Origin of Life
Oparin-Haldane Hypothesis
The Oparin-Haldane hypothesis is a proposed explanation for how life first formed. According to this idea, the inorganic molecules that were present in the early atmosphere were able to form building blocks of biomolecules such as amino acids and nucleotides. These were then able to go on and form self-sustaining biomolecules, such as self-replicating RNA, and through a gradual process, eventually resulted in cells and the first life.
Miller-Urey Experiment
The Miller-Urey experiment was an experiment conducted in the 1950s, and is considered to be the classic experiment looking into abiogenesis (the idea that life first arose from nonliving molecules).
This experiment involved recreating what was believed to be the characteristics found on early Earth. Water, methane, ammonia, and hydrogen gas were present in the experiment. Water was heated to evaporate and create water vapor, which would mix with the other gases in the "atmosphere" and would be exposed to electric sparks which were meant to stimulate lightning. The experiment was set up so that the water vapor would condense back into water and then cycle back towards the heat source, where it would be evaporated and the cycle would continue.
The results from this experiment showed that amino acids could spontaneously form under these conditions, providing evidence to back up the Oparin-Haldane Hypothesis.
The RNA World Hypothesis
Related to these ideas for how life began, the RNA world hypothesis is an idea that self-replicating RNA was the first form of life and that it existed prior to the evolution of DNA and proteins.
While there have been other proposed models for the first type of life to exist, this hypothesis has rather wide acceptance due to the evidence that can back it up.
RNA can store and replicate genetic information, can catalyze chemical reactions (ribozymes are a catalytic RNA molecule, kind of like an "RNA enzyme"), can play a role in regulating gene expression, and RNA is the primary component of the ribosome - the organelle responsible for making proteins that is found in every cell.
As RNA can carry genetic information (which can vary), self-replicate (and therefore pass on the variations), and catalyze reactions (some of which can catalyze better than others), it is possible for RNA as the first form of life to have been undergoing the process of evolution.