The leading theory of how life began just hit a snag

How and where life first emerged on Earth is one of the great unsolved puzzles for scientists exploring the possibility of life on other worlds. For the past decade or so, the prevailing theory has been that the first primitive organisms originated at alkaline hydrothermal vents in the deep ocean. Now, a new paper by Benjamin Tutolo from the University of Calgary in Canada, published in the Proceedings of the National Academy of Sciences, challenges that assumption.

There’s solid reasoning behind the old paradigm. High temperatures at hydrothermal vents, like the famed Lost City Hydrothermal Field on the Atlantic Ocean floor, are critical for organic molecules to come together to form the all-important biomolecules without which life cannot exist. Sulfide, the reduced form of sulfur used to build cell membranes, was also thought to be plentiful in these deep-ocean environments. It seemed the perfect place for nonliving materials to transform into the first living cell.

But Tutolo argues that the required synthesis reactions would not occur at these vents, because the environment would only become alkaline after it cooled down. Furthermore, reduced sulfur would likely be very sparse in ancient ocean vents. There wouldn’t have been enough time for the chemical reactions necessary to create life to occur before the biological building blocks fell apart again.

The new paper doesn’t completely rule out the hydrothermal vent theory, but it shows that scientists’ favorite scenario for the origin of life on Earth may have serious problems. And it has some researchers taking a second look at other hypotheses that have been proposed — and mostly discarded — in the past.

Origin stories

The most common is Darwin’s “little warm pond” theory, which, according to a newer interpretation, has life arising in surface hydrothermal pools like the hot springs at Yellowstone National Park. David Deamer of the University of California at Santa Cruz is among the main proponents of this idea.

In this scenario, the biomolecules critical for life would come together in fresh water rather than seawater. Ultraviolet radiation hitting Earth’s surface would play an essential role in synthesizing certain larger organic molecules such as amino acids. If the warm pond partly dried up occasionally due to evaporation, all the better — the remaining soup of organic molecules would be that much more concentrated, encouraging chemical interaction.

The same potential advantages would be found in tidal flats. The constant cycles of the tides — which would have been stronger four billion years ago, when the Moon was closer to Earth — would alternate dry and wet periods in a muddy environment. This has been suggested as a prerequisite for life’s origin according to the “living pulse” hypothesis, which holds that cycling would help synchronize critical biomolecules.

Organic enrichment would occur not only in hot springs and tidal flats, but also in a cold-water environment like the Arctic. Water residing in microscopic gaps between ice crystals could, in principle, function as incubation sites for biomolecules.

Another intriguing proposal for life’s origin has been advanced by Ulrich Schreiber of the University of Duisburg-Essen, who suggests that tectonic fault zones rich in supercritical carbon dioxide, like the Rio Grande Rift, might have provided a suitable location for the origin of life.

So far, all of these scenarios assume that the life we see all around us originated here on Earth. What if it didn’t? The “panspermia” hypothesis holds that life may have arrived here from somewhere else. It’s difficult to imagine that any form of cellular life could survive a journey through interplanetary space, unprotected from the hostile environment. But what if it were sheltered within a rock? Martian meteorite ALH 84001, which caused quite a stir in the 1990s when scientists claimed to have found fossils inside it, never got heated above 40℃ during its entire 10-million-year journey from Mars to Earth.

If the first life on Earth did in fact come from another planet, Mars was the likely source. Habitable conditions existed there before they did on our own planet, which suffered a collision with a planet-sized object that would have sterilized the surface about 4.5 billion years ago, while creating our large Moon. The transfer of rocks from one world to another is easier from Mars to Earth than vice versa, simply because Martian gravity is weaker. Rocks blown into space by an asteroid impact on Mars would preferentially head toward the center of the Solar System in the direction of Earth, where the impact would be cushioned somewhat by our planet’s relatively thick atmosphere. We have proof that this actually happens: Scientists have recovered more than 100 meteorites from Mars, but not a single recognized one from Venus.

Perhaps the most speculative theory about the origin of life is known as “directed panspermia,” which suggests that some highly evolved alien species intentionally seeded Earth with life. The idea was proposed more than 50 years ago by none other than Francis Crick, the co-discoverer of DNA, and Leslie Orgel. It’s an intriguing notion, and of course can’t be ruled out. But it doesn’t tell us how life arose — just that it might not have originated here.

How common is life in the universe?

For life to have begun on Earth, three things had to happen. In a very short time, and in a relatively confined space, the first organisms needed to (1) form a cell membrane, (2) build a protein or similar biomolecule to produce energy for metabolism, and (3) use a nucleic acid like RNA or DNA to replicate information from one generation to the next. If only one or two of these milestones were achieved, the system would have quickly fallen apart. It seems extremely improbable that all three would come together. Yet we’re here, aren’t we? So there must be a way.

If we knew the answer, we’d have a much better idea of where else to look for life. If it originated in deep-sea hydrothermal vents, we should be focusing on ocean worlds like Jupiter’s moon Europa or Saturn’s Enceladus. If it arose in tidal flats, we probably shouldn’t bother with these icy moons. In this case, Mars might even be ruled out, since the two tiny moons Phobos and Deimos are too small to produce strong tides. It’s also possible that life can originate in different kinds of environments, using different chemical pathways.

Astrobiology, the science of life in the universe, is therefore greatly limited by our lack of understanding of how our own world crossed the threshold from non-living to living. We know some of the conditions that are beneficial for organic molecules to come together and interact, and we can guess some of the chemical reactions that might have occurred on the early Earth. But that’s about as far as we can go right now. We don’t know what kind of metabolism the first cells had, or where they first appeared. Finding out would be the most important step toward estimating how common life is in the universe.

This article is featured on Big Think.