The details turned out to be wrong, since later studies showed that the early Earth's atmosphere had a different mix of gases. But that is almost beside the point.
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In the wake of Miller's experiment, other scientists began finding ways to make simple biological molecules from scratch. A solution to the mystery of the origin of life seemed close. But then it became clear that life was more complicated than anyone had thought. Living cells, it turned out, were not just bags of chemicals: they were intricate little machines. Suddenly, making one from scratch began to look like a much bigger challenge than scientists had anticipated.
By the early s, scientists had moved away from the long-standing assumption that life was a gift from the gods. They had instead begun to explore the possibility that life formed spontaneously and naturally on the early Earth — and thanks to Stanley Miller's iconic experiment, they even had some practical support for the idea.
While Miller was trying to make the stuff of life from scratch, other scientists were figuring out what genes were made of.
By this time, many biological molecules were known. These included sugars, fats, proteins — and nucleic acids such as "deoxyribonucleic acid", or DNA for short. Today we take it for granted that DNA carries our genes, but this actually came as a shock to s biologists. Proteins are more complex, so scientists thought they were the genes.
Charles Darwin: Origins and Arguments
They studied simple viruses that only contain DNA and protein, and which have to infect bacteria in order to reproduce. They found that it was the viral DNA that entered the bacteria: the proteins stayed outside. Clearly, DNA was the genetic material. Hershey and Chase's findings triggered a frantic race to figure out the structure of DNA, and thus how it worked.
The following year, the problem was cracked by Francis Crick and James Watson of the University of Cambridge, UK — with a lot of under-acknowledged help from their colleague Rosalind Franklin.
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Theirs was one of the greatest scientific discoveries of the 20th Century. It also reshaped the search for the origin of life, by revealing the incredible intricacy that is hidden inside living cells. View image of Credit: A. Crick and Watson realised that DNA is a double helix, like a ladder that has been twisted into a spiral. The two "poles" of the ladder are each built from molecules called nucleotides. This structure explained how cells copy their DNA. In other words, it revealed how parents make copies of their genes and pass them on to their children.
The key point is that the double helix can be "unzipped".
Each strand is then used as a template to recreate a copy of the other. Using this mechanism, genes have been passed down from parent to child since the beginning of life.
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Your genes ultimately come from an ancestral bacterium — and at every step they were copied using the mechanism Crick and Watson discovered. This media cannot be played on your device. Crick and Watson set out their findings in a paper in Nature. Over the next few years, biochemists raced to figure out exactly what information DNA carries, and how that information is used in living cells.
The innermost secrets of life were being exposed for the first time. It turned out that DNA only has one job.
Your DNA tells your cells how to make proteins : molecules that perform a host of essential tasks. Without proteins you could not digest your food, your heart would stop and you could not breathe. But the process of using DNA to make proteins proved to be staggeringly intricate. That was a big problem for anyone trying to explain the origin of life, because it is hard to imagine how something so complex could ever have got started. Each protein is essentially a long chain of amino acids, strung together in a specific order.
The sequence of the amino acids determines the three-dimensional shape of the protein, and thus what it does. That information is encoded in the sequence of the DNA's bases. So when a cell needs to make a particular protein, it reads the relevant gene in the DNA to get the sequence of amino acids. But there is a twist. DNA is precious, so cells prefer to keep it bundled away safely. For this reason, they copy the information from DNA onto short molecules of another substance called RNA ribonucleic acid.
Finally, the process of converting the information in that RNA strand into a protein takes place in an enormously elaborate molecule called a "ribosome". This process is going on in every living cell, even the simplest bacteria. It is as essential to life as eating and breathing. Any explanation for the origin of life must show how this complex trinity — DNA, RNA and ribosome protein — came into existence and started working. Suddenly, Oparin and Haldane's ideas looked naively simple, while Miller's experiment, which only produced a few of the amino acids used to build proteins, looked amateurish.
Far from taking us most of the way to creating life, his seminal study was clearly just the first step on a long road. How are we going to find organic chemistry that will make all that in one go? The first person to really tackle this head-on was a British chemist named Leslie Orgel. He was one of the first to see Crick and Watson's model of DNA, and would later help Nasa with their Viking programme, which sent robotic landers to Mars. Orgel set out to simplify the problem.
Writing in , and supported by Crick , he suggested that the first life did not have proteins or DNA. Instead, it was made almost entirely of RNA. For this to work, these primordial RNA molecules must have been particularly versatile.
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For one thing, they must have been able to build copies of themselves, presumably using the same base-pairing mechanism as DNA. The idea that life began with RNA would prove enormously influential. But it also triggered a scientific turf war that has lasted until the present day. By suggesting that life began with RNA and little else, Orgel was proposing that one crucial aspect of life — its ability to reproduce itself — appeared before all the others.
In a sense, he was not just suggesting how life was first assembled: he was saying something about what life is. Many biologists would agree with Orgel's "replication first" idea. In Darwin's theory of evolution, the ability to create offspring is absolutely central: the only way an organism can "win" is to leave behind lots of children. But there are other features of life that seem equally essential. The most obvious is metabolism: the ability to extract energy from your surroundings and use it to keep yourself alive.
For many biologists, metabolism must have been the original defining feature of life, with replication emerging later. Meanwhile, a third group maintained that the first thing to appear was a container for the key molecules, to keep them from floating off. In other words, there needed to be a cell — as Oparin and Haldane had emphasised a few decades earlier — perhaps enclosed by a membrane of simple fats and lipids. All three ideas acquired adherents and have survived to the present day. Scientists have become passionately committed to their pet ideas, sometimes blindly so.
As a result, scientific meetings on the origin of life have often been fractious affairs, and journalists covering the subject are regularly told by a scientist in one camp that the ideas emerging from the other camps are stupid or worse.go to link
Philip Ball on the evolution of Darwin | Books | The Guardian
Thanks to Orgel, the idea that life began with RNA and genetics got off to an early head start. Then came the s, and a startling discovery that seemed to pretty much confirm it. After the s, the scientists on the quest to understand life's origins split into three groups. Some were convinced that life began with the formation of primitive versions of biological cells.
Others thought the key first step was a metabolic system, and yet others focused on the importance of genetics and replication. This last group began trying to figure out what that first replicator might have looked like — with a focus on the idea that it was made of RNA. It is a single-stranded molecule, so unlike stiff, double-stranded DNA it can fold itself into a range of different shapes.
RNA's origami-like folding looked rather similar to the way proteins behave. Proteins are also basically long strands — made of amino acids rather than nucleotides — and this allows them to construct elaborate structures. This is the key to proteins' most amazing ability. Some of them can speed up, or "catalyse", chemical reactions.
These proteins are known as enzymes. Many enzymes are found in your guts, where they break up the complex molecules from your food into simple ones like sugars that your cells can use. You could not live without enzymes.