What is the RNA world hypothesis and what does it postulate?
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All known life forms have three different kinds of chemicals in them: DNA, RNA, and proteins. DNA stores information, RNA takes that information and uses it as a recipe for constructing proteins, and proteins build stuff that actually interacts with the outside world–organs, enzymes, etc.
The RNA World hypothesis is that RNA came first of these three. That there was a time when life used just RNA, not the others.
RNA is capable of storing information, though not as well as DNA, and is capable of interacting with the world, though not as well as proteins, so in principle you could build a life form out of just RNA. And what seems to be the oldest bit of machinery in existing cells–the ribosome which uses the recipe to construct proteins–is suspiciously made out of RNA.
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