Somehow, on this beautiful blue marble we call Earth, the astonishing phenomenon we call life emerged long ago, spreading until it covered nearly every corner of the planet.
One school of thought proposes that the intense asteroid bombardment Earth endured around 4 billion years ago played a role in that process – that without all those space rocks crashing into our world, we might not be here today.
Now, a new discovery in South Korea suggests that the asteroid effect may have been even more complex than we realized.
Beneath a crater gouged by a massive impact around 42,000 years ago, a team led by geologist Jaesoo Lim of the Korea Institute of Geoscience and Mineral Resources (KIGAM) has identified several stromatolites.
Stromatolites are layered structures built by microbial mats, similar to some of the oldest known evidence of life on Earth.
This discovery suggests that the heat generated by the impact may have created a long-lasting hydrothermal environment similar to hot springs, where microbial communities could thrive.
It's possible that during the era of heavy bombardment, billions of years ago, impact craters like these may have created countless temporary refuges for early life across the young Earth.
The story of life's origins is a murky one; it remains unclear exactly when and how non-living components came together in a way that triggered the processes that define biology.
One major clue, however, can be found in stromatolites.
In several places around the world, these structures – the layered mineral scaffolding built by microbes such as cyanobacteria and other microorganisms, similar to the calcium-carbonate bones of corals – have been found dating back as far as 3.5 billion years ago.
That's some of the oldest evidence for life our planet has proffered.
But there's a lot we don't know about how these communities emerged and spread. Figuring that out is sort of like trying to mentally picture a 1,000-piece puzzle with only seven pieces.
The Jeokjung-Chogye Basin in Hapcheon may have just added another handful of pieces by contextualizing discoveries linked to impact craters such as Chicxulub, where evidence of microbial mats had been interpreted as material swept into the crater rather than communities that formed there naturally.
Although the basin is a well-known bowl-shaped feature in the landscape of the Korean peninsula, its identity as an impact structure wasn't known until relatively recently, as revealed in a 2021 paper.
Subsequent analyses showed the mineral signatures of meteoritic material merging with the terrestrial material in the basin, reverse-engineered its shape to understand how the impact took place, used radiocarbon techniques to figure out when it formed, and determined that it once held a vast body of water.
Now, digging under the northwestern part of the crater, Lim and his colleagues found multiple stromatolites, measuring between 10 and 20 centimeters (4 to 8 inches) in diameter.
It has already been established that an impact crater can shatter and heat Earth's crust where it falls, creating a system where the slowly dissipating residual heat warms water that fills the basin left behind – a hydrothermal impact lake.
These stromatolites, the researchers found, probably formed in just such an environment.
The team analyzed their mineral content and found traces of an element called europium that becomes dramatically more soluble in hot hydrothermal fluids.
Europium is usually interpreted as a signature of past hydrothermal activity; it represents a strong signal suggesting the lake that once filled the Jeokjung-Chogye Basin was hydrothermal in nature.
Other signatures support this interpretation, such as high levels of calcium, calcite, and sulfur associated with microbes adapted to hot environments, which were found in the sediment.
The stromatolites, radiocarbon dating of one sample suggested, formed between about 23,400 and 14,600 years ago. That indicates that the hydrothermal lake was present for a few tens of millennia.
It also offers a snapshot into how early Earth may have been primed for life.
The finding shows that an asteroid impact can accidentally create the perfect spa retreat for microbes.
If early Earth was heavily cratered by asteroid impacts during the early bombardment before the inner Solar System settled down, it could have hosted many such havens.
And here's where it gets even more interesting.
Early Earth did not have a lot of oxygen before about 2.4 billion years ago. Scientists think the rise of the first photosynthetic lifeforms, such as cyanobacteria, was at least partially responsible for the air we now breathe today.
There's also evidence to suggest that oxygen may have been a byproduct of the microbial metabolism that built stromatolites.
If that's the case, early bombardment may have created pockets of oxygen production worldwide – what the researchers call "oxygen oases".
"This is the first comprehensive evidence suggesting that stromatolites could form in hydrothermal lakes created by asteroid impacts," Lim says. "Such environments may have provided favorable conditions for early microbial ecosystems."
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This is a little interpretive – the current data are far from proof of what role, if any, stromatolites played in Earth's oxygenation.
But the discovery does further indicate that life on Earth may have arisen through a combination of relatively rare ingredients and events we have yet to find anywhere else in the Universe.
Other impact craters on Earth need to be investigated closely to determine whether stromatolite-rich hydrothermal lakes may have contributed to the rise of oxygen in Earth's atmosphere.
Further, this result suggests that there is still hope to find similar signatures elsewhere. Impact craters on Mars, for example, may be harboring the buried remains of stromatolites, yet to be uncovered.
The research has been published in Communications Earth & Environment.