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.