Origins of life on earth
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The RNA origin of life
Where does life come from? This is one of the most important questions humanity has ever posed. And the scientific answer is: we don't entirely know. You might think that cracking DNA's genetic code should have explained life's origins. And it definitely helped----thanks to our understanding of DNA, we can map out the history of evolution all the way back to single celled life. But that's where we're stuck. The problem is, DNA is a great way to store information, but it doesn't do much else----cells rely on other molecules like proteins to replicate, grow, and survive. Proteins, on the other hand, work great as molecular machines to keep cells alive and healthy, but they can't store information or copy themselves----they need DNA for that. So we have a chicken and egg problem. DNA needs proteins to function, and proteins need DNA to exist. So which came first? Which molecule made life possible? Well, there's a third type of molecule that may hold the answer: RNA. Most scientists think that RNA came first, because RNA can do two jobs: store information and perform various functions that keep cells alive. This idea, that RNA came first, is called the RNA world hypothesis. RNA world suggests that billions of years ago, in some primordial soup of molecules, a self-replicating RNA formed. This may have happened in volcanic vents deep on the ocean floor, or perhaps clay clumps brought the necessary chemical building blocks together. Some scientists have even speculated that early RNAs formed on Mars and hitched a ride on an asteroid to our planet. One way or another, self-replicating RNAs emerged, multiplied, and evolved. Over millions of years they developed into a legion of molecular machines. These microscopic proto-life forms blossomed and competed. The best collections of code lived on, and the weaker ones died out. Survival of the fittest was the name of the game. This competition for survival eventually led RNAs to evolve the ability to build strong, stable proteins, which excelled at carrying out complex biological processes. And somewhere along the line, some critical RNAs mutated into the familiar double helix of DNA. DNA became a stable archive of genetic information that stored blueprints for the most successful RNA and protein molecules. Life became more complex over trillions of tiny steps and happy accidents. And all the while, the RNA lineup grew, alongside lengthening genomes of DNA and complex proteins. And it's all still happening----inside your body. RNAs have adapted to become the Swiss army knives of our cells. Today they can slice, dice, catalyze, build, destroy, code, replicate, and transform. A remarkable diversity from the simplest of beginnings: a single, self-replicating RNA molecule.
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