JETS of sulphurous steam roar out of holes in the ground and an eggy stench hangs in the air. This is Bumpass Hell, a valley of bubbling mud pools in the heart of the Lassen Volcanic National Park in northern California. The valley is ringed with beautiful pine and fir trees climbing up the surrounding slopes, but life seems to have stayed away from the lower reaches. Billions of years ago, though, the opposite might have been true.
I鈥檝e come to Bumpass Hell with David Deamer, a biochemist from the University of California, Santa Cruz, to watch him run an experiment recreating one of the most important episodes in the history of life: when carbon, hydrogen, oxygen, nitrogen and phosphorus came together in the primordial soup to form amino acids, DNA and the rest of life鈥檚 building blocks.
If Deamer is right, then the sort of extreme conditions found here were key to that momentous event. It may be an unattractive and rather dangerous place to work, but to Deamer this is one of the most precious places on Earth 鈥� the closest thing he can get to the cauldron of chemicals from which life might have emerged over 4 billion years ago.
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Researchers have spent decades trying to recreate this magical moment in their labs, and they have made some impressive discoveries along the way. In 1953, Stanley Miller, then at the University of Chicago, was the first to synthesise amino acids by passing high voltages through a cocktail of ammonia, methane, hydrogen and water vapour. In the decades that followed, researchers found other ways to synthesise amino acids and nucleotides 鈥� the building blocks of DNA and RNA 鈥� at temperatures ranging from 80 掳C to -80 掳C. They also discovered many different ways in which these molecules could assemble into larger structures similar to life鈥檚 first proteins and genetic molecules.
Test-tube life forms
Deamer is a veteran of such experiments himself. Along with Jack Szostak, a biochemist at Harvard University, he has created test-tube environments in which fatty acids and similar molecules self-assemble into cell-like structures 鈥� one of the key steps in the emergence of life. These artificial proto-cells are able to survive boiling () and can absorb nucleotides from the environment as they grow chains of RNA within ().
Still, huge gaps remain in our knowledge of how life began. The first genetic material might have been RNA, but equally it might have been some other, unknown molecule. And which of early Earth鈥檚 varied environments was the one that first spawned life 鈥� did it happen in a deep-sea hydrothermal vent, on frigid polar sea ice or in boiling cauldrons of clay and water like the ones at Bumpass Hell? We just don鈥檛 know.
Deamer and a few other like-minded researchers have concluded that lab work alone can get them no further. They have decided to find out which of the experiments that work so well in a squeaky-clean laboratory can be reproduced in the messy real world. 鈥淭he prebiotic world was much more complex than a laboratory situation,鈥� says Deamer. He thinks that doing experiments in places like Bumpass Hell will help narrow down the environments that are realistic candidates for the origin of life. Forget the theory, he says: he wants to see which candidates actually work.
It鈥檚 all too easy to make a false discovery, however. Even in a thoroughly controlled environment, just a few bacteria creeping into the apparatus undetected can ruin the experiment. You could mistakenly conclude that your chemical soup was generating DNA which had in fact come from these trespassing microbes. To prevent such contamination, Miller baked his glassware near its melting point for up to 24 hours.
Swamping contaminants
Of course, Deamer can鈥檛 do that in his mud pools. But unlike Miller, he has a way of distinguishing between synthesised and bacterial nucleotides and RNA. Deamer adds enough bio-material to swamp any belonging to bacteria by a factor of around 100, which means that any synthesised biomolecules will also swamp any signal of bacterial origin. There is no better way to test if a candidate environment could have led to life on early Earth, he says.
Other researchers have tried experiments in cold environments. In 1999, Hauke Trinks, then at the Hamburg University of Technology in Germany, travelled to the Arctic to study the properties of sea ice as an incubator for life. He measured the ice鈥檚 ability to trap and concentrate RNA molecules inside microscopic pockets of unfrozen brine (快猫短视频, 12 August 2006, p 34). Deamer, on the other hand, thinks life emerged in a very different environment.
Bumpass Hell lies near a volcano that last erupted in 1915. The cauldrons that belch boiling mud are a potent reminder that the volcano is merely dormant, not extinct. Water from rain and nearby streams continually drains through fissures in the valley to a spot 4 kilometres below the surface, where it meets molten lava and flashes into steam. In the geological equivalent of an espresso machine, the steam hisses back up through cracks 鈥� carrying sulphuric acid, smelly hydrogen sulphide, iron and other substances to the surface. The pools are a mess of chemicals, including a lot of sulphuric acid, and the edges of the vents undergo regular cycles of wetting and drying, heating and cooling. Industrial chemists have long known that cycles of drying can help kick-start chemical reactions that don鈥檛 work in moist conditions. Deamer thinks these cycles could also drive important biochemical reactions, like RNA synthesis, outside of a cell.
鈥淎s if in a geological espresso machine, steam hisses up through the cracks鈥�
RNA, believed to be a precursor to DNA, consists of a chain of small molecules called nucleotides linked together like the carriages of a train. Inside cells, enzymes catalyse the linking process, but RNA chains are difficult to grow under most natural conditions in the absence of enzymes because the chemical bonds between the nucleotides break as easily as they form (it鈥檚 the same for other biological polymers, like DNA and proteins). Every time two RNA nucleotides bind together they release a molecule of water as a by-product. And when two nucleotides split apart they absorb a molecule of water (see diagram).
One way to manufacture long molecules like RNA is to get rid of the water that the chain releases as it grows, so that it鈥檚 not around to break the bonds after they form. Removing water actually drives the growth of chains by shifting the chemical equilibrium toward forming bonds instead of breaking them. As Deamer puts it, life is basically made by removing water molecules from between nucleotides. A hot place like Bumpass Hell, where water is constantly evaporating, would provide the perfect environment for this.
But does Bumpass Hell really reflect conditions on the early Earth? There may be no trees or grass on this infernal ground, but plenty of bacteria live here. Oxygen represents another wild card: 4 billion years ago the atmosphere contained very little of it.
Furthermore, the composition of the clay at Bumpass Hell is probably radically different from the mud on early Earth. Robert Hazen, a geochemist at the Carnegie Institution in Washington DC, has constructed a history of what he calls Earth鈥檚 mineral evolution (快猫短视频, 22 November 2008, p 14). Of the 4400 minerals known today, he estimates that fewer than 1000 existed 4 billion years ago (). Volcanic clays like those at Bumpass probably did exist, but their composition was likely very different from what is found today. 鈥淭he chemistry of [modern] life will affect these experiments, there鈥檚 just no way around that,鈥� says Hazen. 鈥淚t will not be mimicking early Earth.鈥� Despite this, Hazen still sees value in Deamer鈥檚 experiments as a filter to identify which of the environments that work in the test tube are robust enough to succeed in the more variable conditions that exist outdoors.
So what exactly has Deamer found? Suppose that the chemical soup in which life is thought to have arisen on early Earth contained RNA nucleotides or similar molecules. Water that splashed from a boiling pool could deposit these molecules onto dry land, where the water would evaporate. Deamer thinks this would cause the nucleotides to link into short chains of RNA. New splashes of water would deliver a fresh supply of nucleotides, which would grow the chains as they dried. The process might repeat over and over. 鈥淐omplexity as we know it in life is longer and longer molecules,鈥� says Deamer. So how do you drive that? The most likely way is an environment that cycles between wet and dry, he says.
Deamer and others have grown RNA in such an environment in the lab. 鈥淭hat boiling pool over there with wetting and drying around the edges is what we try to simulate in the laboratory,鈥� he says, nodding at a milky yellow mud pot a few metres away. Now he wants to see if those experiments work in the real world.
He tests the acidity level of various lumps of clay that he has collected: 鈥淭hat鈥檚 almost stomach acid,鈥� he mutters, setting down one of the yellow-white lumps. He then squirts each one with a few drops of liquid, adding nucleotides to some, nothing to others acting as controls, and to yet others he adds RNA 鈥� to see whether it can survive the 80 掳C temperatures and the concentrated sulphuric acid present in the clay. He then sets them in the opening of a vent to cook.
Life in the gutter
This real-world approach is slowly gaining supporters. 鈥淭hese experiments bring something that is sometimes missing in the lab,鈥� says Pierre-Alain Monnard, a biochemist at the University of Southern Denmark in Odense. 鈥淚 think it could lead to a discovery that no one thought about because we are working in clean environments compared to what life evolved in. We are almost working in the hospital, and life started in the gutter.鈥�
A few weeks after the field trip, Deamer鈥檚 preliminary findings are already challenging lab-based results. He now thinks the first chains of RNA may have grown wrapped in blankets of concentrated sulphuric acid. 鈥淵ou remember me kneeling in the clay? The acid was so strong it ate out the knees of my jeans. On the way home both knees fell out.鈥� And yet it seems that, despite the acid and the heat, the nucleotides and RNA Deamer was experimenting with have survived. What鈥檚 more, in the lumps of clay to which he added single nucleotides, he now sees what look like chains of RNA. Other clay samples to which he added nothing show no RNA 鈥� suggesting the new RNA strands are not the result of contamination.
鈥淭he acid was so strong it ate out the knees of my jeans鈥�
He thinks he can explain how this happened: a film of sulphuric acid must cover the surface of the clay. If concentrated enough, the acid will act as a drying agent, sucking water molecules out of any source that it can 鈥� including a growing chain of RNA 鈥� just as evaporation would. 鈥淚t literally pulls water out of the compounds,鈥� says Deamer. In fact, chemists have used sulphuric acid in this way to synthesise organic chemicals in the lab.
The idea of sulphuric acid providing a womb for growing fragile molecules of RNA is controversial, since most people would expect the acid to break the RNA chain apart 鈥� a process that chemists call hydrolysis. But sulphuric acid is an effective drying agent. Some scientists have even suggested that sulphuric acid rather than water could be a medium for life on other planets (快猫短视频, 9 June 2007, p 34).
Deamer needs to re-analyse his samples to confirm that he really did grow RNA inside the clay. But already he is ploughing ahead with lab experiments to grow RNA in sulphuric acid, the way he thinks happened at Bumpass Hell. He also hopes to sample other types of hot springs. Top of his list is the silicon-rich environment around the volcanoes of Hawaii.
Many more independent trials and lab experiments will be needed to determine if his conclusions are correct, but if his idea about the role of sulphuric acid proves true, it will provide a completely new view of the origin of life on Earth. 鈥淭hat鈥檚 the kind of thing that only happens if you actually go out in the field and try these things,鈥� he says.
