WHERE on Earth is the cradle of life? The widespread view is that life began in the oceans, in the water that surrounds deep-sea hydrothermal vents. But that story is being challenged by new evidence, which is deepening a rift between origin-of-life biologists.
Instead, hot springs on land, similar to the “warm little pond” favoured by Charles Darwin, may be a better fit for life’s nursery.
The controversial theory suggests the search for extraterrestrial life must go beyond a hunt for alien oceans (see “Land ho! The search for ET”).
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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 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 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 inside all cells contains much more potassium than sodium. Mulkidjanian thinks that reflects the chemistry of the water life first appeared in, yet seawater is sodium-rich and potassium-poor.
“There is a fundamental problem with the ocean floor hypothesis for life’s origins, and that is salt”
“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 Russia’s Mutnovsky thermal springs are potassium-rich, just like cell cytoplasm (Proceedings of the National Academy of Sciences, ). Life first appeared in similar pools, says Mulkidjanian.
The theory solves another puzzle. Most biologists agree that the earliest life 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.
Others also believe recent evidence calls into question a marine origin for life. “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. Szostak is trying to create artificial versions of the first cells, membranes and all. “Freshwater ponds,” he says, “have lower salt concentrations, which would allow for fatty-acid-based membranes to form.”
While Darwin’s warm little pond appears 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 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 first living cells were equipped to cope with high sodium concentrations.
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, ).
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 the only one looking for a land-based alternative.
Besides Szostak, Paul Knauth, a geologist at Arizona State University in Tempe, also thinks life might not have begun in the sea. He analysed the oxygen isotopes in silica-rich rocks deposited early in Earth’s history, from which you can work out what temperatures were like when the rocks formed. His results showed the entire planet was much hotter than anyone suspected – surface temperatures of 50 to 80 °C 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, 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.
“The oldest fossils so far, 3.43-billion-year-old bacteria, are from non-marine rocks”
This doesn’t mean Mulkidjanian has all the details correct. 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 do underpins most models for life’s beginnings. But the method will always be open to interpretation, which leaves room for alternative explanations.
We might be able to weed out some of the alternatives in the near future. 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 inform 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 when having 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 land 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 the University of Oxford. “If so, the prospects for life on Mars and Titan [where such rocks aren’t found] seem 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.”