0.9 Leagues Under the Sea

Could geothermal vents hold the secret to the origin of life on Earth?

0.9 Leagues Under the Sea

We’ve told many legends about the origin of life. Thousands of years ago, it was of a divine creator crafting mankind from clay and dirt. Today, it depicts a slow self-assembly of organic molecules within a “primordial soup.” But strangely enough, recent developments in marine biology and geochemistry have given rise to a far different story.

According to current theories, life as we know it came about as a result of two scenarios: The first of these two is that early forms of bacteria could have been carried to Earth through a concept called panspermia -- the seeding of habitable worlds with living organisms across the cosmos. Though it seems closer to fiction than reality, the possibility still exists. To analyze why, we must look at the abundance of molecules that could lead to the emergence of life.

As far as we are concerned, all documented life in the universe depends on carbon-based reactions within an aqueous medium. Because of the abundance of hydrogen, oxygen, carbon, and nitrogen in the known universe, it isn’t far-fetched to assume that unique combinations of these elements can emerge in any place, given the conditions are right. In fact, we’ve seen examples of this in our outer solar system: Jupiter’s moon, Europa, for instance, has yielded a plethora of biosignatures, such as liquid water vapor and simple organic compounds, namely amino acids like glycine and adenine. On another note, Saturn’s moon, Titan, is blanketed in a carbon-rich atmosphere of nitrogenous gases, many of which have led to the synthesis of organic molecules like hydrocarbons, nitriles, and cyanides. If present on Europa and Titan, organic chemistry could be present in other areas of Earth’s local neighborhood.

A diagram of Titan's rich atmospheric chemistry, put together by NASA after receiving data from the Cassini-Huygens mission.

With worlds such as these and the chemistry they permit, scientists theorize that, in the early days of our solar system, protoplanetary bodies and other objects may have dispersed carbonaceous and water-rich material throughout the region, causing basic forms of life to manifest. Near the end of this period, around 4.1 to 3.8 billion years ago, an event known as the Late Heavy Bombardment occurred, during which comets, asteroids, and dwarf planets left over from planet formation slammed into the larger bodies of the solar system, delivering materials like silicate rock and water ice to their surface. If this theory is correct, then some of the comets and meteoroids that helped deposit water onto the early Earth may have carried with them biological matter. Far-fetched as it is, though, panspermia has even further evidence to suggest its validity:

Several decades ago, impacts on Mars launched chunks of debris from its surface into the surrounding area, some of which crash-landed on Earth in the mid-1990s. One of these objects, rich in the iron oxides and perchlorates that confirmed its Martian origin, appeared to contain geologic imprints left over from what scientists believed to be primitive forms of microbial life. Though near-impossible to prove, this theory would also suggest that panspermia occurs regularly in outer space and maybe even deep space, carrying the seeds of life across astronomical distances to land by pure chance on worlds like ours.

An image of the Alan Hills 84001 meteorite appears to depict fossilized microorganisms.

The other theory, which is far more straightforward, implies that life on Earth is inherently native to the planet itself, having emerged from a soupy matrix of organic compounds like amino acids, sugars, and nitrogenous aromatic rings like adenine and guanine -- two essential components of nucleic acids. This “primordial soup,” as it is commonly dubbed, would imply that the chemistry required for life as we know it can arise on its own, without the need for chunks of icy material to bring life to us.

While it goes without saying that Occam’s razor would hold the primordial-soup theory to be most likely, the nuances of the idea can be found today, located far below the reach of unaided divers in a shadowy region of the ocean known as the midnight zone. Devoid of sunlight, this eerie sector of the planet is home to a monstrous menagerie of organisms, such as the bioluminescent anglerfish and the gargantuan sixgill shark. Near the bottom of this mysterious realm lies the secret to our question -- a region known to geologists and marine biologists as a hydrothermal vent. Often located near tectonic fissures or volcanically-active areas, these vents spill a steady stream of geothermally-heated water into the surrounding environment, carrying with it a slew of sulfur-rich materials such as hydrogen sulfide and carbonyl sulfide -- an important catalyst in the development of amino acids.

Near the vents lie a small ecosystem of crustaceans and archaebacteria, who feed on the sulfuric materials emanating from the vents. Though unsuited to actually live in the path of the scalding water, the creatures themselves feed on the raw materials leaking from them, along with whatever boiled corpses wash up near the spouts.

A simple, royalty-free diagram of the chemical processes involved in hydrothermal vents and the substances they release into the surrounding water.

According to geologic and biological records, some of the first organisms on Earth were those of the methanotrophs and sulfur-oxidizing bacteria, creatures that fed on the organosulfur molecules of the hydrothermal vents, expelling from them sulfuric acid and carbon dioxide. As these organisms spread across the seafloor, engaging in the first of the biosphere’s many diasporas, some rose to the surface, developing unique systems by which to metabolize carbon dioxide and release oxygen. This evolutionary landmark, which began with cyanobacteria, triggered the Great Oxygenation Event, a geologic period of time during which aerobic microbes gradually inundated the lower atmosphere with diatomic oxygen, leading to one of the worst mass extinction events in the Earth’s history.

Lucky for us, though, life learned to survive, as prokaryotic organisms then evolved past the need for primordial forms of respiration, eventually combining oxygen and glucose to form the versatile molecule known as ATP. With this blueprint, our microbial ancestors could then begin to flourish and move past the Oxygenation Event, slowly developing to the point of endosymbiosis, wherein multiple cells merged and formed the first organelles. From here, the first eukaryotes are believed to have emerged, from which all complex life on the planet subsequently evolved -- from worms and fish to birds and humans.

With this refined version of the “primordial soup” theory, scientists can further postulate how life might form on nearby worlds, such as Europa or Titan. With the presence of hydrothermal activity comes the distribution of essential materials into a water-based medium, which -- given the right salinity and temperature, could lead to a familiar form of biochemistry across the cosmos. Alternatively, this activity could manifest itself in completely different mediums, like ammonia or methanol, within which entirely new forms of life might arise -- forms that will be explored very soon in an upcoming writing project. For more information, contact Billy Bernfeld or your local biology/chemistry teacher.