory in the heated race to discover the structure of DNA; they were the first to grasp its now-iconic double-helix structure, like two spiral staircases embracing one another step-by-step.

  But Crick had overstated their case, though many consider his and Watson's discovery the greatest in twentieth-century biology. The structure of DNA didn't answer the question of the mystery of life; it greatly deepened it. Watson and Crick's discovery paper was published the same month that Urey and Miller published their paper about life's possible origins on Earth from a primordial soup of simple molecules. The combination of a new vision of the molecular basis of life in DNA and thinking about life's molecular origins soon muddied the waters of life's nature and led to a molecular-level chicken-and-egg problem. DNA is often referred to as the blueprint for life because the information it encodes directs the assembling of proteins; however, biochemists soon also realized that DNA itself is assembled by proteins. So the discovery of this molecule of life encoded a conundrum: Which came first, DNA or the proteins that assembled it? Similarly, Benner explains, sitting on a plastic chaise longue, the discovery of DNA also launched what he calls “the great moose debate.” Prior to the 1960s, biologists were still largely tied to a Linnaean vision of life, that of the whole plant or animal—in Benner's example, the great antlered moose. However, the rise of molecular biology created a great divide, he says, between “those who studied moose and those who studied blended moose.” In essence, the discovery of DNA pushed the quest to understand life down to the molecular level—in other words, not a whole moose, or even a mix of its cells, but the mix of chemical juice that makes a moose; that is, chemistry. This was anathema to many biologists, who felt their subject matter was metamorphosing into something unrecognizable, a point Benner drives home by saying that biology has become a subdiscipline of chemistry.

  The extent to which life has become chemistry is denoted in NASA's 1994 working definition of life, a definition adapted from work by Carl Sagan and used by Benner as his guidepost: “Life is a self-sustaining chemical system capable of Darwinian evolution.” There's nothing in this definition that rings true to what many immediately associate with life, such as movement or intelligence. Instead, in developing a prototype cosmic definition of life, astrobiologists have defined life in the broadest possible terms as something that can keep itself alive and have kids who in turn have kids who'll be different from their parents because of a combination of genetic mutations and population-level adaptation to changing environmental conditions.

  Today, scientists generally agree that life on Earth began before DNA; that DNA is the result of life on Earth, rather than its origin. One intriguing pre-DNA possibility is what's called the RNA world hypothesis. Today, life on Earth involves enormous chemical specialization: DNA carries information, and proteins do the work that we call metabolism, primarily catalyzing chemical reactions so that our bodies readily convert sugars into usable energy and use this efficiently to perform cellular bodily functions. But there's strong evidence that the first life on Earth involved chemical multitasking by another key life molecule: ribonucleic acid, or RNA. Today RNA is often introduced in high-school biology class as DNA's less glamorous sidekick, working as a messenger to shuttle information from DNA to ribosomes, the cell's protein manufacturing centers. However, DNA and RNA are very similar. Like DNA, RNA is a string of four different kinds of nucleotide building blocks, the exception being that RNA is a single rather than a double chain, that it uses a slightly different sugar in its molecular architecture, and that it substitutes the nucleotide uracil instead of thymine.

  Although today RNA gets second billing to DNA, RNA is the more dexterous molecule. RNA, unlike DNA, is capable of both carrying genetic information and of getting metabolic work done. RNA is like DNA and a protein rolled into one molecule. This makes RNA a prime candidate for an all-in-one origin-of-life molecule, capable of both reproducing itself and carrying the code to guide the copying. And there's building evidence to support this view. Many of the molecular subunits of RNA are found in carbonaceous chondrites or could have formed through chemical reactions in the Earth's primordial oceans or atmosphere. Many viruses today use only RNA as their genetic molecule, and RNA is the core of ribosomes, our cellular subunits that are factories for protein manufacturing and are known to be older than LUCA, or last universal common ancestor, the hypothesized single-cell ancestor species of all animal life on Earth. As a result, one of the dominant theories on the origins of life now being explored is that our earliest molecular ancestors were all RNA—a form of life that, Benner notes, were we to discover it today on another planet, we'd consider alien.

  LIFE AS A COSMIC CONTINUUM

  As a consequence of the challenge of defining life in a cosmic context, in one of the most interesting rethinkings of the Stardust Revolution, the very question of the definition of life is being turned on its head. Our difficulty in defining li
fe isn't the problem; it's the point. Life cannot be understood as something separate from either the rest of matter or the rest of time. It can be understood only as part of a greater continuum, just as someone's individual existence can be understood only in a genealogical context. The difference here is that this new vision challenges long-held notions of an absolute divide between “life” and “nonlife.” However, in the latest new way of seeing in the Stardust Revolution, what becomes evident is that there's no divide, but there is a new spectrum not of light but of life. “If the origin of life is seen as the evolutionary transition between the non-living and the living, then it is meaningless to attempt to draw a strict line between these two worlds,” says Mexican evolutionary biologist Antonio Lazcano, one of the world's leading thinkers on the origins of life on Earth. “The appearance of life on Earth should, therefore, be seen as an evolutionary continuum that seamlessly joins the prebiotic synthesis and accumulation of organic molecules in a primitive environment, with the emergence of self-sustaining, replicative chemical systems capable of undergoing Darwinian evolution.”

  It's a view that's been slowly building against the tide of dominant thinking, from the very beginnings of the Stardust Revolution. By the end of the nineteenth century, the molecules formerly thought to be produced only by life—sugars, urea, and other carbon- and nitrogen-based compounds—were seen also to originate abiotically, without life. Near the end of his life, Charles Darwin reflected on this chemical revolution and its relationship to the nature of life itself: “Though no evidence worth anything has as yet, in my opinion, been advanced in favor of a living being, being developed from inorganic matter,” Darwin wrote privately to a colleague in 1882, “yet I cannot avoid believing the possibility of this will be proved some day in accordance with the law of continuity. I remember the time, above fifty years ago, when it was said that no substance found in a living plant or animal could be produced without the aid of vital forces.”

  Today, Darwin's notion of “the law of continuity” has been extended from life on Earth to the stars. In looking to traverse this terrain tying atoms to life, stardust scientists see that stars, in their essence, though not “alive,” are biological. This might seem like an outrageous statement. After all, for centuries stars have been the purview of astronomers and physicists. But the central realization of the Stardust Revolution is this: the stars are our direct ancestors. The notion that we are stardust is often taken as an esoteric physical one, in the sense that we're made of the elements formed in stars but somehow still separate from them. In fact, our nature is far more stellar than this. We embody the stuff of stars, and our essential nature is informed by them; we inherit information from them. Life's structure is determined by stars. They don't just forge all the elements heavier than hydrogen and helium; the ratio of these elements determines the nature of life. Stars provide not just the energy for life but also the blueprint. We are CHNOPS beings (named after the six core elements of terrestrial life: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur) not because of the nature of the Earth, but because of the nature of stars. As Steve Benner puts it, “The human genome is nothing more (and nothing less) than a collection of chemical structures, recording how carbon, oxygen, nitrogen, hydrogen and phosphorous atoms are bonded in natural products directly responsible for inheritance.”

  When it comes to elementally differentiating life from anything else in the universe, it's not a question of absolute differences but of relative abundances. One of the most striking examples of this is that your body's elemental composition is just a bunch of hydrogen gas away from being a star. Here's why: the elemental composition of the Sun and the rest of the Solar System is a reflection of the elemental composition of the solar nebula, or cosmic birth cloud out of which the Sun and its solar siblings formed some 4.567 billion years ago. To see just how closely life and the Sun are atomically related, all you have to do is balance out our levels of hydrogen and make what astronomers call devolatized solar abundances (that is, what remains after you boil away the most volatile elements, hydrogen and helium). Then, elementally we see that we're cosmic twins with very similar relative amounts of such key elements as carbon, oxygen, and calcium.

  This stellar ancestry shapes the way astrobiologists think about the question “If we find other cosmic life, to what extent will we see ourselves reflected in this cosmic mirror?” One imagining of this encounter is depicted in the classic 1979 science-fiction movie Alien. The crew of a spaceship returning to Earth with its cargo of ore mined from an asteroid receives a distress signal; soon, however, they have reason to send their own distress signal. They inadvertently stumble across, and become hosts to, a particularly unwelcome and gruesome visitor who the crew, and moviegoers, quickly realize is very much alive. But rather than being carbon-based, this alien is silicon-based, giving it rock-hard, bullet-deflecting skin. Inspired in part by movies such as Alien, many astrobiologists in the past several decades have warned against limiting our view of what life is, and what we search for elsewhere in the cosmos, to what we know on Earth. In The Limits of Organic Life in Planetary Systems, Benner and the other authors argue that we should search in such a way that we'd be able to detect silicon-based life; life that “eats,” or gets energy from, rocks; life that passes on hereditary information in chemicals other than DNA or RNA; and life that uses a solvent other than water, such as a mixture of ammonia and water—what we on Earth would consider a powerful oven cleaner.

  While such alternative forms of life might well exist, the cosmic evidence so far points to some potential common cosmic family traits. If we encounter other cosmic life, we'll be meeting not aliens but distant cosmic cousins with whom we share characteristics rooted in cosmic ecology. The laws of physics and chemistry are universal, and so might be the laws, or at least the broad rules, of universal life. The Stardust Revolution has brought us to the edge of seeing whether, just as with the Copernican and Darwinian Revolutions, we are not exceptional but rather are part of the warp and weave of creation.

  Many astrobiologists believe that there might be a fundamental universal nature to biochemistry. The evidence builds from atoms up to the molecules of life. The four most common life elements on Earth—hydrogen, oxygen, carbon, and nitrogen—are also the most abundant elements in the universe, not counting helium, an element that rarely bonds with others. When these atoms bond, they do so in characteristic fashion throughout the cosmos. And from this bonding, one key, remarkable pattern emerges: cosmic chemistry is built around carbon chemistry. Of the more than fifty cosmic molecules known to date with more than six atoms, all are built on a carbon framework. Prior to the dawn of astrochemistry, we thought of carbon chemistry as the chemistry of life. Now we know that carbon chemistry is the chemistry of the universe. Our carbon nature isn't some terrestrial outlier but is rather a reflection of the carbon-based molecular nature of the cosmos. Silicon is carbon's neighbor on the periodic table and has similar bonding characteristics, but, Alien notwithstanding, complex silicon-based molecules haven't been observed in interstellar space.

  It's even possible that our protein structure and genetic code will share fundamental commonalities with other cosmic life. Stardust scientists have discovered that the twenty amino acids used by life on Earth are far from a random cosmic sample. The amino acids used by all Earth life are those that are energetically most easily synthesized and that similarly represent the greatest possible diversity of size and chemical charges. In other words, the evolution of life on Earth sampled from the smorgasbord of available amino acids, thus achieving maximum diversity while taking the biggest helpings from the largest and easiest-to-access dishes at this cosmic potluck. Finally, abundant evidence from organic materials found in meteorites and from astrochemistry experiments shows that many fundamental cosmic chemical reactions result in hollow, spherical molecular balls that resemble cells. Thus, the ingredients and chemical pathways for carbon-based cellular life have a rich cosmic natural history, one
that might play out similarly elsewhere in the cosmos.

  This is not to say that all cosmic life will have the identical chemical nature of terrestrial life in detail, but rather that we should search for certain fundamental chemical patterns. This is what leading astrobiologist and NASA scientist Chris McKay calls “the Lego Principle” in the search for life. Just as with the popular children's toy, cosmic life is assembled from a limited set of chemical building blocks. But what's critical in spotting life, as with any interesting Lego® structure, is the biological pattern with which the blocks are assembled. On Earth, the three main molecules of life—DNA, proteins, and polysaccharides—are all polymers, chains of simpler building blocks. Every protein—including titin, the largest protein and an awesomely complex interwoven chain of about thirty thousand amino acids that puts spring in your muscles—is built Lego-like from a bucket of just twenty different amino acids. Most impressive, all terrestrial life's incredible diversity—from the snake's pointy tongue to a starfish's wonderful suction cups—is encoded in just five nucleotide bases: adenine, guanine, cytosine, uracil, and thymine; a list biochemists reduce down to the symbols A, G, C, U, and T. Out of this five-letter chemical alphabet emerges the complex polyglot language of life. “Different life forms are likely to have different patterns,” notes McKay, but “a complete analysis of the relative concentration of different types of organic molecules might reveal a pattern that is biological even if that pattern does not involve any of the familiar biomolecules.” In other words, life elsewhere in the cosmos might be pure love, as the NASA engineer I spoke with during the coffee break suggested, but love has its ways, and by looking for these patterns, astrobiologists will be able to recognize the kiss of life.

  In the Stardust Revolution, though, many astrobiologists believe that the debate over the first other cosmic life won't be about cellular biochemistry but rather will be about planetary chemistry. Our first glimpse of life elsewhere in the cosmos—of a possible second cosmic genesis—will come not through a microscope but through a telescope. And the debate will be about whether what we're seeing is an entire world with a heartbeat.