New Discovery Undercuts Origin of Life at Hydrothermal Vents
Most people now recognize that smoking takes years off of one’s life. But if it wasn’t for smokers, life’s start wouldn’t be possible—at least according to some origin-of-life researchers.
One prominent origin-of-life model posits life’s beginnings at deep-sea hydrothermal vents, sometimes referred to as “black smokers.” These structures result when superheated water pushes through the Earth’s crust on the ocean floor to form a sea vent. The water that spews out of the vent is rich in dissolved minerals including sulfides. When the superheated water comes into contact with cold water surrounding the sea vent, it causes the metal sulfides to precipitate. These precipitates look like black smoke emanating from the sea floor. Over time as the sulfides deposit around the vent, a chimney-like structure forms.
A complex, rich ecosystem is associated with black smokers. Some bacteria and archaea thrive near hydrothermal vents. Here the microbes make use of gases like methane and the sulfides through a process called chemosynthesis to generate foodstuff. These chemoautotrophs serve as the primary producers in the hydrothermal vent communities.
Since the discovery of thriving microbial communities near underwater smokestacks, origin-of-life researchers have thought that perhaps life began in similar locales on early Earth. If life can live at hydrothermal vents today, why couldn’t it originate there as well? After all, the microbes thought to be representatives of some of the oldest organisms on Earth are hyperthermophiles and thermophiles, the types of (heat-loving) organisms found at these vents. Additionally, laboratory simulation experiments designed to mimic the chemical and physical conditions of hydrothermal vents have demonstrated that biologically interesting compounds could have been generated in these locales on early Earth. Presumably, these compounds could have formed a complex chemical mix that eventually spawned the first life-forms.
New research, however, indicates that this explanation for the origin of life is probably not correct.
This work took advantage of the typical biochemical makeup of nucleic acids, like DNA and RNA, and proteins of hyperthermophiles and thermophiles to determine (from an evolutionary standpoint) the identity of the last universal common ancestor.
The composition of these important biomolecules serves as a “molecular thermometer” of sorts. For example, hyperthermophiles and thermophiles possess nucleic acids more enriched in guanine and cytosine compared to those of organisms that live at more moderate temperatures. Likewise, the amino acid compositions of thermophilic proteins are depleted within certain amino acids. (For a detailed discussion of the principles behind this analysis go here.)
Using both thermometers, the researchers concluded that the last universal common ancestor must have lived at moderate temperatures. That is, it wasn’t thermophilic. This means that from an evolutionary vantage point, life’s origin probably didn’t take place at hydrothermal vents. (For more information on other problems associated with alternative origin-of-life explanations see Origins of Life.)
It looks like smoking wasn’t all that good for Earth’s first life either.