The collision was so violent it boiled off the top layer of that ocean and, near the impact site, created a tsunami as high as a New York skyscraper. Molten rock rained from the sky. The atmosphere was choked with ash and dust. The planet descended into darkness.
But it wasn’t all bad.
That was the counterintuitive implication of three presentations at the American Geophysical Union’s fall meeting in San Francisco this month. The scientists, working independently, found that the bludgeoning of young Earth by giant space rocks may have primed the planet to be more congenial to life.
Calamity is followed by opportunity: There is a lesson here for everyone, written in the geophysical record. Earth has been through hell and back — smashed, melted, frozen, depending on which eon or epoch is under discussion.
But the first couple billion years of Earth’s history remain hazy. No one knows how, amid the chaos and tumult of that time, life emerged and evolved into more complex organisms.
The story of how this planet became habitable is relevant to the young science of astrobiology, which probes the hypothetical existence of life on other worlds. Our own solar system has abundant examples of planets and moons that are inhospitable. Earth is an anomaly, riotous with living things. What happened to make it so?
As researchers probe the distant past, giant impacts loom large.
“These giant impacts certainly have a negative effect on the atmosphere. If you put a lot of dust in the atmosphere, it gets very dark, and you’re boiling off a lot of the ocean,” Nadja Drabon, an early-Earth geologist at Harvard who presented her research at the AGU meeting, said in an interview.
“At the same time, these negative effects probably didn’t last very long,” she said. And in their wake, conditions for supporting life improved, at least for some period of time.
For example, the tremendous tsunami that was probably created by the impact 3.26 billion years ago would have mixed up the water column in the global ocean and brought more iron, a key nutrient for metabolism, to shallow seas. Other nutrients would have eroded into the ocean from land masses, according to Drabon.
Nick Wogan, a scientist at NASA Ames Research Center who presented at AGU, described how early impacts would have given Earth a “hot steam atmosphere” but also might have triggered life-relevant chemistry.
“Early impacts could have been positive for early life in a number of different ways,” said Simone Marchi, a planetary scientist at the Southwest Research Institute in Boulder, Colo., who also presented at the AGU meeting. “They could have delivered, for instance, key elements or molecules that were missing in the near-surface that perhaps could have been important for early life.”
Young Earth is a mystery because it left little record of itself, and scientists must conjure it with scraps of evidence and clever computer models.
It was a water world, the models suggest. Life was scarce and primitive, and nothing remotely resembling an animal had come into being. Scientists investigating the chemistry of the early atmosphere aren’t sure if the sky was blue or orange.
“Probably there was some orange haze, maybe like Los Angeles in the 1980s,” said John Tarduno, a geophysicist at the University of Rochester.
“You can think of the early Earth as an alien planet,” Marchi said.
It was also a planet under assault from pretty much the day it was born. Early Earth endured a steady bombardment for hundreds of millions of years, including a collision with a Mars-size object that led to the formation of the moon.
Those impacts are key to the planet’s biography. They helped form Earth itself, adding mass. The battering gradually eased as the clutter was cleaned out, but the planet remained vulnerable to globally catastrophic impacts.
Scientists have given the event 3.26 billion years ago the parsimonious name of “S2.” It was among the biggest of the 16 major impacts from 3.5 billion to 2.4 billion years ago that have been identified by scientists, Marchi said.
These events left behind no craters. But crafty scientists can find their signatures in thin layers of sedimentary rock. The “S” of S2 stands for spherules, tiny glassy droplets that rained from the sky after the impact. The spherules and other debris from the impact form a distinct layer in sedimentary rocks that, like a detective’s chalk outline, carry the narrative of a murderous moment.
There wasn’t much to be slain in those early eons, however. The great leap forward came with the evolution of bacteria that pumped oxygen into the atmosphere. Later came organisms that leveraged that oxygen as metabolic fuel. Only in the past billion years did life become multicellular, and the first things that could creep across the sea floor — the first creatures we could call animals — appeared less than 600 million years ago.
Would life on Earth and its endless forms most beautiful have happened without early impacts? No one can say. When they happened there were no people around, or dinosaurs, or even jellyfish — just bacteria.
But microbes were potentially well-positioned to benefit from catastrophe. They are remarkably hardy, capable of surviving under extreme conditions, such as in the hydrothermal pools of Yellowstone or around hot vents at the bottom of the sea.
“Back then, life was basically very, very simple organisms. So perhaps that’s why they managed to survive,” Marchi said. “If we had evolved life like you and I back then, we’d probably have been extinct.”
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