Ancient Climate on Earth Reveals Origins of Life--But Not with Nitrogen

Why did life evolve when it did on Earth? That's a mystery that scientists have been trying to piece apart for years. Now, this riddle may have gotten a bit more complicated. Researchers have now ruled out a theory as to why the planet was warm enough to sustain Earths' earliest life forms when the Sun's energy was roughly three-quarters of the strength it is today.

Life is estimated to have evolved on our planet during the Archean, a time between 3.8 and 2.4 billion years ago. At the time, though, the weak Sun should have meant that the planet was simply too cold for life to take hold. That's why researchers have tried to find an explanation for how life was possible.

"During the Archaen the solar energy received at the surface of the Earth was about 20 to 25 percent lower than present," said Ray Burgess, one of the researchers, in a news release. "If the greenhouse gas composition of the atmosphere was comparable to current levels then the Earth should have been permanently glaciated, but geological evidence suggests there were no global glaciations before the end of the Archean and that liquid water was widespread."

Scientists have come up with a theory, though. It's possible that greenhouse gas levels were significantly higher during the Archean than they are today. This would mean that the carbon dioxide in the atmosphere could counter the effect of the weaker sun. That said, the gas levels would have had to be as much as 1,000 times higher than they are at present. Fossil evidence shows that, in fact, the levels weren't this high.

Another theory, though, was that the amount of nitrogen could have been higher in the ancient atmosphere. This would have amplified the greenhouse effect of carbon dioxide and would have allowed the Earth to remain ice-free. In order to test this theory, the scientists analyzed tiny samples of air trapped in water bubbles of quartz.

"We measured the amount and isotopic abundances of nitrogen and argon in the ancient air," said Bernard Marty, one of the researchers, in a news release. "Argon is a noble gas which, being chemically inert, is an ideal element to monitor atmospheric change. Using the nitrogen and argon measurements we were able to reconstruct the amount and isotope composition of the nitrogen dissolved in the water and, from that, the atmosphere that was once in equilibrium with the water."

The findings revealed that, in fact, excess nitrogen couldn't have been the cause of the warming. It was far too low in the atmosphere at the time to enhance the greenhouse effect of carbon dioxide. While this doesn't explain exactly how the warming occurred, though, it does rule out one possibility.

The findings are published in the journal Science.