The first life may have arisen in ponds, not oceans

A new study from MIT reveals that primitive ponds may have created an environment conducive to brewing the earliest forms of life on Earth, more than the oceans.

The researchers reported that shallow water masses, on the order of 10 centimeters deep, could have contained high concentrations of what many scientists believe to be a key ingredient in life on Earth: l & # 39; nitrogen.

In shallow ponds, nitrogen, in the form of nitrogen oxides, would have had a good chance of accumulating enough to react with other compounds and give rise to first living organisms. In the much deeper oceans, nitrogen would have had more trouble establishing a meaningful, life-catalyzing presence, researchers say.

"Our message is this: If you think that the origin of life requires fixed nitrogen, as many people do, it's hard to make sure that the origin of the Life is happening in the ocean, "says lead author Sukrit Ranjan, a postdoc in the MIT Department of Earth. Atmospheric and Global Sciences (EAPS). "It's a lot easier than it happens in a pond."

Ranjan and his colleagues published their results today in the journal Geochemistry, geophysics, geosystems. Co-authors of the article are Andrew Babbin, Doherty Assistant Professor of Ocean Utilization in EAPS, as well as Zoe Todd and Dimitar Sasselov of Harvard University, and Paul Rimmer of the University. from Cambridge.

Break a link

If primitive life actually resulted from a key reaction involving nitrogen, scientists believe that this could have happened in two ways. The first hypothesis concerns deep oceans, where nitrogen, in the form of nitrogen oxides, could have reacted with the carbon dioxide that emanates from hydrothermal vents to form the first molecular building blocks of life.

The second hypothesis on the origin of life based on nitrogen involves RNA – ribonucleic acid, a molecule that today helps to code our genetic information. In its primitive form, RNA was probably a free-floating molecule. Some scientists believe that RNA could have been chemically induced to form the first molecular chains of life in contact with nitrogen oxides. This process of RNA formation could have occurred in the oceans or in shallow lakes and ponds.

Nitrogen oxides have probably been deposited in waters, including oceans and ponds, as remnants of the degradation of nitrogen in the Earth's atmosphere. Atmospheric nitrogen consists of two nitrogen molecules, linked by a strong triple bond, which can only be broken by an extremely energetic event, namely lightning.

"Lightning is like a very intense bomb going off," says Ranjan. "It produces enough energy to break this triple bond in our atmospheric nitrogen gas, to produce nitrogen oxides that can then be deposited in the water."

Scientists believe that there could have been enough lightning cracks in the primitive atmosphere to produce an abundance of nitrogen oxides in order to fuel the origin of life in the oceans. Ranjan says the scientists have assumed that this nitrogen oxide supply generated by lightning was relatively stable once the compounds had entered the oceans.

However, in this new study, he identifies two significant "sinks", or effects that may have destroyed a significant portion of the nitrogen oxides, particularly in the oceans. He and his colleagues studied the scientific literature and discovered that nitrogen oxides in the water could be broken down via interactions with ultraviolet light from the sun, as well as with dissolved iron removed from the primitive oceanic rocks.

Ranjan says that ultraviolet light and dissolved iron may have destroyed a significant portion of the nitrogen oxides in the ocean, returning compounds to the atmosphere as gaseous nitrogen.

"We have shown that if you include these two new wells that people have not thought of before, it would reduce by 1,000 times the concentration of nitrogen oxides in the ocean, by compared to what people were calculating before, "says Ranjan.

"Build a cathedral"

In the ocean, ultraviolet light and dissolved iron would have made nitrogen oxides much less available for the synthesis of living organisms. In shallow ponds, however, life would have had a better chance of becoming established. This is mainly due to the fact that the basins have much less volume on which the compounds can be diluted. As a result, nitrogen oxides would have accumulated much higher concentrations in ponds. All "wells", such as ultraviolet rays and dissolved iron, would have had a lesser effect on the overall concentrations of the compound.

According to Ranjan, the smaller the pond, the more likely it is that the nitrogen oxides had to interact with other molecules, including RNA, to catalyze the early organisms. living.

"These ponds could have a depth of 10 to 100 centimeters and an area of ​​several tens of square meters or more," says Ranjan. "They would have been similar to the Don Juan Pond in Antarctica, whose seasonal depth is about 10 centimeters in summer."

This may not seem to be a significant body of water, but it says it's precisely the goal: In deeper or larger environments, the nitrogen oxides would have simply been too dilute, excluding any involvement in the chemistry of the origin of life. Other groups estimated that about 3.9 billion years ago, just before the first signs of life on Earth appeared, about 500 square kilometers of ponds and shallow lakes in the world.

"It's quite tiny, compared to the extent of the lake we have today," Ranjan says. "However, in relation to the surface required for prebiotic chemists, the postulate is necessary for life to begin, it is entirely adequate."

The debate over whether life began between ponds and oceans is not entirely resolved, but Ranjan says the new study provides convincing evidence for the first.

"This discipline is less like overthrowing a row of dominoes, but rather building a cathedral," says Ranjan. "There is not really any moment" aha ".This is more like a patient construction from one observation to the other, and the picture that is emerging is that in there is not a moment. together, many prebiotic synthetic pathways seem to be chemically easier in ponds than in oceans. "