As they say in real estate, location, location, location.
Though scientists cannot define life, most are convinced that, like consciousness (also hard to define), it must have arisen from inanimate nature, as the result of an as-yet-unknown law or a cascade of unusual events. Or it arises, but only at key moments, from the unexplored power to self-organize. Whichever of these options one favors, the explanatory gap of even the most discussed theory, RNA world, can plausibly be compared to magic.
But location matters. The location that matters in early Earth’s geography is deep-sea thermal vents, discovered by the submersible Alvin in 1977. Researcher Nick Lane suggests that the very high CO2 levels of Earth’s early oceans “may well have generated natural proton gradients similar to those that drive ATP production in modern organisms.” So maybe RNA world could arise there, if not anywhere else. Lane focuses on white smokers, calling them “the only model that makes the emergence of life look like a probable and deterministic outcome of geology, geochemistry and thermodynamics.”
There are also black smokers to consider. Fossils of deep-sea microbes (1.43 billion years old) have been found around their chemical-rich “chimneys,” which host microbes today. Chemist G�nter W�chtersh�user and others argue that the exotic conditions near such volcanoes once hosted life midwifed by metal catalysts, rather than by the substances or environments that we regard as natural today. Some dispute the details, arguing that volcanic sulfur, floating pumice, or thermal hot springs played a key role.
Critics argue that hydrothermal vent fields are too hot and too acid for a promising soup of free-floating amino acids. A critic (who advocates ancient mud volcanoes instead) charges, “It’d be like trying to make life evolve from hot Coca-Cola.” Stanley Miller of Miller-Urey experiment fame told Discover Magazine in 1992 that overall, “The vent hypothesis is a real loser. I don’t understand why we even have to discuss it.” One difficulty is that the oldest known fossils are stromatolites, clumps of bacteria from 3.5 billion years ago, which suggests that life began in shallow seas, not deep ones.
But what if element-rich comets struck a location deemed favorable for other reasons? Comet shockwaves, possibly producing amino acids, might produce the needed shock synthesis of chemicals. Some argue for dust orbiting the sun before the planets formed as a source for organic molecules.
But the gulf between the molecules that could form life and actual life remains huge. And, inconveniently, crashed comets might destroy more than they create (ask the dinosaurs). Still, to test the thesis, some study the tardigrade (water bear) — yes, you caught a glimpse of one in the first episode of Cosmos — an animal which can survive near absolute zero and above boiling point, as well as massive doses of radiation, to see if life forms could be swapped between planets.
Generally speaking, theories like these, which hold that life did not originate solely from materials available on Earth, are called panspermia. Some of them aim at expanding backward the time available for the origin of life. That’s because life on Earth apparently got started quite soon after the planet cooled. Recent evidence from a study of zircons suggests that Earth firmed up only 160 million years after the solar system was formed. Indeed, there is even a “faint young Sun paradox” involved: The sun is thought to have been too faint between between 3.8 and 2.4 billion years ago (Archaean) for life to have taken hold, but clearly it did.
Harvard astrophysicist Abraham Loeb argues that life might have originated only fifteen million years after the Big Bang, over 13 bya. In a warm environment, “rare ‘islands’ packed with denser matter” could have formed massive, short-lived stars in the early universe, producing the first rocky planets. That said, a recent study from Tel Aviv University says that the universe heated up later, not earlier than thought. However, a paper last year, citing a version of Moore’s Law, argued a similar position to Loeb’s, but pegged the origin at 9.2 billion years ago. It is significant that the reasoning behind both papers (that there is not enough time for a naturalistic origin of life on Earth) is not generally dismissed, as it surely would be if researchers had much confidence in other theories.
Assuming that comets brought life to Earth, some ask whether it could have started on Mars. Steven Benner, chemist at the Westheimer Institute for Science and Technology, pursues this lead because, he admits, the chances of life forming on Earth are poor:
The molecules that combined to form genetic material are far more complex than the primordial “pre-biotic” soup of organic (carbon-based) chemicals thought to have existed on the Earth more than three billion years ago, and RNA (ribonucleic acid) is thought to have been the first of them to appear.
Simply adding energy such as heat or light to the more basic organic molecules in the “soup” does not generate RNA. Instead, it generates tar.
Generally, the public learns about problems like these when a radically new research emphasis is proposed. Benner opts for Mars because the minerals most effective at templating RNA would not dissolve in oceans there and elements such as boron and molybdenum, “key in assembling atoms into life-forming molecules,” were available.
“A panspermia solution, after all, produces another panspermia problem,” he said. “If a Martian microbe did make it from Mars to Earth, maybe it would be as if it landed in Eden. But just as likely, it would quickly die.”
In any event, some recent findings suggest that many meteorites from Mars are only about 200 million years old, not four billion, as needed for these purposes.
The truth is, it is mainly fresh frustration, not fresh findings, that have given panspermia new traction. As National Geographic News reported in 2013,
The reemergence of the theory of panspermia is intertwined with progress (or lack of progress) in a long-term scientific quest to find out how life began on Earth, a question that synthetic biology experts such as Benner have been working on for decades. Despite some advances, the field has come up against chemical walls that are proving impossible to climb.
Seth Shostak of the SETI Institute has described finding clues to life’s earliest moments on Earth as “tougher than overcooked road kill.” Yet it happened somehow. The variety of models that seek to explain how it did so are a testimony to human creativity.
Editor’s Note: Here are links to the whole “Science Fictions Origin of Life” series.
Photo source: cletch/Flickr.