By Robert F. Service from Science Magazine — Mar. 2, 2020 , 12:30 PM
Studies of the origin of life are replete with paradoxes. Take this doozy: Every known organism on Earth uses a suite of proteins—and the DNA that helps build it—to construct the building blocks of our cells. But those very building blocks are also needed to make DNA and proteins.
The solution to this chicken-and-egg conundrum may lie at the site of hydrothermal vents, fissures in the sea floor that spew hot water and a wealth of other chemicals, researchers report today. Scientists say they have found that a trio of metal compounds abundant around the vents can cause hydrogen gas and carbon dioxide (CO2) to react to form a collection of energy-rich organic compounds critical to cell growth. And the high temperatures and pressures around the vents themselves may have jump-started life on Earth, the team argues.
The new work is “thrilling,” says Thomas Carell, an origin of life chemist at Ludwig Maximilian University of Munich who was not affiliated with the new project. The organic molecules the study generated include formate, acetate, and pyruvate, which Carell calls “the most fundamental molecules of energy metabolism,” the process of converting nutrients into cell growth. The new results support a long-held idea about the origin of life known as “metabolism first hypothesis.” It posits that geochemical processes on early Earth created a stew of simple energy-rich compounds that drove the synthesis of complex molecules, which eventually provided the materials for Darwinian evolution and life.
A clue to this primordial metabolism came in 2016. Researchers led by William Martin, an evolutionary biologist at Heinrich Heine University of Dusseldorf, scanned the genomes of thousands of bacteria and archaea, identifying 355 proteins encoded by shared genes that likely belonged to a microbial Eve, the last universal common ancestor of all life. Those proteins suggest this primordial microbe thrived in scalding temperatures and ate hydrogen gas, using its electrons to convert inorganic CO2 dissolved in the ocean into energy-rich organic compounds. That supports the notion that the microbes lived near hydrothermal vents, where those conditions would have been present.
That idea is bolstered by the fact that modern organisms still combine hydrogen and CO2 to make organic molecules in a process known as the acetyl–coenzyme A (acetyl-CoA) pathway. This process feeds essential organic molecules into biochemical processes that drive the production of proteins, carbohydrates, and lipids, which is at the heart of energy metabolism in cells. The problem, however, is that modern organisms run the acetyl-CoA pathway using 11 enzymes made up of a combined 15,000 amino acids, all finely positioned to carry out their work. And without the right protein machinery or catalyst, if you put hydrogen and CO2 together, Martin says, “Nothing will happen.”
So how could organisms have spontaneously developed their prowess to run the acetyl-CoA pathway? Two years ago, researchers led by Joseph Moran, a chemist at the University of Strasbourg, suggested at least a partial answer. They reported that pure metals, including iron, nickel, and cobalt, could catalyze the reaction of water (water molecules contain hydrogen) and CO2 to form acetate and pyruvate, key members of the acetyl-CoA pathway. That finding suggests the earliest life could have simply fed on these organic compounds to get a toehold, and over time evolved a suite of proteins to make the reactions even more efficient.
Still, Martin notes, converting water and CO2 into needed organics isn’t how microbial Eve’s most closely related modern brethren do it. Rather, these organisms start with hydrogen gas and CO2. “We wanted to see if we could get this pathway to work without enzymes,” Martin says.
He and his colleagues knew hydrothermal vents continually spew out hydrogen gas, driven by reactions between water and metals deep below Earth’s crust. And researchers previously determined that CO2 in early Earth’s oceans was about 1000 times more abundant than it is today. So, Martin wondered whether metal-rich minerals common around hydrothermal vents could cause hydrogen to react with CO2.
To find out, Martin’s and Moran’s teams joined forces to investigate three iron-rich minerals found near vents: greigite, magnetite, and awaruite. They added these to a water solution and bubbled in hydrogen and CO2 at 100°C and 25 bars of pressure, conditions common around deep-sea vents. All three minerals catalyzed a reaction of hydrogen and CO2 to form a mix of organics including formate, acetate, and pyruvate, the group reports today in Nature Ecology & Evolution. “What we have here is a sustained source of chemical energy, and it generates these energy-rich molecules used in metabolism,” Martin says.
So, was this mix of organics life’s first meal? It’s a fair bet, says Steven Benner, a chemist at the Foundation for Applied Molecular Evolution. For evolution to begin, life would have needed both a food source and some form of protogenetic molecule to transmit information from one organism to its progeny. How they came together is still unclear. However, any early Darwinian system would need to feed. And, Benner says: “The process described by [Martin’s and Moran’s team] could certainly have been the source of some of their food.”