Evolution is the generally-accepted explanation for how life on Earth became so complex, but there's one thing it can't explain – how life emerged from non-living matter in the first place. The prevailing hypothesis is that life got started in the ocean, where hydrothermal vents provided just the right chemical reactions. But a new MIT study has found that ancient oceans probably didn't have enough nitrogen – but shallow ponds might have.
Nitrogen is often credited as a key part of the transition from non-life to life. The story goes that when nitrogenous oxides and primitive RNA mixed, the RNA was chemically induced to start forming the first amino acids, which then developed into the first simple organisms. These nitrogenous oxides are thought to have rained down on the surface after lightning strikes split the bonds of nitrogen in the atmosphere.
But according to the new study, nitrogenous oxides likely wouldn't have lasted long enough to reach the deep-sea hydrothermal vents, and ultimately kickstart life. The team found two previously-overlooked factors that break down nitrogenous oxides in water: ultraviolet light from the Sun, and dissolved iron from rocks.
"We showed that if you include these two new sinks that people hadn't thought about before, that suppresses the concentrations of nitrogenous oxides in the ocean by a factor of 1,000, relative to what people calculated before," says Sukrit Ranjan, lead author of the study.
So if the oceans didn't have enough nitrogen in one place, where did life get started? According to the MIT team, shallow ponds are better candidates. Because there's simply less volume for the compounds to be diluted across, higher concentrations of nitrogenous oxides could build up in these ponds, giving them a better chance of interacting with molecules like RNA.
"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," says Ranjan. "It's much easier to have that happen in a pond. These ponds could have been from 10 to 100 cm (3.9 to 39 in) deep, with a surface area of tens of square meters or larger."
The idea that life didn't arise in the deep ocean isn't a complete game-changer – it's been hypothesized many times before that life could have gotten started in shallow ponds or hot springs. The researchers admit that the latest study doesn't resolve the debate, but it does provide an intriguing new piece of evidence to sway the argument towards life emerging from a shallower bowl of primordial soup.
The research was published in the journal Geochemistry, Geophysics, Geosystems.
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