IN 4.5 billion years of Earthly history, life as we know it arose just once. Every living thing on our planet shares the same chemistry, and can be traced back to “LUCA”, the last universal common ancestor. So we assume that life must have been really hard to get going, only arising when a nigh-on-impossible set of circumstances combine.
Or was it? Simple experiments by biologists aiming to recreate life’s earliest moments are challenging that assumption. Life, it seems, is a matter of basic chemistry – no magic required, no rare ingredients, no bolt from the blue.
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And that suggests an even more intriguing possibility. Rather than springing into existence just once in some chemically blessed primordial pond, life may have had many origins. It could have got going over and over again in many different forms for hundreds of thousands of years, only becoming what we see today when everything else was wiped out it in Earth’s first ever mass extinction. In its earliest days on the planet, life as we know it might not have been alone.
And what about life on other planets? Read about the search for life in the solar system’s other seas
Just to be clear, what we are talking about came long before animals or plants or even microbes. We are going right back to the start, when the only things fitting the description of “life” were little more than molecular machines. Even then, having stripped away bodies, organs and cells and reduced everything down to the essential reactions, things appear devilishly complex. At a bare minimum, life needs some kind of code, it needs to use that code to make useful molecular machines, and then the code must be able to make copies of itself.
“What if life were easy? No magic, no rare ingredients, no bolt from the blue“
Over the decades, people have invoked all sorts of external forces to explain how some of the starting components were made. In the famed Urey-Miller experiments of the 1950s, the trigger was a zap of electricity mimicking a lightening bolt striking water (see “Bolt from the blue“). Other theories have invoked extraterrestrial delivery by meteorites or comets.
More recently, chemists interested in the origins of life have taken a more methodical approach to the problem. By breaking down the very beginning into its component stages (see “Four steps”), they are stripping away the mystique that surrounds that initial spark of life. What they have discovered points to a very different beginning.
For of the MRC Laboratory of Molecular Biology in Cambridge, UK, the difference between life and non-life is genetic code. “Biology has memory, while chemistry does not,” he says. “To me, the origin of life is really the origin of information.” Many biologists subscribe to the RNA world hypothesis for the beginnings of life, which says that before DNA, this information was embodied in its close relative, RNA. Both molecules are long strings made of repeating units, or “letters”. So step one to building life, in this version of events, has to be making the building blocks of RNA.
In May, , a biomolecular chemist at the Ludwig Maximilian University of Munich,Germany, announced that his team had found a very easy way to make some of these units, from substances that could have been abundant on early Earth. “You don’t need much – just take hydrogen cyanide, ammonia and formic acid, and there you go,” he says.
Carell’s building blocks are precursors of the ones found in cells today, meaning they are nearly but not quite the finished thing. Even so, his reactions hint that an RNA world could have been made relatively easily. “That’s the beauty of our work,” he says. “You really don’t need special conditions. These reactions can happen everywhere – a little pond, the deep sea, wherever.”
So we have the letters of the code, though they’re not much use in free-floating form. The next stage might be easy, however. Twenty years ago, British chemist Leslie Orgel showed that if you could only get the building blocks of RNA to form they would spontaneously assemble into chains. All he needed was clay. That’s because crystals in some clays carry a natural electrical charge which appears to pull RNA letters in and encourage them to line up and stick to each other.
From code to machines
We are still some way from life, though. Code is only useful if it acts as a template to make things like proteins – the building materials and engines of all living things. In our bodies, this is known as gene expression, and is an incredibly complicated process overseen by sophisticated molecular machines. They are very unlikely to have emerged just so from the primordial mud. How do you do away with all that in the earliest life forms? at the University of Colorado in Boulder believes he has the solution. “We spent several years in the wilderness, doing experiment after experiment,” he says. “We did it frozen, we did it dry, we did it in solutions – we did it every way we could think of.”
Finally, his team found a surprisingly simple reaction which they say looks like very rudimentary gene expression. By mixing repeating strands of RNA in water with extra free-floating RNA letters, they found that the letters would spontaneously arrange themselves to form new molecules. The original strand of RNA seemed to act as a template. Intriguingly, , called coenzymes.
Today, coenzymes do very little on their own. Their job is mostly to help other, larger enzymes do theirs, but perhaps they are relics from life’s earliest stages. Many are made from the same types of components as RNA and DNA. “It’s reasonable to argue that they could be molecular fossils,” says Holliger. Yarus says his team was able to create nicotinamide adenine dinucleotide (NAD), a coenzyme that is used by all living cells today to generate energy from sugar, though the work has yet to be published.
“This would have been the very first mass extinction, billions of years ago“
Just like Carell’s RNA components, these kinds of reactions happen relatively easily. “You don’t need magic, you don’t even need exotic chemistry. You just need things that are lying around in almost every chemistry lab and you can go right to gene expression,” says Yarus. He says his experiments allow you to see “a continuous line of descent going right back to the origin event”.
Then again, they might not. The trouble with trying to figure out exactly how life began is that we can never truly know the answer, only make educated guesses. , an evolutionary chemist at Harvard Medical School says he suspects coenzymes like NAD could form spontaneously in water without RNA templates. If that is true, it weakens Yarus’s claim that he has reproduced a precursor of gene expression.
Szostak also has reservations about Carell’s reactions, saying they wouldn’t necessarily have worked on early Earth. “It’s a step,” he says, “it’s just not the final answer.” Szostak does, however, have his own piece of the puzzle to add.
Ask anyone what they think the key properties of life are and sooner or later most will say “reproduction”. Living things make copies of themselves, inert things like rocks do not. Without reproduction, life is a dead end.
Today, we have special enzymes tasked with replicating DNA. But , Szostak’s team showed that RNA can efficiently copy itself without help from any enzymes. They mixed an RNA template with free-floating RNA building blocks just as Yarus did. But Szostak added a few RNA fragments that matched parts of the template. And that made all the difference. The fragments seemed to kick-start a replication process and soon the team had reasonably faithful copies of the templates.
“These reactions happen pretty easily,” says Szostak. Others have been able to copy RNA without enzymes before, but these reactions are faster. He says the small booster fragments are so short they could have formed spontaneously 4 billion years ago.
Szostak’s team thinks these reactions or similar ones could have been an early form of replication, even though they might not have generated perfect copies each time. Eventually, new templates would have evolved, coding for really useful inventions like building cell walls. At that point, better copying would have become important. “There’s strong selective pressure to evolve replication machinery once there’s something worth replicating,” says Szostak.
Multiple beginnings
Taken together, all these findings suggest that building a rudimentary RNA world may not have been the special, once-in-a-universe occurrence it is popularly made out to be. This raises an intriguing possibility: that life’s earliest stages didn’t happen just once, but over and over again. “If all the steps leading to life are easy, then life could have been starting up many times in many places on early Earth,” says Szostak.
If this is true, then life’s first epoch was one of great experimentation. Many different kinds of live molecular machines would have popped up in the primordial soup, some more successful than others. For a time, they would have coexisted, but eventually only the most successful remained, either because it was better than everything else, conditions changed and favoured it, or by sheer chance. This would have been Earth’s very first mass extinction, billions of years ago.
We have very few relics left to tell us about the age of experimentation, or its end. We do know that LUCA must have emerged as the ultimate winner, going on to give rise to all future living things. “LUCA must have been an exceptionally successful organism, because if there are any other branches on the tree, they have not left any other descendants in modern biology,” says Holliger.
An important development over the past 20 years has been the realisation that LUCA was quite an advanced organism, says , an evolutionary biologist at the University of Maryland in Baltimore County. LUCA probably already used DNA instead of RNA, and was packaged inside a membrane which created a micro-environment for building complex proteins. Holliger speculates that innovations like these may have meant it survived when all alternative forms of life died out.
But with everything else long gone, test tubes are our only hope for a glimpse of how life got started. “The only way is to try to grow these systems and see what could form and what could work,” says Szostak. Who knows – along the way, it’s just possible we could create whole new kinds of life ourselves.
A bolt from the blue
In the 1950s, two chemists – Stanley Miller and Harold Urey – were the first to show that some of the essential building blocks for life can be made from simpler materials. The critical step was electricity. They mixed water with gases they thought would have been present on early Earth and zapped them with simulated lightning. This produced amino acids, the molecules that all modern proteins are made of.
Life did not necessarily need proteins to get started, but they certainly became necessary at some stage, and all living things that exist today rely on the same 20 or so amino acids to make proteins. It is becoming clear that amino acids must form very easily indeed: they have been found almost anywhere in space astrobiologists care to look. “About half are given to us free by the universe,” says at the University of Maryland. Some have been found on meteorites and by the Rosetta probe orbiting comet 67P/Churyumov-Gerasimenko.
Experiments still suggest there must have been some input of energy to get these amino acids – be it shock waves from a meteorite impact or heat transmitted from deep in the Earth via a hydrothermal vent. So although building a genetic code may not have required a bolt from the blue, building proteins probably did.
Where on Earth?
As the steps that could have created life become clearer, we might get a better idea of where it all began.
We know that clay can help join life’s building blocks together to form strands of RNA (see main story). Unfortunately, that’s one of the world’s worst clues, says Michael Yarus of the University of Colorado in Boulder: “Clay is everywhere.” But life has many more requirements, and there are going to be a limited number of locales where you could have brought them all together, he says.
So far, his experiments suggest life could have begun in water that would sometimes get injections of different chemical elements to feed new reactions. at the University of Toronto, Canada, studies isolated pockets of ancient water trapped deep inside some of the world’s oldest rocks. Fractures in the rocks open and close up again, she says, providing those kinds of conditions.
Rocky fractures like this could have also provided surfaces where reactions could take place while protecting nascent life from the UV radiation and heavy asteroid bombardment that would have made a lot of Earth’s surface inhospitable.
What are the odds of finding relics of life’s cauldron? Sherwood Lollar’s rocks are mostly a few billion years old – too young for the first living things. “We do have a very small amount of rock on this planet that still exists from 4 billion years and older,” she says. But it would be an incredible stroke of luck if it happened to harbour life’s origins: in total, these very ancient rocks now make up an area no bigger than Manhattan.
This article appeared in print under the headline “Life, spontaneously”
