Primordial Haze Theory Competes with Primordial Soup in Debate over Life

Composite view of Titan built with Cassini images taken on Oct. 9 and Oct. 25, 2006. CREDIT: NASA/JPL/University of Arizona

Life on Earth may have originated high up in the atmosphere rather than in the surface waters of oceans or pools, researchers have found.

Scientists simulating possible chemical reactions occurring in the upper atmosphere of Titan, Saturn's largest moon, found that amino acids and nucleotide bases — the building blocks of life as we know it — could form without much prodding. A similar process may have taken place on Earth, they said.

Meanwhile, an unrelated study released today (Nov. 10) in the journal Nature suggests that Earth's atmosphere had enough oxygen to support complex life forms on the surface as early as 1.2 billion years ago. This study found evidence in ancient sediments in Scotland that the oxygen concentration of the atmosphere was sufficient.

Earth vs. Titan

While Titan and Earth aren't exactly twins — the Saturn moon is much colder, with average surface temperatures around minus 290 degrees Fahrenheit (minus 179 degrees Celsius) — they share a thick, nitrogen-rich atmosphere.

And long ago, Earth's upper atmosphere probably looked a lot like Titan's does today, many scientists think, so similar reactions could have taken place here.

"It's pretty likely there was a haze similar to Titan's before life existed," said researcher Sarah Horst, a University of Arizona graduate student, during a conference last month. "Probably it was a similar kind of gas mixture."

Horst presented her team's findings at the 42nd meeting of the American Astronomical Society's Division of Planetary Sciences, in Pasadena, Calif.

Conditions just right high up?

The primordial-soup idea posits that life on Earth took root in the planet's waters.

Complex molecules sloshing about in primeval oceans were broken apart and recombined, the theory goes, by some energy jolt coursing through the broth — perhaps lightning strikes. These reactions eventually gave rise to self-replicating molecules: life as we know it.

This may well have happened, Horst said, but her team's results suggest another plausible life incubator: Earth's upper atmosphere.

Big, complicated molecules, some containing 1,000 carbon atoms, are known to exist in the high altitudes of Titan and could have swirled about in Earth's upper air as well. They could have served as starter molecules for all sorts of interesting reactions, Horst said.

Plenty of energy to spur those reactions would have streamed into the upper atmosphere every day from the sun, she added. And high up, there may have been just the right mix of oxygen: enough to be incorporated into some newly forming molecules, but not enough to combust everything.

While we may never be certain how and where life took hold on Earth, studying Titan may help us make some inferences and educated guesses, scientists said.

On Titan, "we know this is happening, and we know we can study it further," said Roger Yelle of the University of Arizona, Horst's Ph.D. supervisor and a member of the research team.

Pushing the clock back

Wherever Earth's first organisms evolved, they were simple and remained so for a long time. Scientists think life first took root around 3.8 billion years ago or so but didn't really explode into a panoply of complex, multicellular organisms until oxygen levels in Earth's atmosphere increased substantially.

Until recently, researchers thought this oxygen boost occurred about 800 million years ago. But the new Nature study suggests that oxygen levels were high enough to support complex life much earlier.

The researchers found evidence in Scottish rocks that bacteria were using oxygen to generate energy and stay alive 1.2 billion years ago.

"Evidence of this chemical reaction tells us that the levels of oxygen in the atmosphere were at this key point for evolution, at this much earlier stage in Earth's history," study leader John Parnell of the University of Aberdeen said in a statement.

• Gallery: Saturn and Titan
• Exotic Life Could Sprout from Titan Chemistry
• Earth's Oxygen Could Support Complex Life Earlier Than Thought