First life form on Earth may have bred in ponds, finds study

A new study has found that first life form on Earth may have bred in ponds than in oceans.

In the study published in the journal Geochemistry, Geophysics, Geosystems, researchers have reported that shallow bodies of water with 10 centimetres depth could have held a high concentration of nitrogen, a key factor for life on earth.

According to the study, nitrogenous oxides in the shallow ponds had a good chance of reacting with other compounds to give rise to the first living organism. However, in deeper oceans, it was harder for nitrogen to establish a significant, life-catalysing presence.

"Our overall message is, if you think the origin of life required fixed nitrogen, as many people do, then it's tough to have the origin of life happen in the ocean. It's much easier to have that happen in a pond," said lead author Sukrit Ranjan.

 

If primitive life indeed sprang from a key reaction involving nitrogen, there are two ways in which scientists believe this could have happened.

The first hypothesis involves the deep ocean, where nitrogen, in the form of nitrogenous oxides, could have reacted with carbon dioxide bubbling forth from hydrothermal vents, to form life's first molecular building blocks.

The second nitrogen-based hypothesis for the origin of life involves ribonucleic acid (RNA) a molecule that today helps encode our genetic information.

In its primitive form, RNA was likely a free-floating molecule. When it got in contact with nitrogenous oxides, some scientists believe, RNA could have been chemically induced to form the first molecular chains of life. This process of RNA formation could have occurred in either the oceans or in shallow lakes and ponds.

Nitrogenous oxides were likely deposited in bodies of water, including oceans and ponds, as remnants of the breakdown of nitrogen in Earth's atmosphere. Atmospheric nitrogen consists of two nitrogen molecules, linked via a strong triple bond that can only be broken by an extremely energetic event namely, lightning.

"Lightning is like a really intense bomb going off. It produces enough energy that it breaks that triple bond in our atmospheric nitrogen gas, to produce nitrogenous oxides that can then rain down into water bodies," Ranjan said.

Scientists believe that there could have been enough lightning crackling through the early atmosphere to produce an abundance of nitrogenous oxides to fuel the origin of life in the ocean.

However, in this new study, Ranjan identified two significant 'sinks', or effects that could have destroyed a significant portion of nitrogenous oxides, particularly in the oceans. He and his colleagues looked through the scientific literature and found that nitrogenous oxides in water can be broken down via interactions with the sun's ultraviolet light, and also with dissolved iron sloughed off from primitive oceanic rocks.

Ranjan said, "Both ultraviolet light and dissolved iron could have destroyed a significant portion of nitrogenous oxides in the ocean, sending the compounds back into the atmosphere as gaseous nitrogen."

In the ocean, ultraviolet light and dissolved iron would have made nitrogenous oxides far less available for synthesising living organisms. In shallow ponds, however, life would have had a better chance to take hold. That's mainly because ponds have much less volume over which compounds can be diluted. As a result, nitrogenous oxides would have built up too much higher concentrations in ponds. Any 'sinks' such as UV light and dissolved iron, would have had less of an effect on the compound's overall concentrations.

Ranjan said, "The more shallow the ponds, the greater the chance nitrogenous oxides would have had to interact with other molecules, and particularly RNA, to catalyse the first living organisms."

"These ponds could have been from 10 to 100 centimeters deep, with a surface area of tens of square meters or larger. They would have been similar to Don Juan Pond in Antarctica today, which has a summer seasonal depth of about 10 centimeters," said Ranjan.

That may not seem like a significant body of water, but Ranjan said that's precisely the point: In deeper or larger environments, nitrogenous oxides would simply have been too diluted, precluding any participation in origin-of-life chemistry.

Other groups have estimated that, around 3.9 billion years ago, just before the first signs of life appeared on Earth, there may have been about 500 square kilometres of shallow ponds and lakes worldwide.

"That's utterly tiny, compared to the amount of lake area we have today. However, relative to the amount of surface area prebiotic chemists postulate is required to get life started, it's quite adequate," Ranjan said.

The debate over whether life originated in ponds versus oceans is not quite resolved, but Ranjan said the new study provides a convincing piece of evidence.