# Chemical Evolution Theory --- How did life arise? The theory of **chemical evolution** states that, through a series of [[chemical reaction|chemical reactions]] the formation of increasingly complex carbon-containing substances led to the original [[biology|biological]] molecule. Once that happened, and once natural selection took over and produced a metabolically active being with a [[plasma membrane]], the five characteristics of life were met, and life began. This is when chemistry was driving everything, there was no biology yet. There are two competing theories about how life on earth began. The first is that proteins came into existence and started replicating themselves, and the second is that RNA came into existence and started replicating themselves. ### The Urey-Miller Experiment The **Urey-Miller experiment** demonstrated that amino acids could form spontaneously under certain conditions. This experiment suggests that amino acids--like [[carbohydrates|sugars]] and [[nucleotide|nitrogenous bases]] of DNA--are inherently stable. They put in very small molecules (like NH$_3$, CH$_4$, H$_2$) in a closed system with water and sparks and heat. ![[urey-miller experiment.png]] In the end, they they found things like, HCN, CH$_2$O, and [[amino acids]] had spontaneously formed. Bear in mind, this is not a proof that this is how it happened, but it does confirm it is a possible mechanism. ## RNA World Hypothesis The **RNA world hypothesis** states that [[RNA]] was the first amino acid to form. It's simple structure and high rate of mutation would be a benefit in primitive cells, until the benefit of a more stable information-storage molecule outweighed the need for versatility and gave way to the rise of [[DNA]]. ![[RNA world.png]] ___