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 āwarm little pondā favoured by Charles Darwin, may be a better fit for lifeās nursery.
The controversial theory suggests the search for extraterrestrial life must go beyond a hunt for alien oceans (see āLand ho! The search for ETā).
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Life appeared sometime before 3.8 billion years ago, towards the end of a turbulent phase in our planetās 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 lifeās very first stages comes from chemical signatures in rocks.
This hasnāt 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 OsnabrĆ¼ck 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.
āThere is a fundamental problem with the ocean floor hypothesis for lifeās origins, and that is saltā
āThe ancient sea contained the wrong balance of sodium and potassium for the origin of cells,ā 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 Russiaās 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. āYou donāt get UV light around deep-sea vents,ā says Mulkidjanian.
Others also believe recent evidence calls into question a marine origin for life. āI do not think the oceans were a favourable environment for the origin of life ā freshwater ponds seem more favourable,ā 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. āFreshwater ponds,ā he says, āhave lower salt concentrations, which would allow for fatty-acid-based membranes to form.ā
While Darwinās 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 couldnāt cope with life in sodium-rich water. Early cells could have pumped out sodium ions, he says. āThis is exactly what many methanogens and acetogens do,ā 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 ŠÓ°ÉŌ““ labelled Mulkidjanianās 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 Earthās 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 ā surface temperatures of 50 to 80 Ā°C may have been common. The seas were also twice as salty as today, because so-called āevaporiticā deposits ā which locked away vast quantities of salt ā had not begun to form. āThe early ocean was a deathtrap of hot salty water,ā he says. āI like the idea of a non-marine origin.ā
Then there is the fossil evidence. Although the fossil record doesnāt capture events at the origin of life, it does record some slightly later chapters, which origin-of-life researchers āignore at their perilā, 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. āI am coming round to the opinion that we may be wrong about the ocean as the mother of life,ā says Brasier.
āThe oldest fossils so far, 3.43-billion-year-old bacteria, are from non-marine rocksā
This doesnāt 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 ā 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 lifeās 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. Brasierās discovery last year paves the way for fossil hunting in even older non-marine rocks ā 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 ā and say whether Darwinās hunch was correct after all. āThe rock record,ā says Brasier, āis the only safe witness we have.ā
Land ho! the search for ET
āFollow the water,ā NASA astrobiologists like to say when having conversations about the search for extraterrestrial life. āThe problem,ā says Paul Knauth, a geologist at Arizona State University in Tempe, āis that chlorine follows the water better than any astrobiologist.ā
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 lifeās early stages.
Whatās 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. āIf so, the prospects for life on Mars and Titan [where such rocks arenāt found] seem a bit bleak.ā
The same rules probably apply elsewhere in the galaxy. āSo a pale blue dot would be an exciting discovery,ā says Knauth. āBut one with brown spots would be more encouraging.ā