Life on Earth two billion years ago was literally pond scum, but from low-lifes like this, all other life forms we know have evolved. In fact, it is known that these simple, single celled proto-plants were forced to develop from opportunistic nitrogen-eaters to diligent nitrogen-makers during the Archaean age.
How this critical evolutionary leap happened has been a mystery until now: Researchers at NASA Ames Research Center and the Universidad Nacional Autonoma de Mexico (UNAM) found that atmospheric changes in the ancient air forced bacteria to be more self-sufficient, more productive, and in the end, more adaptable to other environments.
And the finding has implications for astrobiology and questions about when and how life arose on our planet, and elsewhere.
"We are used to thinking of the environment and life as steady and unchanging," said Chris McKay, one of the NASA researchers on this project. "But the early Earth was quite different. Major changes in the atmosphere occurred and life had to adapt. To me the interesting follow-up question is: Did it happen on other planets, too?"
Earth was very different during the Archaean age. Instead of the atmosphere containing 80 percent nitrogen and 20 percent oxygen as it does now, the air instead was closer to a 50-50 mix of carbon dioxide and nitrogen.
Along with carbon dioxide, water and light, plants need nitrates to survive. The ancient, higher levels of atmospheric nitrogen provided more than enough nitrates for the plants because lightning, it was theorized by the Ames/UNAM researchers, catalyzed an atmospheric carbon dioxide and nitrogen reaction that made nitrate foods for Archaean-era life forms.
This changed when carbon dioxide levels dropped dramatically. Ancient soil samples show this drop lasted over 100 million years during the Archaean Era, drying up the nitrate supply along the way. This had to have forced the plants to find a way to make nitrogen on their own.
An atmospheric reenactment
McKay said the cause of the carbon dioxide drop is still a mystery, but that its effect -- the loss of nitrates in the environment -- is no longer thanks to the Ames/UNAM experiment.
"The nitrate fertilizers were once made naturally, but that was depleted, so plants were forced to make it on their own. Plants were forced to develop the capacity of developing their own fertilizer," said McKay.
To figure this out, the researchers simulated all possible Archaean atmospheric combinations of carbon dioxide and nitrogen in the lab. A high-powered laser was used to simulate lighting. The results of this reenactment were published in the July 5 issue of the journal Nature.
The experiment proved that lightning really did produce nitrates in the early atmosphere, and more importantly, explained McKay, as carbon dioxide levels were lessened, nitrate production also went down. The experiment proved plants had to have gone through what McKay calls an evolutionary "choke point," a precipitous and life-threatening drop in fuel.
"This would have been a long time compared to the life of a plant, so the drop in carbon dioxide levels would have forced life to change," said McKay.
What doesn't kill you
While there is no direct evidence that Archaean plants actually did change, McKay said, "the system obviously didn't die."
This advance in plant evolution would have allowed for plants to colonize more environments on Earth, he said, and this proliferation eventually raised oxygen levels in the air and made an environment suitable for animals later.
"Once life invented the ability to fix (produce) nitrogen, life's new invention made it stronger," he said. "When life responds to changes, that life is stronger than before."
As far as this sort of evolution occurring in our solar system, McKay said that Saturn's moon Titan, the best contender, is just too cold for life to exist.
"But on other plants around other stars, life would probably need nitrogen as well, and may need to 'invent' nitrogen fixation," said McKay.
But if other planets experience a similar decline in carbon dioxide levels, the invention of nitrogen fixing would likely follow. This, McKay said, would help life grow into alien multicellular organisms and eventually a global biosphere.
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