[Global] In spite of a handful of modest advances in understanding, the origin of life has remained largely shrouded in mystery.
The transition from lifeless inorganic matter to basic organic molecules is a relatively small first step, most scientists agree. Amino acids, the building blocks of proteins, after all, have been detected in interstellar clouds and manufactured in the laboratory with little ado.
It is a very much bigger, and rather little understood, second step to get from organic molecules to independent life forms – even in the shape of single-celled microorganisms such as amoebae, bacteria or microalgae.
(Basic life-forms like these can self-replicate and capture energy from their environment through the mechanics of metabolism – all the biochemical processes resulting in an organism’s growth, energy cycling, waste elimination and so forth.)
New research suggests that a small, common and rather dull molecule may turn out to be the secret to life’s origins. “It has only one extra oxygen atom than water and no-one usually gives it the time of day,” ANU mathematical chemist Associate Professor Rowena Ball says.
The miracle molecule, if one can call it that, is hydrogen peroxide – used for anything from bleaching hair and disinfecting toilets to powering the plastics industry.
“Its properties were determined by the 1950s, in the context of possible use in rocket fuel during the Cold War,” Ball says. “With such familiarity, it was not on scientists’ radar as a key ingredient in primordial soup until we published our latest research in the Journal of the Royal Society Interface on hydrogen peroxide’s role in the origin of life.”
The thermal activity of deep-sea hydrothermal vents on the early Earth may have provided the energy for making hydrogen peroxide.
(Water, heated by magma deep within Earth, flows from these vents, which are basically fissures on the sea floor. The water typically reaches temperatures of 400 degrees and often contains dissolved minerals that, over time, build into underwater stacks of minerals, known as chimneys.)
“Our research reveals that hydrogen peroxide and, to a lesser extent, thiosulfate, which also occurred in the ancient oceans, provide the answer. ”
ANU mathematical chemist Associate Professor Rowena Ball
“The hydrogen peroxide then underwent reactions that produced periodically varying acidity that was vital for pre-cellular life to replicate, evolve, amplify and spread throughout the oceans,” Ball explains.
More than 3.8 billion years ago, there was no deoxyribonucleic acid, or DNA, the self-replicating material present in nearly all living organisms that carries genetic information. Instead, non-cellular life forms were based on a simpler, ribonucleic acid, or RNA – a long, single-stranded chain of molecules.
“Until now, no one knew of a periodic energy source that could power the replication and amplification of RNA without cells,” Ball says. “Our research reveals that hydrogen peroxide and, to a lesser extent, thiosulfate, which also occurred in the ancient oceans, provide the answer.”
Both these chemicals undergo a reaction in which heat and acidity cycle back and forth every couple of minutes, providing the perfect conditions for early life to evolve, she explains.
“These oscillating reactions were studied for years as a curiosity – but nobody realised that their fluctuating acidity and heat could have powered early life before DNA and proteins evolved,” Ball says.
All forms of cellular life are powered by acidity gradients, known as the proton motive force, she adds. “Processes such as respiration and photosynthesis depend on this electrochemical gradient across selectively permeable membranes.”
“Now we know that the proton motive force is extremely ancient – and it was present before the evolution of cells,” Ball says.
As well as accounting for the variations in acidity, the reactivity of hydrogen peroxide was important for the development of life, she adds. “Hydrogen peroxide has just enough oxidising power to cause mutations every now and then, which would drive evolution.”
The shape of the hydrogen peroxide molecule also solves another puzzle: “Hydrogen peroxide occurs as two mirror-image forms,” she adds.
“Specific interactions of molecules with one or other of these forms may explain why key biological molecules developed as only one mirror-image form, something that has puzzled scientists for generations.”
How they did it
Ball and co-researcher Professor John Brindley from Leeds University in Britain brainstormed the problem, scribbling diagrams and maths and chemistry on the backs of envelopes, on paper napkins at cafes, and on whiteboards, and every Friday over the phone for more than six months.
Among other things, they brought together knowledge of the hydrogen peroxide and thiosulphate reaction from organic chemists, who had studied the acidity cycles, and chemical engineers who had investigated the reaction’s heat cycles.
“Waves of acidity and heat could spread away from the original heat source, which would liberate life from narrow regions around these hydrothermal vents,” Ball says.
The third and final step in the evolution of life is the rise of complex life forms, such as bigger plants and animals. This stage probably entailed less of a leap than establishing life in the first place.
Monash University evolutionary ecologist Associate Professor Bob Wong says large-scale events go some way to explaining the rise of complex life forms: “Even some of the most significant milestones in the history of life have occurred more than once, such as the ‘leap’ from single-celled organisms to multicellular ones, or the colonisation of land.”
Yet there are also examples where, if conditions change, apparent trends might cease or reverse, he explains.
“Thus, while it is tempting to imagine that, under the right conditions, life could emerge more than once, it doesn’t mean that there is an intrinsic drive towards some end goal,” Wong says.
Given the difficulties in creating organisms with metabolisms and the ability to reproduce, one is bound to ask: is life a one-off act of biochemical conjuring – a sort of freak chemical accident?
Although many scientists reckon it was accidental, others say it followed a law-like path. This refers to the idea that life is somehow written into the basic laws of nature – and that it will emerge quite spontaneously under the right conditions.
Some scientists believe that higher-order laws of physics, of which they are still unaware, probably govern such processes as the origins of life.
In his book At home in the universe, complexity theorist Stuart Kauffman suggests that higher-order laws of physics, as yet unidentified, might govern such processes as the origin of life – and perhaps even consciousness.
If scientists were able to identify such laws, this might lead to an understanding of how life emerged from “self-organising structures”. These are basically collections of small, interdependent units that spontaneously form organised structures or patterns without external intervention by a central authority.
Melbourne University botanist Professor Geoff McFadden believes that life is inevitable. “If a specific set of criteria are met, then life will evolve,” he says. “It will never play out exactly the same way and will not lead to humans inevitably. That’s just our anthropomorphism, I believe.”
The key, McFadden explains, is the ability to replicate and pass on a template for that replication. “If these criteria are met, then we have a system where Darwin’s natural selection will ensue.”
Once it starts, selection will guide ever-more-successful replicators. They could take any form and need not necessarily be cellular, McFadden says. “They might be just a big molecular soup of complex molecules able to orchestrate their own template’s replication.”
Scientists might struggle to recognise such a soup as being alive, but this sort of system is the likely precursor to cellular life, he adds.
“If they can replicate themselves from a template that undergoes slight errors, then natural selection will occur and evolution starts. Where it goes is not random but will be different each time unless conditions are identical, which is unlikely.”
Photo: A remotely operated robotic arm breaks away part of a mineral rich “chimney” from a hydrothermal vent in the Bismarck Sea, off the coast of Papua New Guinea. Water, which has been heated by magma deep within the Earth, flows from these fissures on the seafloor. Photo: Nautilus Minerals
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