Russian hot springs point to rocky origins for life

 

Kamchatka peninsula, a perfect place for life (Image: Anna S. Karyagina)

It's a question that strikes at the very heart of one of the deepest mysteries in the universe: how did life begin on Earth? New evidence challenges the widespread view that it all kicked off in the oceans, around deep-sea hydrothermal vents.

Instead, hot springs on land, similar to the "warm little pond" favoured by Charles Darwin, may be a better fit for the cradle of life.

The controversial new theory suggests the search for extraterrestrial life must go beyond a hunt for alien oceans (see Land ho! The search for ET, below).

Life appeared sometime before 3.8 billion years ago, towards the end of a turbulent phase in our planet's early history dubbed Hadean Earth. Exactly where and how this happened is still a mystery. The first fossils are about 3.4 billion years old, and all we know about life's very first stages comes from chemical signatures in rocks.

This hasn't stopped endless speculation. Conventional wisdom has it that hydrothermal vents on the ocean floor offered an ideal chemical environment for the earliest life. Deep, dark oceans would also have protected the delicate cells from the harmful ultraviolet light that bathed early Earth before the ozone layer formed.

Case closed? Not quite. Armen Mulkidjanian at the University of Osnabrück in Germany says there is a fundamental problem with the ocean floor hypothesis: salt. The cytoplasm found inside all cells contains much more potassium than sodium. Mulkidjanian thinks that chemistry reflects the chemistry of the water life first appeared in, yet salty seawater is sodium-rich and potassium-poor.

"The ancient sea contained the wrong balance of sodium and potassium for the origin of cells," says Mulkidjanian. Now, after extensive field studies, he claims to have found the one place on Earth where that balance is right: in the thermal springs of Kamchatka in far-east Siberia. Mulkidjanian found that puddles condensing from the hydrothermal vapour at Siberia's Mutnovsky thermal springs are potassium-rich, just like cell cytoplasm (Proceedings of the National Academy of Sciences, DOI: 10.1073.pnas.1117774109). Life first appeared in similar pools, says Mulkidjanian.

And while early life would have been damaged if over-exposed to UVs, Mulkidjanian's theory solves another puzzle. Most evolutionary biologists agree that life at this stage would have been little more than floating strands of DNA and RNA. The nucleotides that make up DNA and RNA are all surprisingly stable when exposed to UV light, suggesting they evolved in an environment where UV exposure weeded out all but the most photostable molecules. "You don't get UV light around deep-sea vents," says Mulkidjanian.

"I do not think the oceans were a favourable environment for the origin of life – freshwater ponds seem more favourable," says Nobel laureate Jack Szostak at Harvard University, a key player in the field. "Freshwater ponds have lower salt concentrations, which would allow for fatty acid based membranes to form."

While Darwin's warm little ponds appear to be coming back in vogue, this is a highly polarised field of research and many origin-of-life researchers are not convinced. Nick Lane at University College London disputes the claims that the first cells couldn't cope with life in sodium-rich water. Early cells could have actively pumped out sodium ions, he says. "This is exactly what many methanogens and acetogens do," he points out, referring to microbes that are thought to be among the earliest cellular life forms. This, says Lane, is good evidence that the earliest living cells did indeed actively pump out sodium ions.

Carrine Blank, a geologist at the University of Montana in Missoula says life was unlikely to survive on land 3.8 billion years ago, at a time when meteorites were pummelling Earth. Mulkidjanian counters that some geologists now question whether the late heavy bombardment, as it is known, really happened at that time (Elements, DOI: 10.2113/gselements.5.1.23).

Others contacted by New Scientist labelled Mulkidjanian's ideas absurd and declined to comment. Undoubtedly, most researchers still favour the sea as the cradle of life. Still, Mulkidjanian is not alone in looking for a land-based alternative.

Paul Knauth, a geologist at Arizona State University in Tempe, also thinks life may not have begun in the sea – which he says has ramifications for the search for extraterrestrial life. He has analysed the oxygen isotopes in the silica-rich rocks deposited early in Earth's history, from which you can work out temperatures at the time the rocks formed. He says that the entire planet was much hotter than anyone suspected – surface temperatures of 50 to 80 0C may have been common. The seas were also twice as salty as today, because so-called "evaporitic" deposits - which locked away vast quantities of salt - had not begun to form. "The early ocean was a deathtrap of hot salty water," he says. "I like the idea of a non-marine origin."

Then there is the fossil evidence. Although the fossil record doesn't capture events at the origin of life, it does record some slightly later chapters in life's history, which origin-of-life researchers "ignore at their peril", according to Martin Brasier at the University of Oxford. Last year Brasier unearthed the oldest fossils so far: 3.43-billion-year-old bacteria. He found them in Australia, in non-marine rocks that formed on a beach. "I am coming round to the opinion that we may be wrong about the ocean as the mother of life," says Brasier.

This doesn't mean that Mulkidjanian has all the details correct, though. Brasier agrees with Lane that early cells probably could pump out enough sodium from their cytoplasm to survive in sodium-rich environments – so life might have emerged in salty pools or shorelines rather than in Siberian-style thermal springs.

Using observations from living cells to work out what the first cells could- – and could not – do underpins most models for life's beginnings. But there will always be a degree of interpretation in how we re-construct history based on observations of living things, and that leaves room for alternative explanations.

This situation might soon change, though. Brasier's discovery last year paves the way for fossil hunting in even older non-marine rocks – something previously considered a waste of time. Studies of early rocks will take some big steps forward in the coming decade, predicts Brasier. The evidence locked inside them might help settle the debate – and say whether Darwin's hunch was correct after all. "The rock record," says Brasier, "is the only safe witness we have."

Land ho! The search for ET

"Follow the water," NASA astrobiologists like to say in conversations about the search for extraterrestrial life. "The problem," says Paul Knauth, a geologist at Arizona State University in Tempe, "is that chlorine follows the water better than any astrobiologist."
Knauth says chlorine-rich salts made the seas on early Earth far too saline for life to emerge. Only once large quantities of salt had evaporated and were locked safely away in land-based deposits could complex life take off in the oceans, suggesting rocks played a key role in life's early stages.
What's more, many of the elements life relies on probably came from the weathering of rocks, like granite, that form only on continents, says Martin Brasier at Oxford University. "If so, the prospects for life on Mars and Titan [where such rocks aren't found] seems a bit bleak."
The same rules probably apply elsewhere in the galaxy. "So, a pale blue dot would be an exciting discovery," says Knauth. "But one with brown spots would be more encouraging."

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