Evidence of life in 3.48-billion-year-old Australian rocks pushes back the earliest known terrestrial life by billions of years. The discovery also supports the theory that life evolved on land, not in the sea.
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The fossils were found in the Pilbara region in northwestern Australia, famous mainly for its iron-rich red earth and ancient rock formations, reports the team in Nature Communications.
Even older evidence was reported to have been found in Canadian rocks from the primordial deep sea. But that dating is contested, says Dr. Martin Van Kranendonk, director of the Australian Centre for Astrobiology and head of the UNSW school of Biological, Earth and Environmental Sciences. Also, the evidence provided of life, which would have developed by deep-sea vents on the seabed, is controversial, he says.
As for claimed hints of life found in 3.8-billion year old rocks in Greenland, well, it's a theory.
In the Pilbara hot spring deposits, the scientists discovered layers of fossilized bacterial mats, called stromatolites. The rock also had well-preserved bubbles that are thought to have been trapped in a sticky substance of microbial origin.
Van Kranendonk explains why life developed on land, in the opinion of a growing community of scientists.
Freshwater hot springs, or deep-sea vents
Earth itself is believed to be around 4.6 billion years old, and to have spent a couple of hundred million years cooling down to the point where water could condensate on its surface and life could even begin to develop without getting fried.
Previously, the earliest known life found on land were cells from 2.7 to 2.9 billion years old, detected in deposits in South Africa.
The 3.48-billion-year-old rocks in the Pilbara had been part of a terrestrial hot springs system. The scientists didn't find cells: they found "potential biological signatures" in rocks.
How do they know the rocks formed on land, not in the ocean? The stone contained geyserites, a mineral deposit that forms under conditions of near boiling, silica-rich, water found only in terrestrial hot spring environments.
Previously, the oldest known geyserite had been identified from 400-million-year-old rocks. In other words, the team not only pushed back the age of terrestrial life by half a billion years. The team has pushed back the oldest know terrestrial geyser by 3 billion years, Van Kranendonk says. "It's a paradigm shift," he told Haaretz. "Our discovery opens up a whole new window for understanding where life may have originated."
He also points out that life could have started way, way earlier than even this discovery indicates, it's just that we haven't found the evidence yet.
Nobody's arguing that water wasn't necessary for life to evolve. It was. But seawater or fresh?
One factoid supporting the terrestrial-evolution theory is cellular cytoplasm has a distinct ratio of salts, like potassium and sodium, that is not like seawater. It's like freshwater. Even the most primitive of cells have the cytoplasmic ratio of freshwater.
For that matter seawater is too salty for organic molecules to functionally survive, so it was unlikely to enable the development of pre-biotic chemistry, Van Kranendonk points out.
Another argument in favor of life developing in freshwater ponds on land, not by hydrothermal vents in the sea, is that the development of long, complicated organic molecules like RNA and DNA need the power of wetting and drying cycles, says Van Kranendonk, and explains.
Research by other scientists on "organic soups" – freshwater mixed with small organic molecules - finds that under the type of conditions characterizing deep-sea vents, the molecules never get very long or complex, he says.
"But on the surface, given a warm little pond that undergoes cycles of evaporation and drying, the organic molecules at the edge of the pond expel their water, which forces them together and makes them long and complex," says Van Kranendonk.
"We know hot springs on land have wetting and drying cycles, as much as 10 to 20 times a day. Old Faithful [in Yellowstone, the U.S.] is like that, for instance. The edge of the pool expands with each eruption," he says.
May still adhere to the deep-sea hypothesis, not least because of the compelling presence of life, including impressively large arthropods, in the seemingly uninviting environment of deep-sea vents. But the ocean is actually an extreme environment, says Van Kranendonk.
"The discovery that we found ancient life in a hot spring terrestrial environment is incredibly exciting, because it shows for first time that these environments existed on early earth. We didn't know that before. The earliest hot spring deposit we knew about before was only about 400 million years old," says Van Kranendonk. Our discovery pushes that back 3 billion years, and has signs of life in it."