It’s one of life’s most profound and puzzling mysteries: how on earth did life get started from simple, inanimate matter?
We’ve known for some time that it must have been sparked by a chemical reaction, but it’s never been clear what triggered it, and from where the energy required for those reactions was drawn.
Now, researchers at the Heinrich Heine University Düsseldorf (HHU), Germany, say they’ve found the smoking gun. The secret ingredient that fuelled the origins of life is the modern era’s cleanest, greenest energy carrier: hydrogen gas (H2).
The team, led by William Martin at the Institute for Molecular Evolution at HHU, runs chemical experiments to investigate reactions between hydrogen and carbon dioxide (CO2), using catalysts and conditions found in submarine hydrothermal vents. It also uses a computational method known as molecular archaeology to analyse the traces of primordial life found in the proteins and DNA of modern cells.
For this particular study, published today in Frontiers in Microbiology, the researchers wanted to understand what kind of chemical environment fostered the earliest metabolism – the processing of energy that keeps living things alive. So they looked at the Last Universal Common Ancestor, or LUCA, of all living things on Earth.
The team identified 402 metabolic reactions that have been virtually unchanged since the origins of life some four billion years ago, and which all occur among some of the present day’s simplest life forms – bacteria and archaea. By looking at these extraordinarily well-conserved reactions, PhD researcher and lead author on the study Jessica Wimmer was able to piece together LUCA’s metabolic activity – peering through the veil at the functioning of primordial life billions of years in the past.
“We wanted to know where the energy came from that drove primordial metabolism forward,” says Wimmer. “At the very onset of metabolic reactions some four billion years ago, there were no proteins or enzymes to catalyse reactions because they had not yet evolved.
“Metabolism had to arise from reactions that could take place in the environment, perhaps with help from inorganic catalysts. But catalysts or not, in order to go forward, the reactions have to release energy. Where did that energy come from?”
Wimmer says no one had thought to look at the metabolic reactions themselves to find the source of catalytic energy.
To find the source of energy in these primordial metabolic reactions, the team calculated the amount of free energy – also known as Gibbs energy – released in each reaction.
What they found was startling. LUCA didn’t need UV light, meteorite impacts, volcanic eruptions or radioactivity to spark the earliest earthly metabolism. Instead, the energy for LUCA’s reactions came from its own metabolism. What that means is that LUCA’s metabolic reactions liberated its own energy, and this probably occurred in an environment much like modern-day, submarine hydrothermal vents.
“That is exciting,” says Martin, “because the 402 interconnected reactions of central metabolism, which seem so hopelessly complex upon first encounter, suddenly reveal a natural tendency to unfold all by themselves under the right conditions.”
It means that the origin of life is perhaps less complex than we once thought. All it needed was the right chemicals to be present, and the right set of local conditions. Computer analysis of the 402 key reactions found they occur optimally in regions with an alkaline range around pH 9, and a temperature of 80°C – much like hydrothermal vents in the ocean, which produce abundant H2.
“This is almost exactly what we see at Lost City, a H2-producing hydrothermal field in the Mid-Atlantic,” Martin says. “In an environment like that, about 95 to 97% of LUCA’s metabolic reactions could go forward spontaneously – that is, without the need for any other source of energy.
“In the abyssal darkness of hydrothermal systems, H2 is chemical sunlight.”
Hydrogen is crucial in these reactions.
“Without hydrogen, nothing happens at all, because hydrogen is required to get carbon from CO2 incorporated into metabolism in the first place,” says Wimmer.
So the energy needed to ignite that initial spark of life is really quite simple.
“We need no sunlight, no meteorites, no UV light, just H2 and CO2, plus some ammonia and salts,” Wimmer says.
Martin muses that humanity, now starting to harness the energetic power of hydrogen, is in many ways going back to its roots. “Modern energy research exploits exactly the same properties of hydrogen as life does,” he says. “It is just that life has four billion years of experience with hydrogen technology, while we are just getting started.”